Download Alchemist`s Cookbook Student Part 2 (final)

Honors Chemistry
The Alchemist’s Cookbook
2014-15
Semester 2
© Steven G Sogo 2015
Table of Contents (Spring 2014)
Page #
Assignment
146
147
148-152
153
154
155
156-157
158
159-161
162-163
164-167
168
169
170-171
172-173
174
175
176
177
178
179
180-181
182-183
184-185
186
187
188-189
190
191-192
193-195
196
198-200
201-203
204-205
206
207-208
209-210
211
212-213
214
215-216
217-220
221-224
#86 Thought Quiz on Electrons in Atoms
#87 Web-Based Rutherford Experiment Worksheet
#88 May the Force Be With You (computer sims of forces in atoms)
#89a Trends in Atomic Radii
#89b Ionization Energies
#89c A Graph of Ionization Energies
#89d Ayeeh!
#90 Quantum Chemistry: An Introduction
#91 The Bohr Model
#92 Interpreting Equations (Lab)
#93 The Photon Game
#94 Photon Game Questions
#95 The Colors of the Rainbow
#96 Atomic Orbitals (parts a & b)
#97 Wir Bauen Auf (aufbau diagrams and electron configurations)
#98 Songs without Words (periodic table orbitals)
#99 Singing the Song (the orbital song)
#100 Orbital Architecture
#101 Sir Charles (invents a gas law)
#102 A Bag of Hot Air
#103 ATM Stands for Atmosphere
#104 Exploring Boyle’s Law with Syringes (MINILAB)
#105 Under Pressure
#106 The Lab with Many Flasks (LAB)
#107 Applying Gas Laws
#108 One Law to Rule Them All (the ideal gas law PV=nRT)
#109 Gases in Chemical Equations
#110 Problem Solving using a Graphing Calculator
#111 The Giant Muffin (gas stoichiometry)
#112 The Volume is Right! (LAB)
#113 Gas Density and Simple Gas Laws (Practice Problems)
#114 Launching Rockets using Secret Fuel #1 (LAB)
#115 The Other Side of the Mountain (activation energies)
#116 Calculating your Carbon Footprint
#117 Scientific Notation Introduction
#118 Mr. Toad and Moley
#119 Pink be Gone (LAB)
#120 Molecular Structure Practice Page
#121 Acids Rock!
#122 Bases Rule!
#123 Through the Looking Glass (visualizing acid-base rxns)
#124 The Love Song of J. Alfred Proton (LAB)
#125 Should I Stay or Should I Go? (Acid Base Simulations)
Table of Contents (continued)
Page #
Assignment
225
226-227
228
229-230
231-232
233-236
237
238-239
240
241
242
243-244
245-247
248
249-252
253-254
255-256
257
258-260
261-262
263-265
266
267
268-269
270
271
272
273
274
275-276
277
278
279
280-281
282-283
284
285
286-288
289-290
291
292
293-294
295
296-297
#126
#127
#128
#129
#130
#131
#132
#133
#134
#135
#136
#137
#138
#139
#140
#141
#142
#143
#144
#145
#146
#147
#148
#149
#150
#151
#152
#153
#154
#155
#156
#157
#158
#159
#160
#161
#162
#163
#164
#165
#166
#167
#168
#169
Follow-up Questions for the Acid-Base Sims
The Nature of Chemical Equilibrium
Introduction to Ka Values
Cover Your Bases! (relating base strength to acid strength)
Breaking Up is Hard to Do (conductivity in aqueous solutions)
Good Titrations (LAB)
I Can Do Logarithms!
Powers of Ten
Phun with pH (A pH chart)
Introduction to Percent Dissociation (strong vs. weak acids)
Playing the Percentages
How do you Spell KaT?
The Color of X (LAB) (Equilibrium is all Around Us)
Review of pH Calculations
The Equilibrium Island
Introduction to Le Chatelier’s Principle
Applying Le Chatelier’s Principle
How Buff is your Buffer?
Building a Buffer (LAB)
Acid-Base Summary
Less Than Zero (LAB)
Note-Taker for Nuclear Chemistry
Alpha Beta
All My Children (the family tree of uranium)
Mr. Fusion
Cloud Chambers (MINILAB)
Alien Blood Lab
I’ve Got Your Number (intro to oxidation numbers)
LEO says GER
Redox Mini-Lab (MINILAB)
Summary of Oxidation and Reduction
Wolves and Goats (introduction to balancing redox rxns)
Feed the Wolf (practice balancing redox rxns)
Color by Number (LAB) (permanganate rxn with oxalic acid)
Two Halves make a Whole
The Kid’s Got Potential
Redox Practice Prior to E4E
A Shadow of Doubt (LAB) (oxidizing anions)
How to Build a Battery
Shock Therapy (intro to electricity)
Signor Volta
Let There Be Light! (LAB)
Introduction to Electrolytic Cells
Self-guiding Analysis of Electrolysis
Table of Contents (continued)
298
299-300
301
#170 Redox and Electrochem Concept Review
#171 Electrolysis Mini-Lab (MINILAB)
#172 Electrolysis of a Nickel (LAB)
146
Assignment #86: Thought Quiz on Electrons in Atoms
We have fun learning what we don’t know. . .
1. Which of the following best describes the motion of electrons within an atom:
a) completely random
b) they go around in orbits, kind of like the solar system
c) they live in houses, at least most of the time
d) sometimes they disappear entirely
2. Electrons in atoms are often said to reside in "shells" around the
nucleus. In general, more complex atoms have more electron shells. The
number of shells present in a Uranium atom (atomic number = 92) is:
a) 1
b) 3
c) 5
d) 7
e) 9
3. Which of the following atoms has the largest diameter:
a) Li
b) C
c) N
d) F
e) their diameters are all the same
Shell #
1
2
3
4
5
6
ee3p
e-
diameter
4. A neutral sodium atom contains 11 protons, 12 neutrons, and 11 electrons. A
sodium ion (Na+) contains 11 protons, 12 neutrons, and only 10 electrons.
Compared to a neutral sodium atom's radius, the radius of a sodium ion (Na+) is:
a) about 10% larger
b) almost the same
c) about 10% smaller
d) about 50% smaller
5. The difference between green light and yellow light is:
a) green light is a mixture of light colors
b) yellow light has a different energy than green light
c) yellow light travels faster than green light
d) yellow light doesn’t really exist—it’s just a figment of human imagination
6. If light of the proper wavelength is absorbed by a hydrogen atom:
a) the electron in the atom may move to a higher energy level
b) the atom may turn into an H+ ion
c) the same wavelength of light may be emitted by the atom some time later
d) energy will be conserved
Capacity
2 e8 e18 e32 e50 e72 e-
147
Assignment #87: Web-based Rutherford Experiment Worksheet



Search Google for “Rutherford Experiment Animation”
Choose the “Gold Foil—MHHE” site
The URL for this website is shown below:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
The website has a narrator (so you’ll need sound) and 4 separate animations. The first animation
appears when you first visit the site—the others are accessed by clicking on the appropriate
buttons. Replay the animations as many times as necessary to gain an understanding of the methods
employed and the overall significance of the Rutherford Experiment. Feel free to visit other
websites to improve your understanding.
Answer the following questions as you gain understanding.
1. J.J. Thomson’s Plum Pudding model (proposed in 1904) was the best explanation for
atomic structure that scientists possessed prior to the discovery of the proton (in 1917) and
neutron (in 1932). Therefore, if you had been a chemistry student in 1906, you would have
drawn atoms using the Plum Pudding model. In the space below, sketch a picture of a Plum
Pudding atom and explain how it is different from the pictures of atoms you have been trained
to draw as a 21st century student of chemistry.
2. Explain why Rutherford expected his alpha particles to pass through the gold foil he used in
his famous experiment. Hint: consider what the term "pudding" connotes. . .
3. Use a sketch to explain why a few (about 1 in 20,000) of Rutherford’s alpha particles were
deflected in his experiment. Hint: your sketch should show the “special event” that had to occur to
cause an alpha particle to bounce back.
4. How did the Rutherford experiment change the way scientists sketched pictures of atoms?
148
Assignment #88: May the Force Be With You
Investigating Atomic Structure Using Interactive Physics
Overview: In today’s activities, you will investigate how ELECTROSTATIC FORCES and VELOCITIES
interact to influence an electron’s motion within an atom. The word VELOCITY is used to describe motion
with a particular speed and direction. In today’s sims, velocity will be measured in both the x-direction
and the y-direction (as depicted on a coordinate plane). Electrostatic forces are attractive and repulsive
forces based on CHARGES. Both forces and velocities can be represented by arrows called vectors.
Task 1: Building an Atom from Scratch
1) In the Chemistry Sims folder, find the Atomic Structure subfolder and open the simulation entitled
“Sim #1: Building an Atom”. You should see a red circle named “Nucleus” and a smaller blue circle
named “Electron” on your screen. Click on the run button atop the screen to observe the simulation.
Rerun the simulation at least once so that you understand the electron’s motion. In the space below,
provide two sketches that display the vectors (both force and velocity) that describe the electron’s
motion at two time points: just after motion has begun and just before collision occurs. Note: you
should ponder why the nucleus does NOT move noticeably in this simulation!
Sketch of force and velocity vectors
just after motion commences
Sketch of force and velocity vectors
just before collision
2) Double-click on the electron and change its Vx value to +1.5 m/s. This will give the electron a
in the x-direction (i.e. the electron will start the simulation moving towards the right of the
screen). Run the simulation. Record your observations and discuss how this simulation illustrates the
general truth that FORCE MODIFIES VELOCITY.
VELOCITY
Nucleus
+1
Force
e-
Initial velocity
3) Experiment with other Vx values as suggested in the table below. FILL IN THE TABLE to describe how
the velocity of the electron affects whether it will stay bound to the nucleus or not.
Electron
velocity
Resultant motion of electron
Zero
e- collides with nucleus
Final KE of
electron
Moderate (2 m/s)
High (5 m/s)
4) Use the slider control to increase the number of protons in the nucleus to five instead of one. Then
rerun the simulation with an x-velocity of 5 m/s. Describe the results of this simulation and explain why
the results are different from those recorded in step #3.
149
Assignment #88: May the Force Be With You (continued)
In a simple system involving two electrically charged bodies, the potential energy of the system can be
calculated using the mathematical formula shown below:
K = electrostatic constant
Q1 = charge of first object
Q2 = charge of second object
d = distance between the charged objects
In the simulation you have just run, Q1 (charge of proton) = +1, Q2 (charge of electron) = -1, K = 90,
distance between objects (initially) = 4 m.
5) Based on these values, calculate the PE of the original system. Note: your answer should be a
negative value, indicating that the objects are bound together in a low potential energy state.
6) The electron’s kinetic energy = ½ mv2. In this simulation, the electron’s mass is 5 kilograms (which
is unrealistic, but mathematically convenient). Calculate the speed (velocity) that the electron would
require to escape the negative PE calculated in (#5). Then run the simulation using your calculated
velocity and describe the results.
Calculation:
Simulation results:
7) Suppose the simulation incorporated a two-proton nucleus instead of a 1-proton nucleus. Which of
the following statements best expresses the truth about the velocity an electron would need to escape from
this nucleus? If possible, provide evidence to support your choice.
a) The electron would require double the velocity to escape the 2-proton nucleus.
b) The electron would require double the kinetic energy to escape the 2-proton nucleus
c) The two statements given above are equivalent!
8) Restore the electron’s Vx value to 0. Then change its Vy value to +0.5 m/s. This will give the
electron a velocity that sends it toward the TOP of the computer screen. Run the simulation several times
to observe the path followed by the electron. Sketch the results in the space below, and try to explain
why the path traveled by the electron curves: Note: to trace the path of the electron, go to the WORLD
menu, choose TRACKING, and click “every 4 frames”. To erase the track, type Control-E
Note: include initial force and velocity vectors in your sketch—it is the interaction of these
vectors which causes the electron’s path to curve!
150
Assignment #88: May the Force Be With You (continued)
9) Every simulation you have run thus far has resulted in the electron crashing into the nucleus. In a real
atom, electrons orbit the nucleus at a discrete distance. Experiment with altered electron velocities to
make the electron ORBIT AROUND THE NUCLEUS AT A DISTANCE (similar to the way the Earth orbits the
Sun). Make a sketch of your “atom” in the space below and fill in proper descriptions for the results
observed when the velocity is too fast or too slow. Note: your orbit may end up being elliptical rather than
circular—you should experiment with different velocity values to see how you can get closer to a circular orbit. If
you haven’t yet turned on TRACKING from the WORLD menu, you’ll certainly want to do so now (track every 4 frames).
If velocity is too slow:
Sketch for “just right” velocity (stable orbit)
Note: try to get close to a circular orbit here!
If velocity is too fast:
10) In question #5, you calculated that an electron at a distance of 4 meters from the nucleus has a
potential energy of -22.5 Joules, indicating a bound state. Which one of the following is a reasonable
value for the total energy (kinetic + potential) of an electron orbiting at a constant distance of 4 meters?
Explain.
a) +5 joules
b) -11 joules
c) -22.5 joules
d) -35 joules
11) Your next goal is to investigate how the charge of the nucleus affects the velocity of an orbiting
electron. Using a velocity that creates a circular orbit, increase the nuclear charge by moving the slider
to a larger number of protons. Then adjust the velocity of the electron so that it can orbit in a circle
around this more highly charged nucleus. Use the data table below to record the velocity values that
worked for a number of different nuclear charges and TRY TO DISCOVER A MATHEMATICAL
RELATIONSHIP relating # of protons to electron velocity. Note: use the graph paper grid to plot
your data!
# of protons
e- velocity
151
Assignment #88: May the Force Be With You (continued)
Task 2: Looking at an atom with more than one electron
Load Simulation #2: Atom with Two Shells. Run the simulation and note the motions of the two
electrons.
13) In the space below, make a sketch of what you observe in the simulation. In your sketch, use the
words “inner shell” and “outer shell” to refer to the paths followed by the two electrons.
14) Imagine that the energy well for an electron orbiting a nucleus can be represented by a funnel as
shown below. In this funnel, sketch two circular electron orbits, one corresponding to the inner shell
and one corresponding to the outer shell of the atom represented in this simulation.
0
Negative PE
+
15) How would increasing the number of protons affect the size or shape of the funnel shown in
question #14?
16) In this simulation, the inner shell electron possesses a greater kinetic energy, while the outer shell
electron has a greater potential energy.
Which of the following ideas correctly describes the overall relative energy between the inner and outer
shells? Explain briefly.
i. The inner shell electron would need to GAIN ENERGY to get farther away from the nucleus
ii. The inner shell electron would need to LOSE ENERGY to get farther away from the nucleus
152
Assignment #88: May the Force Be With You (continued)
Task 3: Adding and Subtracting Electron Energies
Open the simulation entitled “Sim #3: Energy Boost with a Baseball Bat”.
Run the simulation and observe how the baseball bat interacts with the electron in the simulation.
17) The first collision between the baseball bat and the electron gives the electron an extra BOOST of
energy. What effect does this energy boost have on the SIZE of the electron’s orbit?
18) When an electron in an atom is boosted up to a higher energy orbit, the atom is said to be in an
“EXCITED STATE”. Do you think an excited state is more or less stable than the original state of the atom
(called the “GROUND STATE”)? Hint: recall that stability is related to the potential energy of an atom.
19) In reality, a baseball bat is not the appropriate tool to boost an electron’s energy level within an atom.
How do you think a chemist could realistically boost the energy of an atom’s electrons?
20) Consider an electron in an excited state returning to its ground state. Would such an event be an
exothermic or an endothermic process? Explain briefly.
21) Reset the simulation and experiment with other velocity values for the baseball bat. Record any
interesting findings in the table below: Note: there is an on-screen SLIDING CONTROL for the baseball
bat’s velocity.
Baseball bat velocity
Results
22) When an electron gets completely knocked out of an atom, the atom is said to be “IONIZED”. Use the
letters below to fill in the blanks to make a meaningful definition: A E E I I I O O Y N
_ _ N _ Z _ T _ _ _ _ N _ R G _ = amount of energy needed to knock
an electron completely out of an atom
153
Assignment #89a: Trends in Atomic Radii
Excerpted from Zumdahl Chemistry (6th edition) page 915
The numbers in this chart represent ATOMIC RADII measured in picometers (10-12 meters)
One shell
Two shells
3 shells
4 shells
5 shells
6 shells
1. Each row of the chart corresponds to a particular number of electron shells (e.g. third row elements all
have 3 shells). Provide an explanation based on electrostatic forces for why the elements shrink in
radius from left to right across a particular row. Hint: discuss protons!
e-
4 e-
e3p
e-
2. Explain why each successive ROW of elements is larger than the row that preceded it. Sketch a
picture to support your answer. Hint: discuss shells!
e6p
e-
154
Assignment #89b: Ionization Energies
In this data table, the values for IE1 represent the amount of energy needed to
remove the atom’s OUTERMOST electron. Values for IE2 represent the amount of energy
needed to remove a SECOND electron from the atom in question. The values for IE3 and
IE4 represent the energy needed to remove a 3rd or 4th electron from the atom.
IE1 = energy
needed to
remove this
outermost
electron.
ee3p
e-
Lithium atom
3. The picture shown to the right represents the electron shells of a lithium atom.
a) According to the data table, what quantity of energy is required to remove lithium’s outermost
electron?
b) What quantity of energy is needed to remove a SECOND electron from a lithium atom?
c) Provide an explanation based on ELECTROSTATIC FORCES that accounts for the large increase in
energy needed to remove a second electron from lithium. Hint: your answer should focus on the
distance between protons and electrons.
4. Provide an explanation based on electrostatic forces for why helium’s first ionization energy (IE1) is
so much greater than hydrogen’s (IE1).
5. Which element in this table has the WEAKEST hold on its outermost electron? Provide a rational
explanation for why this element’s hold is so weak. Hint: consider the number of SHELLS that this
element possesses.
IE2 = energy
needed to
nd
remove a 2
electron.
155
Assignment #89c: A Graph of Ionization Energies
6. Elements in the first column of the periodic table are called the ALKALI METALS. What common
feature of the alkali metals is evident in the graph of ionization energies? Note: the alkali metals are
highly reactive metals—try to relate this to the data shown in the graph.
7. NOBLE GASES are the least reactive elements on the periodic table. Describe how the ionization energy
graph explains the non-reactivity of the noble gases.
8. The term “PERIODIC” describes something which REPEATS every so often. Explain how the graph
shown above expresses the “periodic” nature of chemical elements.
9. Suppose you were to extrapolate to make a PREDICTION for the ionization energy of Rb (#37 on
periodic table). What value would you estimate for the ionization energy of Rb? Explain briefly.
156
Assignment #89d: Ayeeh!
Applying information from the charts to understand atomic behaviors
1. Draw pictures that show the number of PROTONS in the nucleus and the number of ELECTRON
SHELLS for each of the following atoms/ions and EXPLAIN why one atom’s RADIUS is larger than
the other:
a) Na vs. Na+ Hint: neutral sodium has 11 electrons, while the sodium cation has only 10 e-.
Your explanation for why the sodium
ion has a smaller radius than a
neutral sodium atom:
11+
2 e8 e1 e-
Neutral sodium
Sodium Ion
b) Al+3 vs. Mg+2 Hint: Al+3 is missing all three of its original valence electrons, while Mg+2 is missing two
of its original valence electrons. Take this into account when you draw the electron shells.
Which has a bigger radius:
Al3+ or Mg2+?
+1
2. The pictures below represent a neutral oxygen atom and an O ion.
a) Based on the data shown in the chart on assignment #83b, comment on which of these pictures
represents a higher energy state. Also discuss whether this represents an increase in kinetic energy
or in potential energy.
8+
2 e6 e-
Neutral oxygen
8+
2 e5 e-
Oxygen +1 ion
b) What energy changes would you expect to occur if an electron were added to an O+1 ion? Recall
that energy can never be created or destroyed.
Continued on next page!
157
Assignment #89d: Ayeeh! (continued)
3. Use your IONIZATION ENERGY CHARTS (shown in Assignments #83b and #83c) to help you answer the
following questions:
a) Use the table of ionization energies (#83b) to provide the data requested below:
Sodium IE1 = __________ kJ/mol
Neon IE1 = __________ kJ/mol
Sodium IE2 = __________ kJ/mol
Neon IE2 = __________ kJ/mol
b) Explain why Sodium’s IE1 is much lower than neon’s IE1.
c) Provide an argument based on ELECTROSTATIC FORCES that explains why sodium’s IE2
is higher than Neon’s IE2. Hint: try to define the location of the electron being removed in each
case.
d) Provide an explanation for the finding that the second ionization energy of neon is higher than the
first ionization energy of neon.
e) Provide an explanation for why sodium is commonly found as a +1 ion in nature, whereas neon is
never found as a +1 ion in nature.
f) Provide an explanation for why sodium (unlike iron) is NEVER found as a +2 or +3 ion.
158
Assignment #90: Quantum Chemistry: An Introduction
The early part of the 20th century witnessed the birth of quantum chemistry, a field of
science describing the behavior of very small particles, such as atoms. The “rules” for quantum
chemistry seem very strange to humans that are familiar with the “rules” for the behavior of
everyday objects. This page is an attempt to instruct you in the fundamental principles of
quantum chemistry and to see how they are different from the “normal” behavior of objects in
the macroscopic world.
The fundamental concept underlying all of quantum chemistry is that certain measurable
quantities can exist only in pre-determined “packets” (each packet is called a quantum).
Entities that are quantized (i.e. exist in pre-determined quantities) include:
a) the charge of an atom’s nucleus
b) light
c) energy levels for electrons within an atom
Here are some examples that may help you understand the concept of quantum chemistry:
1. As an everyday life example, money is quantized. When you carry money in your pocket,
you can have $1.00, $1.01, $1.02, $1.03, etc. It is impossible for you to have $1.02734 in your
pocket. The smallest “packet” of money in the USA is a penny. Therefore, one cent can be
considered a single QUANTUM of money in the United States.
2. Human height is NOT quantized. If you line up a bunch of people, they could have heights
as shown below:
152 cm (about 5-feet tall)
152.004 cm (a smidge taller than the previous value)
152.027 cm
152.140 cm
In this example, there is no minimum unit of height that separates one human from another.
This means that any value of human height (even 152.01759302 cm) is permissible!
When a measurement can have any possible value (i.e. is NOT quantized), it is called a
CONTINUUM.
3. Test your understanding:
Which of these mathematical concepts represents a continuum, and which is quantized?
 All positive integers (i.e. the counting numbers) =
 All positive real numbers (includes fractions, decimals, etc) =
4. Now imagine the strange nature of a world in which human height is quantized. Let us
suppose that the minimum quantum of human height is 1-foot.
a) Cross out the height listed below that would be impossible on such a world.
4-feet tall
5-feet tall
5’6” tall
6-feet tall
b) Suppose you were a child growing up in a world where height is quantized (in 1-foot
increments). Which one of the following choices describes a “legal” way to become taller on
this world?
 Suddenly, during a volleyball game, you become 1-foot taller
 Gradually, during a 24-hour period, you grow from 4-feet to 5-feet tall
 Each day, over a period of a month, you gain 0.4-inches in height
159
Chemistry Assignment #91: The Bohr Model
An Introduction to Quantum Theory
There is an equation that describes the energy spacing of electron shells within the hydrogen
atom. This equation is given below:
1312
E   2 kJ / mol
n
n
where n indicates the number of the energy level (shell) in question
Note: energies in this equation are calculated as NEGATIVE VALUES, thus indicating the BINDING ENERGY of the
electron.
1. This equation can be used to calculate exact numbers for the energy levels 1 through 4 of the
hydrogen atom. Fill in the table shown below with appropriately LABELED energy values.
Figure 1
n
Energy (En)
1
E1 =
2
E2 =
3
E3 =
4
E4 =
2. In Figure 2 shown below, label each shell
with its proper “n” value and its proper
energy value.
3. Follow your instructor’s guidelines for
labeling Figure 3.
E4 =
n=4
n=3
E3 =
E2 =
Increasing Energy
n=2
+
n=1
E1 =
n=2
E2 =
n=3
E3 =
Figure 2
n=1
E1 =
Figure 3
Continued on next page!
160
Chemistry Assignment #91: The Bohr Model (continued)
Use your labeled diagrams (Figure 2 and Figure 3) to answer the following questions:
4. How many electrons can “fit” in the energy level labeled n = 2? Hint: this represents the second shell
of the atom!
5. Suppose you glance at an atom and observe an electron residing in the first shell (n=1) at a given
moment in time. A few seconds later, you look back at this atom and find that the electron now resides in
the second energy level (n=2). Which of the following best describes what happened to the electron in
the time between your first glance and your second glance? Explain.
a) The electron must have gained energy from somewhere
b) The electron must have lost some of its original potential energy
c) The electron randomly jumped from shell #1 to shell #2 (jumping from shell to shell is a natural
occurrence and happens spontaneously from time to time)
6. Looking at Figure 2 and Figure 3, which of the following statements appear to be true? (mark all that
you feel are true).
a) An electron in level n = 1 is closest to the nucleus
b) An electron in level n = 4 is more tightly bound than an electron in level n = 1
c) Electrons prefer to reside in lower energy levels (such as n = 1)
d) If an electron were to move from the third shell to the second shell, the atom would possess a
greater amount of POTENTIAL energy.
7. Normally, electrons are bound to nuclei to create atoms, but sometimes, an electron can ESCAPE the
pull of the nucleus to become an unbound electron.
a) How much energy would be needed to liberate an electron from the 1st shell of a hydrogen atom
and allow it to become a free, unbound electron? Express your answer in kilojoules per mole.
b) Would the hydrogen atom still be an ATOM if its electron escaped from the pull of the nucleus?
8. If an electron in the 1st shell of a hydrogen atom were offered a quantity of energy equivalent to 500 kJ
per mole, would the electron be able to move to the 2nd energy level (n=2)? Explain.
9. If an electron in level n = 4 dropped down to n = 2, energy would be RELEASED from the atom. In
what form do you think this released energy would appear?
a. heat
b. light
c. music
d. gamma rays
e. kinetic energy (i.e. the atom would begin to move faster)
Continued on next page!
161
Chemistry Assignment #91: The Bohr Model (continued)
10. The next simplest atom after hydrogen is the He+ ion. This ion has ONE electron, just like
the hydrogen atom has. The sketch below shows an energy well for the He+ ion next to the
energy well for a hydrogen atom. Both wells are drawn to the same scale.
E2 = -328 kJ/mol
E3 = -584 kJ/mol
E1 = -1312 kJ/mol
e-
E2 = -1312 kJ/mol
E1 = -5248 kJ/mol
e-
a) In the diagrams drawn above, show where the nuclei of the hydrogen and helium atoms belong. Note:
the hydrogen nucleus contains 1 proton, while the helium nucleus contains 2 protons. Show this in the
diagram when you sketch the location of each nucleus.
b) The diagram shows you that the electron in He+ is more tightly bound than the electron in H. Provide
an explanation based on ELECTROSTATIC FORCES for why a helium ion’s energy well is much deeper
than a hydrogen atom’s energy well.
c) The arrangement of electrons in the helium atom shown below is unusual (some might say it is
“wrong”). Try to define what makes this picture unusual, and explain how it might be possible to create
a helium atom with this particular arrangement of electrons.
eeHe
162
Assignment #92: Interpreting equations
Objective: In this lab, you will be asked to carry out several chemical reactions without knowing the
identities of the reactants and products. By using your knowledge of descriptive chemistry and some
deductive logic, you will identify each reaction as one of the equations listed on the next page. You may
find the following considerations useful:
a) classification of a substance as a metal, salt, or gas
b) color of the substance
c) solubility of the substance
d) the type of reaction that occurs (Forms a precipitate? Forms gas
bubbles??)
e) energetics of the reaction (exothermic/endothermic/thermoneutral)
Reaction Procedures (In all cases, agitation to mix contents is a good idea):
Reaction AB. Test the solubility of solid A by mixing a modest scoop (0.5-1.0 g) of solid A with 10
ml distilled water in a small beaker. Stir vigorously and observe (you may want to centrifuge a sample
to verify solubility). Then add 10 ml of liquid B to the beaker.
Reaction CD. Put about 5 ml of liquid C in a test tube. Add about 5 ml of liquid D to the tube.
Reaction EF. Place about 5 ml of liquid E in a test tube. Drop in a piece of solid F.
Reaction GH. Add about 5 ml of liquid G to a test tube. Add an equal volume of liquid H to the tube.
Reaction IJ. Measure out 0.4 grams of solid I in a small beaker. Test its solubility by adding 10 mL
of distilled water to the beaker and stirring (you may want to centrifuge a sample to verify solubility).
Then add 10 mL of liquid J to the beaker and stir.
Clean-up procedure: You may dump all liquids down the drain. Rinse with much water. Try not to
dump solids down the drain as they tend to clog up the sink.
Some Guidelines for Experimental work:
Proper sequence of tasks:
1. Choose a reaction to run (best to
start with what’s at your table!)
2. Run the reaction according to the
instructions.
3. Record important clues—these
clues include describing what you
started with!
4. Figure out which equation fits the
clues. Note: some reactions take
time to fully develop, so don’t
dispose of your evidence too soon!
5. Write up your thoughts following
the example on the next page. (one
answer page to be turned in per
group)
6. Go back to step 1 (trade with the
buffet table to get new reagents)
Legal tools
 Centrifuge
 Pipets
 Vortex mixer
 Thermometer
 Magic bean
Illegal tools (NO, NO!)
 Bunsen burner
 pH paper
 other reactions not specified in the
instructions
163
Assignment #92: Interpreting equations (continued)
Equations: Note: it is possible that some of the substances in these equations are (aq)!
1)
K2CO3 + HCl  KCl + H2O + CO2
2)
Ca + H2O  Ca(OH)2 + H2
3)
MgCO3 + H2O  no reaction
4)
CaCl2 + Na2CO3  CaCO3 + NaCl
5)
CuCl2 + Na2CO3  CuCO3 + NaCl
6)
Zn + HCl  ZnCl2 + H2
7)
NaCl + K2SO4  2 KCl + Na2SO4
8)
Mg(OH)2 + HCl  MgCl2 + H2O
9)
MgCO3 + HCl  MgCl2 + H2O + CO2
10)
Mg + H2O  no reaction
11)
Zn + SnCl2
12)
NiSO4 + NaCl  NiCl2 + Na2SO4
 ZnCl2 + Sn
H = -415 kJ/mol
H = -155 kJ/mol
H = -108 kJ/mol
H = -120 kJ/mol
Grading the Lab: There is no take home write-up for this lab. Your lab group must turn in an official
answer page at the end of the hour. For each reaction, you must DESCRIBE your observations of the
reaction, CHOOSE an equation, and EXPLAIN your reasons for choosing that equation. The job of WRITING
up a particular reaction should be ROTATED among group members. Note: You are allowed one “lifeline”
use of the Merck Index—ask your teacher to clarify what this means.
Example of how to write up your experiments:
In reaction MN, we mixed a yellow powder (solid M) with water and vortexed. The
yellow powder dissolved in the water, suggesting that solid M is a soluble salt.
We next mixed aqueous M with liquid N, a transparent liquid that looked like water.
After centrifuging the mixture, we were left with an orange precipitate and a clear
supernatant (see sketch).
In matching this reaction to an equation, we focused on equations that would form a
precipitate. We narrowed down our choices to the equations shown below.
SrCl2 + Na2SO4  SrSO4 + NaCl
OR
FeCl3 + Na3PO4  FePO4 + NaCl
Both these equations begin with soluble salts and form precipitates. We chose the 2nd
equation as our final answer because ferric ions have a yellow/orange color, and our
precipitate was yellow/orange. We think we formed ferric phosphate as the precipitate.
164
Assignment #93a: The Photon Game!!!
Key Concepts:
1. You will work with a partner in this game. Each of you will need to start with a game board
(which represents the quantum energy levels that exist in a particular atom).
2. You will need a marker (such as a penny) to represent an electron. In this game, the electron
will be jumping between various energy levels within each atom.
3. Take a look at your gameboard and notice that there are energy values written on the board.
Based on these energy values, try to complete the sentences below:
In any atom, the lowest energy level is labeled as n = ___. This corresponds to the innermost shell
of the atom. As n increases, energy approaches _____ kJ/mol. This means that an outer shell is
at a relatively high / low energy level compared to an inner shell. If an electron ever reaches the
energy level of zero kJ/mole, the electron has _____________ from the pull of the atom’s nucleus.
4. For an electron to climb higher in the energy well, it must ABSORB energy. This results in an
increase in POTENTIAL energy. Sources of energy in the PHOTON GAME include:



Bunsen burner
The Sun!
Energy-carrying photons (infrared, visible, and ultraviolet)
5. If an electron FALLS DOWN in the energy well, it will create a
PHOTON of a particular color. This is a conversion of potential
energy to light energy! The energy table shown on each game
board specifies the particular photon ENERGIES that are associated
with various COLORS.
e-
Ex: This drop
of ____ kJ/mol
will create a
_________ colored photon.
-600 kJ/mol
-820 kJ/mol
Fi
6. Your instructor has a set of powerpoint slides which will specify possible changes
for the electron’s position on the game board. For example, a slide may say “jump to
level 4”. Alternatively, the slide may say something like “gain up to 500 kJ/mole of
energy”. You must move your electron within the energy well to follow the changes
that the slides specify.
Color
kJ/mole
Orange
191-210
Yellow
211-230
Green
231-250
7. When your team has successfully produced ALL the colors of the rainbow (as well as ALL the
“colors” that lie above and below the visible spectrum, you are a WINNER in the photon game!
Your prize is a pair of photon-viewing glasses (one pair per person).
165
#93b: The Photon Game!
Version P (for Practice)
0 kJ/mol
n=5
-100 kJ/mol
n=4
-250 kJ/mol
-430 kJ/mol
n=3
-650 kJ/mol
n=2
A PHOTON is produced
only when an electron
FALLS within the
energy well.
An electron must
reside on one of the
indicated levels (or on
the 0 kJ “freedom”
level. When an
electron jumps from
one level to another, it
is called a QUANTUM
LEAP.
Scoreboard
-910 kJ/mol
As you create
photons, color here!!!!
n=1
Photon
Color
Infrared
Energy
(kJ/mol)
<170
Red
170-190
Orange
191-210
Yellow
211-230
Green
231-250
Blue
251-270
Violet
271-300
UV-A
301-370
UV-B
371-420
UV-C
421-1200
166
#93c: The Photon Game!
Version C
0 kJ/mol
n=5
-170 kJ/mol
n=4
-200 kJ/mol
-280 kJ/mol
A PHOTON is produced
only when an electron
FALLS within the
energy well.
An electron must
reside on one of the
indicated levels (or on
the 0 kJ “freedom”
level. When an
electron jumps from
one level to another, it
is called a QUANTUM
LEAP.
n=3
-460 kJ/mol
n=2
-800 kJ/mol
As you create
photons, color here!!!!
n=1
Scoreboard
Photon
Color
Infrared
Energy
(kJ/mol)
<170
Red
170-190
Orange
191-210
Yellow
211-230
Green
231-250
Blue
251-270
Violet
271-300
UV-A
301-370
UV-B
371-420
UV-C
421-1200
167
#93d: The Photon Game!
Version D
0 kJ/mol
n=5
-40 kJ/mol
n=4
-120 kJ/mol
-290 kJ/mol
A PHOTON is produced
only when an electron
FALLS within the
energy well.
An electron must
reside on one of the
indicated levels (or on
the 0 kJ “freedom”
level. When an
electron jumps from
one level to another, it
is called a QUANTUM
LEAP.
n=3
-510 kJ/mol
n=2
Scoreboard
-1030 kJ/mol
As you create
photons, color here!!!!
n=1
Photon
Color
Infrared
Energy
(kJ/mol)
<170
Red
170-190
Orange
191-210
Yellow
211-230
Green
231-250
Blue
251-270
Violet
271-300
UV-A
301-370
UV-B
371-420
UV-C
421-1200
168
Assignment #94: The Photon Game Questions
1. A particular atom (such as a helium atom) can emit only certain colors of photons. In other words,
helium atoms might be able to make R, Y, and V photons, but not O, G, or B photons. What is your
explanation for why helium atoms can only emit particular colors of photons?
2. You probably know that ultraviolet “rays” can be damaging to your skin (and eyes). Why do you
suppose UV light is so damaging to skin and eyes?
3. Label each of the following statements as TRUE or FALSE:
Photons can never be created or destroyed
An atom in its GROUND STATE cannot emit a photon
Photons can go through glass
Photons carry energy
Photons can be created only after an atom has GAINED energy
4. Which of the following objects are photon producers? Circle all that apply. . .
a) The sun
b) A light bulb
c) A hedgehog
d) A glow in the dark Frisbee
e) A rock
5. In order to remember the relative energies of the various colors of the rainbow, an acronym known as
ROY G BIV is sometimes employed. Fill in the spaces in the table below.
Abbreviation
Color name
Wavelength
Energy
(high vs. low)
Long
Low
Short
High
R
O
Y
G
B
I
Indigo
V
6. Add in the “color” that belongs before R and the “color” that belongs after V in the table shown above.
169
Assignment #95: The Colors of the Rainbow
An introduction to the Balmer Series of photons
1. Use the Bohr Model equation shown above to
calculate energy values for the first 8 shells of a
hydrogen atom.
Energy
8
-20.5 kJ/mole
E4 =
E3 =
E2 =
Increasing Energy
Shell #
E5 =
7
6
5
4
E1 =
3
2
1
Energy well for a hydrogen atom
2. Using the energy values that you calculated above, find the COLORS of photons produced
from the following electron transitions that could occur within a hydrogen atom:
Transition
Potential Energy Loss
Photon color emitted
31
1166 kJ/mole
Ultraviolet (UV-C)
63
42
3. The visible spectrum of a hydrogen atom consists of only four bright lines. The photons
emitted are red, teal green, and two closely spaced violet lines. Determine the electron
transitions that result in these 4 colors and explain why no other visible colors can be emitted.
Violet (1)
Violet (2)
Teal
Red
62
292 kJ/mol
Explanation:
170
Assignment #96a: The Chart of Atomic Orbitals
6p
5d
4f
6s
5p
4d
5s
4p
3d
ENERGY
4s
3p
3s
SUMMARY
2p
1st shell =
nd
2s
2 shell =
3rd shell =
4th shell =
5th shell =
6th shell =
1s
orbitals =
e-
171
Assignment 96b: Orbital charts for practice with circles and arrows
Example #1
Element = __________________
Number of electrons = _______
2p
High energy
2s
Electrons always go to
the lowest energy
orbital available.
Low energy
1s
If orbitals of equal
energy are available,
electrons will spread
out as much as possible.
4p
3d
4s
3p
Orbitals fill with electrons from
bottom (lowest energy) to top
(highest energy). You can imagine
a rising water level “flooding” the
lower energy orbitals, while the
higher energy orbitals remain
“dry”.
3s
2p
2s
1s
Example #2
Element = __________________
Number of electrons = _______
172
Assignment #97: Wir Bauen Auf!
“Aufbau” means “build up” in German
1. Fill in (with arrows) the orbital diagrams for the elements shown below and summarize the electron
configuration with a string of numbers and letters (e.g. 1s2 2s2 etc.).
a) F (9 electrons)
b) Si (14 electrons)
3p
3p
3s
3s
2p
2p
2s
2s
1s
1s
2. Write-out the shorthand electron configuration for each of the elements shown below:
a) Ca (20 electrons) = 1s2 2s2 . . .
b) Ni (28 electrons) =
4p
c) Rb (37 electrons) =
3d
4s
3p
3. Electrons that are alone in an orbital are called
“unpaired electrons”. Determine the number of
unpaired electrons in each element shown in
question #2.
a) Ca has ____ unpaired electrons.
3s
2p
2s
b) Ni has ____ unpaired electrons
c) Rb has ____ unpaired electrons
1s
Continued on the next page!
173
Assignment #97: Wir Bauen Auf! (continued)
4. An ORBITAL electron configuration for an atom contains the information needed to place electrons in
SHELLS.
For example, the orbital electron configuration for iron (Fe) is 1s22s22p63s23p64s23d6
 The first shell consists of only one orbital: 1s2. Therefore, there are 2 electrons in the first shell
of an iron atom.
 The second shell consists of four orbitals: 2s22p6. Counting up the total number of electrons
here will show you that there are 8 electrons in the second shell of an iron atom.
 The third shell (complicated!) consists of nine orbitals: 3s23p63d6. Counting up the total
number of electrons here will show you that there are 14 electrons in the third shell of an iron
atom!
 The fourth shell consists of 16 orbitals, but in Fe, only one orbital is occupied: 4s2. Therefore,
there are 2 electrons in the fourth shell of an iron atom.
Summary of electron shells for IRON:
Shell
Number
1
2
3
4
Occupied Orbitals
Total number of electrons
1s2
2s22p6
3s23p63d6
4s2
2
8
14
2
Using the electron configurations shown below, complete each atomic picture to show the number of
electrons in EACH SHELL of a calcium atom and a nickel atom. Hint: follow the format of the example
sodium atom shown.
11+
2 e-
Ni
Ca
8 e1 e-
Example: sodium atom
Ca = 1s22s22p63s23p64s2
Ni = 1s22s22p63s23p64s23d8
5. Write out electron configurations for the following IONS: (remember that a positive charge indicates
MISSING electrons,
while a negative charge means EXTRA electrons)
Ex: Na+ (10 electrons) = 1s2 2s2 2p6
a) K+ (18 electrons) =
-
b) F ( ___ electrons) =
6. The ions shown in question #5 possess “NOBLE GAS CONFIGURATIONS”. This means that each of the
ions has the same electron configuration as a noble gas on the periodic table. Identify which noble gas is
emulated by each of the ions shown above. Note: “emulate” means to imitate or try to equal someone
else. . .
Ex: Na+ has the same electron configuration as the noble gas Neon.
174
Assignment #98: Songs without Words
175
Assignment #99: Singing the Song (the orbital song)
1. Using your best singing skills, predict the electron configurations of the following atoms:
Note: do this using only your colored periodic table—not the orbital chart)
a) As (arsenic, #33 on the periodic table)
b) Ba (barium, #56 on the periodic table)
c) Hg (mercury, #80 on the periodic table)
Hint: make sure you take the detour down to the 4f orbitals!
3. Use the space to the right to sketch a picture with
circles and arrows (drawn at appropriate relative
energies) to indicate which ORBITALS are occupied by
electrons in an OXYGEN atom.
Hint: you’ll need 5 circles in your diagram!
Increasing Energy
2. Using any method you wish, predict the electron configuration of the Sr2+ ion.
4. If enough energy is applied to an oxygen atom, it can become a positive ion (O+1). Which one of the
electrons drawn in question #3 would be LOST when oxygen becomes a positive ion? Explain your
reasoning.
Look
at
#89c
Look
at
#89a
5. Although question #4 suggests that oxygen can become a positive ion, this almost never occurs in the
natural world. Explain why it is very difficult to find O+1 in the natural world.
6. Suppose the electron configurations shown below belong to three neutral elements on the periodic
table. Rank these elements from smallest radius to largest radius. Explain your reasoning.
a) 1s22s22p6
b) 1s22s22p63s1
c) 1s22s22p63s23p5
7. A metal ion with a partially filled d-sublevel is likely to be a colorful ion. On the basis of this
statement, which of the following metal ions would you expect to be colorful?
Co2+
Ba2+
Rh3+
176
Assignment #100: Orbital Architecture
For interactive simulations of orbital shapes, visit the Orbitron website from University of
Sheffield:
http://winter.group.shef.ac.uk/orbitron/
For a review of the Bohr model with quantized energy levels:
http://www.colorado.edu/physics/2000/quantumzone/bohr.html
Representation of orbital shapes for the 1st and second shells:
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Molecular_Orbital_The
ory
s is for __________________
p is for ____________________
d is for ____________________
f is for _____________________
177
Assignment #101: Sir Charles (invents a gas law)
(use a sharp pencil for all your work on this page!)
Sir Charles has a sample of gas in an expandable container. He measures the volume of this gas sample at
a variety of temperatures, as shown in the data table below.
Temp (C)
25
75
150
250
Volume (Liters)
1.00
1.17
1.42
1.76
1. Plot these data points on the graph paper shown below.
2L
1L
-300C
-200C
-100C
0
100C
200C
300C
2. If you have plotted the points well, you should see that the graph is linear. Use a straight edge to
connect the points, extending the line all the way to the x-axis.
3. Based on your line graph, estimate the y-intercept of the line. (put this label on the graph)
4. In Sir Charles’ experiment, what does the y-intercept of the line represent?
5. Determine the equation of the line. Write the equation on the graph. Note: the equation of a line can
be expressed as y=mx+b, where m is the slope of the line, and b is the y-intercept of the line.
6. Use your equation to determine the x-intercept of the line. Compare your calculated value to the
graphical representation of the x-intercept. If the calculated value is very different from the graphical
value, you have probably made an error. Write the x-intercept value on the graph.
7. In Sir Charles’ experiment what does the x-intercept of the line represent?
8. Demonstrate, using a pair of data points from the graph, that the Volume of the Gas is NOT
proportional to Celsius temperature.
9. In order for two quantities to be proportional, the following conditions must be met:
 the function must be linear
 y-intercept must equal zero
 equation of the line must be y = mx
Invent a new temperature scale in which Volume of Gas will be proportional to Temperature.
178
Assignment #102: A Bag of Hot Air
Suppose you have a 2.9-gram sample of nitrogen gas (N2) in an expandable
container as schematized in the margin to the right. The gas occupies a volume of
2.5 liters at 22C and a “normal” pressure of 1 atmosphere.
1. Show how to calculate the Kelvin temperature of the gas in the cylinder.
2. The mass of N2 gas present in the cylinder is 2.9 grams.
a) Show a calculation (either PROPORTION or DIMENSIONAL ANALYSIS) that
converts 2.9 grams of N2 into MOLES of N2. Hint: use your periodic table to
find the MOLAR MASS of N2!
Mass = 2.9 g N2
Volume = 2.5 liters
Temp = 22°C
= _____ K
b) Calculate the density of the nitrogen gas in the cylinder. Express your answer in GRAMS PER
Hint: use the information provided in the picture! The correct answer is between 1 and 2 g/L.
LITER.
3. Now suppose the nitrogen gas in the cylinder is warmed to a temperature of 100°C.
a) Calculate the Kelvin temperature of the gas at 100°C.
b) Warming the gas from 22C to 100C will cause the gas to expand. Use the set-up shown below to
calculate the VOLUME of the expanded gas. Note: “K” stands for Kelvin temperature! The picture
provided above shows you the volume of the gas at 22 C!!!
c) Calculate the density of the heated gas in units of GRAMS PER LITER. Hint: the mass of the gas is
still 2.9 grams!
4.* A hot air balloon exploits the difference in density between hot air and cold air to lift objects off the
ground. The amount of lift achieved in a hot air balloon is equal to the DIFFERENCE IN MASS between the
hot gas inside the balloon and an EQUAL VOLUME of unheated air. Assuming that air is pure nitrogen gas
(actually, air is about 80% N2), calculate the mass that could be lifted by a 425-Liter balloon assuming
that the air inside the balloon is 100°C and the air outside the balloon is 22°C. Hint: Start by DRAWING
A PICTURE! You’ve already calculated the density of N2 at these two temperatures. . .
179
Assignment #103: ATM stands for atmosphere
Working with various units of pressure
1. Use the conversion factors provided on your periodic table page to set up dimensional
analysis (or ratio and proportion) to make the following conversions:
a) convert 36 PSI to atmospheres
b) a “normal” blood pressure is considered to be about 120 mm of Hg. Convert this value into PSI.
Note: if blood pressure gets too high, it can lead to the rupture of a blood vessel (e.g. a stroke).
2. The picture shown below shows a method for measuring gas pressure using a column of
mercury. The gas is in the flask exerts pressure on the liquid mercury, which makes the mercury
rise in the left-hand tube.
a) The height that the mercury rises is a direct measurement of the pressure of the gas. If
h = 275 millimeters, the pressure of the gas is said to be 275 mm of Hg. Use this information to
calculate the pressure of the gas within the flask in the units of atmospheres and PSI.
b) Assume that the pressure you calculated in part (a) is valid for the gas sample at room temperature
(about 20C). If this gas were heated to 40C, how many PSI will exist within the flask? Hint: the
pressure of the gas will be PROPORTIONAL to its temperature measured in Kelvin.
c)* What would happen in the original picture if the tube on the left hand side were punctured at its top
so that the Hg (l) in the tube is exposed to the atmosphere? Hint: atmospheric pressure is 760 mm Hg.
i. The liquid mercury would rise on the left because now there is an opening for the mercury to escape
ii. The atmospheric pressure would push the mercury down (back towards the flask)
iii. There would be no change in the mercury column because the pressure in the flask has not
changed
180
Assignment #104: Exploring Boyle’s Law with Syringes (mini lab)
1. At the lab table, you should find one 12 mL syringe for each person.
2. Pull out the plunger on the syringe to the 12 marking. This means that there are 12 mL of air in the
syringe. Since the syringe volume is exposed to the atmosphere, the pressure of the gas is 1-atmosphere.
3. Place your finger (or thumb) over the opening of the syringe and try to squeeze the volume of air down
to 6 mL. You should feel resistance as you do this—the resistance is a measure of the pressure building
up inside the syringe. When the volume is squeezed down to 6 mL (from an original volume of 12 mL),
the pressure inside the syringe doubles to 2-atmospheres.
4. Try to squeeze the air in the syringe into an even smaller volume.
 If you can reach a volume of 4 mL, the pressure inside the syringe is 3 atmospheres.
 If you can reach 3 mL, the pressure is 4 atmospheres.
 If you can reach 2 mL, the pressure is 6 atmospheres.
5. Release your finger or thumb to let the air in the syringe escape.
12
6. Complete the data table shown below. Then graph the data.
11
Pressurevolume product
(P x V)
12 mL
1 atm
12
6 mL
2 atm
12
4 mL
3 atm
10
Pressure
(in PSI)
9
Pressure (atm)
Volume
Pressure
(in atm)
8
7
6
5
4
3
2
1
3 mL
0
0
2 mL
1
2
3
4
5
6
7
8
9 10 11 12
Volume (mL)
7. Write a sentence that summarizes how the PRESSURE changes as the VOLUME of air in the syringe
decreases. Note: you should also express what mathematical quantity remains the same as you do these
experiments!
8. Complete the picture of syringe B by sketching the spacing of the gas molecules in the syringe when
the syringe plunger is depressed. Then write a brief explanation for how invisible gas molecules can be
stronger than a human.
A: Original syringe state
B: Plunger depressed
9. Show a mathematical calculation for the pressure inside the syringe when the volume is compressed
to 5 mL. Hint: recall that the pressure-volume product (P x V) is a constant!!!!
Continued on next page!
181
Assignment #104: Exploring Boyle’s Law (continued)
10. Now restart your syringe with the plunger set to 1 mL. Place your finger or thumb over the opening
of the syringe and try to pull the plunger out, expanding the volume of the gas in the syringe.
11. Ponder what force is fighting against you as you pull the plunger out. Describe your thoughts in the
space below. Hint: you aren’t fighting against anything INSIDE the syringe.
12. Release the plunger from its extended position without letting air enter the syringe. I hope that you
will observe the plunger move back to 1 mL. Note: some syringes do this better than others. . .
13. In the diagram below, add gas molecules to figure B both inside and outside the syringe and explain
what force “magically” moves the plunger back in to the 1 mL mark. Hint: try to use the term
“equilibrium” here.
B: Plunger pulled out
A: Plunger originally set at 1 mL
Atmospheric
molecules
14. Complete the data table shown below:
Volume
Pressure
(atm)
1 mL
1 atm
2 mL
1/2 atm
3 mL
1/3 atm
5 mL
PxV
Pressure
(PSI)
If you have some time
left on the clock, you
should experiment with a
water-filled syringe—try
to compress/expand the
volume of the water.
See what you can
discover!
10 mL
15. The relationship you have discovered in these experiments is called Boyle’s law. Boyle’s law states
that the volume of a gas is inversely proportional to the pressure of the gas. Try to explain what the
term “inversely proportional” means.
16. Farmer Bill and Farmer Fred have identical 40-liter propane tanks. Farmer Bill fills his tank with
propane gas at a pressure of 3-atmospheres, while Farmer Fred fills his tank with propane gas at a
pressure of 5-atmospheres. Will the two cylinders have the same weight after being filled with propane
as described above? Explain.
182
Assignment #105: Under Pressure
1. Suppose two flasks are connected with a rubber hose as shown below. Flask A contains
molecules of an unknown gas. Flask B has a vertical column of liquid that acts as a pressure
gauge.
Suppose each of the following manipulations were done to flask A. Which of these
manipulations would INCREASE THE PRESSURE IN FLASK B? If the pressure will increase, explain
on a molecular level what causes the higher pressure.
Assume each of these manipulations is done independently—not in sequence.
a)
b)
c)
d)
e)
f)
warming flask A over a burner = pressure increases because molecules move faster
adding a teaspoon of water to flask A =
adding an ice cube to flask A =
turning flask A on its side =
running an exothermic chemical reaction in flask A =
running a reaction that creates a fizzy gas in flask A =
Would any of the above manipulations create a PERMANENT pressure increase (as opposed to one
that will revert to the original pressure after a few minutes (or hours))?
2. A balloon has a volume of 2.13 Liters at room temperature. What volume will this balloon
have when it is chilled in a freezer to -20C? Hint: use a proportion. . .
3. If a sample of gas in a plastic bag occupies a volume of 500 mL at a pressure of 1 atm, what
volume will it occupy when the bag is placed in an environment where the external pressure is 3
atmospheres? Hint: this is an example of Boyle’s Law! Think about using an inverse proportion!
Continued on the next page!
183
Assignment #105: Under Pressure (continued)
4. Convert the following units of pressure measurements using proportional reasoning or
dimensional analysis:
a) The eye of a hurricane can have an atmospheric pressure as low as 662 mm Hg. Convert this
pressure to atmospheres. Note: this is as low as atmospheric pressure can ever be expected to get (at
sea level).
b) The pressure in a propane tank at about 70° Fahrenheit is about 7 atmospheres. Convert this
pressure into units of pounds per square inch (PSI)
c) Convert a pressure of 22 PSI to torr (mm Hg)
5. Consider two round flasks connected by a valve as schematized below. (Picture from Zumdahl,
Chemistry 2nd Edition, p. 740.) Assume that the black dots in the picture represent gas molecules.
a) Explain why the right hand flask appears to have a higher pressure than the left hand flask. Hint: try
to base your answer on the number of molecules bouncing around, colliding with the walls of the flask.
b) If the valve connecting the flasks were opened, what would you expect to happen? Hint: think
“whoosh”. . .
c) Smart Sally suggests that although it is LIKELY that the gas in the right-hand flask is exerting a higher
pressure, it is POSSIBLE that the gas in the left-hand flask is actually exerting a higher pressure. She says
that there are factors that affect gas pressure that cannot be seen in this picture. Can you think of any such
factors?
6. The weight of a full grown elephant is approximately 16,000 pounds. Estimate the surface area of an
elephant’s footprint (in square inches). Then calculate the pressure (in PSI) that an ant would feel if the
elephant were to step on it with one of its four feet. Hint: you’ll need to do some division here.
184
Assignment #106: The Lab With Many Flasks
A Chemical Engineering Project
In this experiment, you will attempt to design a system that will accomplish a fairly complex task
as defined below. Instead of a paragraph-based write up for this lab, you will make a neatly
organized full-page sketch that shows how your system works (a SCIENTIFIC DIAGRAM). Your
grade in this lab will be based on how well your system works and on the quality of your
scientific diagram.
1. You will be provided with three flasks and appropriate stoppers. The flasks will be arranged
in a fashion similar to that schematized below.
A
B
C
2. You will put a colored liquid in container C.
3. You may initially place either liquids or solids in containers A and B.
4. Your ultimate goal is to make the liquid in container C change colors (your choice of colors)
when you add a substance (solid or liquid) to container A. Ideally, this color change will be
abrupt and dramatic (like a magic show).
5. There must be no direct transfer of material from container A to container C. Material may
be transferred from A to B and from B to C.
6. Material transfer is to be accomplished in an "automated" fashion (i.e. you are not supposed
to pick up flask A and pour its contents into flask B).
7. You will use glass and plastic tubing to connect the containers to each other.
8. The flasks must be securely clamped to your ring stand so that they will not tip over.
9. Your system must involve the use of GAS PRESSURE to drive the transfer of liquid from
flask B to flask C.
10. Your system must have a pressure release point so that the flasks do not explode. This
means you must leave an open hole on one of the stoppers in your set-up.
11. You are discouraged from creating toxic gases.
12. Use of human lung pressure is NOT allowed.
13. Use of the natural gas valve as a source of pressure is NOT allowed.
14. Use of HEAT is allowed, but you are encouraged to find ways to create gas pressure without
heat. If you use a Bunsen burner, place a wire screen under the securely clamped flask.
15. The ideal system will have a well-controlled time required for the color change.
Note: If you find a quick solution to this lab, your instructor will present you with suggestions
for an increased level of challenge.
185
Assignment #106: The Lab with Many Flasks (continued)
Color Changes
The buffet table will provide many color-changing dyes, such as:
Congo Red
Each of these dyes can be different colors
depending on the pH of the environment!
Therefore, you can make the color change
“magically” by altering the pH of the solution in
which the dye exists.
Bromcresol Green
Methyl Orange
Alizarin
Several acids and bases will also be provided at the buffet table:



0.1-molar HCl
Strong Acid
pH  1
0.1 M Acetic Acid
Weak Acid
pH  3
0.1 M Ammonia
Weak Base
pH  9
0.1 % NaOH
Strong Base
pH  12
Zinc Metal is
provided on the
Buffet Table.
Other materials
may be made
available if you
The dyes are HIGHLY CONCENTRATED—add ONLY A FEW DROPS of dye. You may
dilute the dyes with water or any of the acid/base solutions present on the buffet
table.
A good way to start the lab is to use TEST TUBES to experiment with the various need them.
dyes and the various acids and bases. Your goal is to discover what materials can
1 mole of gas
be mixed together to create a dramatic color change.
occupies about
Once you know what chemicals you want to use for your magical color change,
22.4 liters. Be
you can think about how to incorporate them into your engineering set up.
sparing with your
use of metal!
Guidelines for Scientific Diagram (each individual must make a diagram!). Your
diagram is due on Monday.
A good scientific diagram will be NEAT, EASY TO FOLLOW, and LARGE enough to show DETAIL
within each flask. The diagram should communicate by means of skillful illustration WITHOUT
requiring supporting paragraphs. The diagram should be done with as much artistry as possible.
Minimum size for diagram = 8.5 x 11”
Maximum size = 11 x 17”
The diagram you draw should clearly show
 Details of how the flasks are connected with tubing & stoppers
 What chemicals are present in each flask? (quantities and concentrations are important!)
 What molecules are moving??
 BALANCED CHEMICAL EQUATIONS for any chemical reactions occurring
 pH values in the various flasks
 Your diagram should show ACTION! Try to convey the dynamic nature of your system.
Color is a powerful tool in scientific diagrams. Try to ADD COLOR where appropriate to make
your diagram more attractive and communicative.
186
Assignment #107: Applying Gas Laws
Consider the sample of gas shown in the diagram to the right. Assume that the
piston in this cylinder is moveable.
1. Describe the effect on the volume of the gas if additional molecules were
pumped in through the valve near the bottom of the cylinder. Present an
argument for whether the molecular spacing in the gas sample after addition will
be more crowded or less crowded than the original state.
2. Describe the effect on the volume of the gas if the temperature increases.
Provide a molecular explanation for this change in volume.
3. Describe the effect on the volume of the gas if a 50-pound object were placed on the top of
the piston (e.g. atop the handle). Provide a molecular explanation for why the volume will
change, but then equalize at a predictable level.
4. Suppose the original gas sample has the initial conditions specified below, but then undergoes
three changes as outlined in the table. Provide an argument (supported by clear calculations) for
the predicted final volume of the gas.
Original
Modified
Temperature
22°C
48°C
Weight placed
on handle
0 pounds
32 pounds
Cross-sectional
area of piston
16 in2
16 in2
Moles of gas
molecules
0.52
1.04
Volume
12.8 L
187
Assignment #108: One Law to Rule Them All. . .
An introduction to the ideal gas law
1. Use one of the simple gas laws (i.e. Charles’ Law or Boyle’s Law) to solve the following
problems. SHOW YOUR SET UP AND CLEARLY LABEL EVERY NUMBER!
Charles’ Law =
Boyle’s Law = P x V is a constant value
a) A sample of gas in an expandable container (e.g. a heat-resistant balloon) is heated from room
temperature (22°C) to 175°C in a typical household oven (175°C is a good cookie-baking temperature).
If the original volume of the gas is 1700 cm3, to what volume will it expand when heated in the oven?
b) A 45-mL sample of gas in a sealed syringe is initially at a pressure of 14.7 pounds per square inch.
The plunger is then depressed using a force equivalent to 33 PSI. Calculate the final volume of the gas.
2. Utilize the IDEAL GAS LAW to solve the following questions arranged in order of increasing
difficulty. In each case, start by defining the 5 letters present in the ideal gas law. Then solve
using appropriate algebra.
a) Calculate the pressure exerted by 0.16 moles of neon gas contained in a 1.5-Liter flask at 310 K.
P = _____
V = _____
n = _____
R = _____
T = _____
b) A 20-liter steel cylinder containing pure carbon dioxide reads a pressure of 160 PSI at 22°C.
Calculate the number of GRAMS of carbon dioxide present in the cylinder.
P = _____
(must be in atm!)
V = _____
n = _____
(unknown # of moles)
R = _____
T = _____
Final
answer asks
for grams
of CO2!
(must be in Kelvin!)
c) Calculate the DENSITY (in grams per liter) of nitrogen gas (N2) at -20°C and 0.75 atmospheres of
pressure (such conditions would exist atop a tall mountain on a cold winter’s day). Note: the density will be
the same for any sized sample of nitrogen. Choose a quantity that you think is convenient (e.g. 10 grams, 1-mole, 4liters, etc.) and run Pivnert to calculate the quantity you don’t know. . .
188
Assignment #109: Gases in Chemical Equations
Using your new friend Pivnert to do gas stoichiometry
Consider the following chemical equation:
C2H5OH (l) + 3 O2 (g)  2 CO2 (g) + 3 H2O (g)
H = -1240 kJ/mol
1. What type of chemical reaction is this? Hint: the correct word starts with a “c”. . .
2. Suppose you burn 25 grams of ethanol (C2H5OH) according to the chemical equation shown above.
The steps below will lead you through a method to determine how many LITERS of oxygen would be
needed to burn all this ethanol.
a) Use MOLAR MASS to convert 25 GRAMS of ethanol to MOLES of ethanol. Show either dimensional
analysis or a proportion as your method for solving this problem.
b) Now use a MOLE RATIO (the coefficients in the balanced chemical equation!) to determine how
many MOLES OF OXYGEN GAS would be needed to burn the moles of ethanol you calculated in (i).
c) To convert MOLES of oxygen gas to LITERS of oxygen gas, you will use the IDEAL GAS LAW
(pivnert). Assume that the pressure of the oxygen is normal air pressure and the temperature is
normal room temperature.
d) If you tried to burn 25-grams of ethanol in a steel box that contains only 5-liters of oxygen gas (at
normal pressure and room temperature), incomplete combustion would occur. Explain.
3. Go through a process similar to what you did in (b) to determine how many LITERS of CARBON
DIOXIDE gas (measured at 27C and 0.89 atm) would be produced if you burned 100-GRAMS of ethanol.
4. The delta-H value for this reaction is -1240 kilojoules per mole of C2H5OH. The negative sign means
that the reaction is EXOTHERMIC, converting potential energy into heat and light. Calculate how much
energy is converted into heat and light when 85-grams of ethanol are burned.
Continued on the next page!
189
Assignment #109: Gases in Chemical Equations (continued)
A chemical equation that you should recognize is shown below:
6 CO2 (g) + 6 H2O (l)  C6H12O6 (aq) + 6 O2 (g)
H = +2520 kJ/mol
5. What is the 5-syllable word that this chemical equation describes? Hint: it starts with the letter “p”
6. Which molecule in the chemical equation represents sugar (glucose)?
7. What does the positive sign on the H value tell you?
8. Suppose a scientist has a mango tree in a specially designed greenhouse. The greenhouse has a
volume of 8,000 liters. The scientist then fills the greenhouse with CO2 gas at a pressure of 850 mm Hg
and maintains a constant temperature of 30C.
a) Calculate the number of moles of carbon dioxide originally present in the greenhouse.
P = _____
(must be in . . .?)
V = _____
n = _____
R = _____
T = _____
(must be in . . .?)
b) Plants use the photosynthesis reaction (shown at the top of the page) to convert CO2 and H2O to
sugar and O2. Use a MOLE RATIO to calculate how many moles of sugar (C6H12O6)could be
produced assuming the plant uses 100% of the CO2 originally provided in the greenhouse.
c) Suppose that it takes 50 grams of glucose for the tree to produce one juicy mango. Calculate the
maximum number of mangos that the tree can make using the CO2 present in the greenhouse. Note:
dimensional analysis (DA) would work well here!!!!
d) Which of the following statements best describes the role of sunlight and carbon dioxide
molecules in the creation of juicy mangos? Explain.
) The carbon dioxide and sunlight both provide the ENERGY needed to make a mango
) The carbon dioxide provides the ATOMS needed, while the sun provides the ENERGY needed to
make a mango.
) The carbon dioxide provides the ENERGY needed, while the sun provides the ATOMS needed to
make a mango.
e) Would the PRESSURE inside the greenhouse RISE or FALL as the plant uses up the carbon dioxide?
Explain.
9. Suppose you were asked to calculate the VOLUME occupied by 1.44 moles of solid C6H12O6 at a
temperature of 25C and a pressure of 1 atmosphere. Would it be appropriate to use the ideal gas law to
calculate an answer to this question? If not, what would be a better way to get an answer?
190
Assignment #110: Problem Solving using a Graphing Calculator


Consider a sample of hydrogen gas that exerts a pressure of 1 atmosphere at 20C.
This sample is slowly compressed in a large syringe. A sample of air at 20 C (density =
1.21 grams per liter) is simultaneously warmed in an expandable container.
If the rate of heating is 5C per minute, and the rate of compression is 0.200
atmospheres per minute, at what time will the densities of the gas samples be equal?
Numerical Data for the Hydrogen Gas Sample compressed in a syringe: Temp = 20C
Time (min)
Pressure
Volume of
1 mole
sample
0
1
atmosphere
24.0-L
1
1.2 atm
20.0-L
2
1.4 atm
3
1.6 atm
5
2.0 atm
Density
(2g = 1 mol
H2)
.083 g/L
Visualize:
An equation that expresses Density at a given time t: Hint: this is a linear equation!
D=
Numerical Data for the sample of air being warmed:
Starts at 20C and rises 5 per minute
Time (min)
Temperature
Volume of
1.21 gram
sample
(leave as an unsolved expression?)
0
293 K
1-Liter
1.21g/1 L
8
333 K
1.14-L
1.21g/1.14 L
24
413 K
Density
Visualize:
48
96
An equation that expresses Volume at a given time t:
V=
An equation that expresses Density at a given time t:
D=
Plot both equations for Density on your graphing calculator. Set the WINDOW to
span: X-axis domain from 0 to 100 minutes, Y-axis range from 0 to 1.2 gram
per liter. Sketch a rough representation of the graph in the space to the right and
use your calculator to find the point of intersection. Interpret the meaning of the
point of intersection.
Rough graph:
191
Assignment #111: The Giant Muffin
Using gas laws of many kinds
1. Suppose you have a 3.2-liter balloon filled with helium gas at a temperature of 25C.
a) If the balloon described is warmed to 40C (a typical summer temperature in Palm Springs) with no
change in pressure, what will its new volume be? Note: You should do this problem using a
PROPORTION, NOT the ideal gas law (PV=nRT).
b) If the original pressure in the balloon was 0.94 atm, calculate the moles of gas in the balloon. Now
you should use the ideal gas law. The MOLES of gas in the balloon will not change as it is warmed up.
Therefore, you can calculate using either the original temperature or the hotter temperature!
2. Given a 10-gram sample of CO2 and a 10-gram sample of N2, each at STP (standard
temperature and pressure):
a) Make a guess as to which sample will have a larger volume (or whether they will be the same).
Then CALCULATE the volume of each sample of gas. Hint: you should know how to calculate
volumes at STP using a shortcut instead of relying on Pivnert.
b) Use the PICTURES shown below to discuss which gas sample is DENSER (in terms of GRAMS PER
Answer this question CONCEPTUALLY, without calculating actual densities.
LITER).
10 grams CO2
in small volume
10 grams N2 in BIG volume
c) Use your answers from part (a) to calculate the density (in grams per liter) of each of the gas
samples.
10
____
/
3. If a balloon has a volume of 6.25 liters at 0.80-atm of pressure, what will its volume be if the
atmospheric pressure is reduced to 0.20-atm? Hint: think about which gas law to apply here!
Continued on next page!
192
Assignment #111: The Giant Muffin (continued)
4. Baking soda (sodium bicarbonate, NaHCO3) is a commonly used leavening agent in household
recipes. When heated to about 100C, the sodium bicarbonate decomposes to form sodium carbonate,
water, and carbon dioxide. The CO2 (g) formed in the reaction will cause a baker’s dough to “rise”,
creating light and fluffy muffins.
Suppose you are trying to make a giant muffin (your choice of flavor). The initial batter has a volume
of 400 liters, and you want the baked muffin to occupy a final volume of 1000 liters. You have decided
to use baking soda as the sole leavening agent. The muffin will be baked at 350°F (175°C)
What quantity of baking soda would be needed to produce enough CO2 to raise the muffin batter up to
a volume of 1000 liters? Note that one teaspoon of baking soda is approximately 5 grams. Hint: START
WITH A BALANCED CHEMICAL EQUATION!
Then consider the picture shown below that describes the muffin batter
before and after baking. It is CO2 (g) that “inflates” the muffin.
P = _____ (muffin baked at normal pressure)
V = _____ (look at the picture!)
n = _____ (moles of CO2)
R = _____
T = _____ (must use Kelvin!)
5. A rigid cylinder is filled with pure Cl2 gas at 25C and 740 mm Hg (740 torr).
a) If the volume of the cylinder is 15 liters, how many GRAMS of Cl2 are in the cylinder?
b) The cylinder of chlorine gas is then placed in a hot oven. If the temperature in the oven reaches 250
C, how high will the pressure in the cylinder get? (you may express your answer using any of the pressure
measurements discussed in class)
c) Now suppose the cylinder has a few ml of water in it when it is filled with Cl2. At room temp, this
water has little effect on the pressure within the cylinder, but when this "wet" cylinder is heated to 250C,
the pressure inside is found to be much greater than the number you calculated in (b). How can you
explain the appearance of this "extra" pressure? (you need not do any calculations here--just provide a
conceptual answer)
d) Explain how the situation described in part (c) is analogous to what goes on when making popcorn.
Hint: a popcorn kernel has a small amount of water inside it!!!
193
Assignment #112: The Volume is Right!
(LAB)
Overview: The title of this lab comes from the game show "The Price is Right". In this experiment, you
will be trying to create a specific quantity of hydrogen gas (H2) between 300 and 600 mL (your
instructor will assign you a target volume). The hydrogen gas will be generated from the reaction of
magnesium metal with HCl. Your objective is to come as close as possible to your target volume. To
accomplish this, you will make use of stoichiometric relationships, including the ideal gas law (Pivnert).
Procedure: General scheme of the lab:

Phase 1: Make calculations (follow guidelines on next page)

Phase 2: Set up a practice run. Figure out how to set-up the gas collection system and try out
your reaction using the quantities you think are correct. You may only do one practice run
(unless your instructor specifically tells you otherwise).

Phase 3: Rethink your methods. During this phase, you may change your mind on how to run
the reaction. You should also introduce methods to make your system “fool-proof” for the actual
competition. YOU MUST CLEARLY DESCRIBE YOUR METHODS THAT YOU ARE USING
FOR THE COMPETITION. INCLUDE A SKETCH OF YOUR SET-UP.

Phase 4: Set-up for competition. You will need to have all materials set up for the competition
about 10 minutes before the end of the period. Your instructor must read the starting volume of
water in your collection cylinder.

Phase 5: Competition time! Near the end of the period, all groups will simultaneously start their
reactions and collect the hydrogen gas produced. You will be allowed 3-minutes to run your
reaction. If you aren’t ready to go when the competition is scheduled to begin, you will probably
need to stay after class to complete your experiment.
Grading: Your grade for this lab will be based on how close you come to your target volume. Each
group member should keep a well-organized note page for the lab that will be turned in (as a table) at the
end of class. If a person fails to turn in a quality note page, that person will be penalized 1 point.
Grading Rubric
H2 gas collected
in cylinder as a
percentage of
target volume
Accuracy of gas production
Best in Class
Lab Score
10 points
Within 5% of target volume
9.5 pts
Within 10% of target volume
9 points
Within 20% of target volume
8.5 points
Within 30% of target volume
8 points
Within 40% of target volume
7.5 points
Worse than that. . .
7 points
Notes:
1. Please staple your group’s note pages together at the end of class and leave them on your lab table for
your instructor to pick up. Your base score (from the rubric above) may be adjusted up or down
depending on the quality of your individual note page.
2. If your group CHEATS during the contest, expect to receive 5 points for the lab (at most).
3. If you scored less than 8.5 points according to the grading scale shown above, you may do a write-up
for the lab to raise your individual score. In your write-up, show all your calculations and thoughtfully
discuss the “Factors to Consider” shown on page 2 of the lab. Try to provide a thoughtful error analysis.
Ideally, your write-up should be able to guide other students to avoid all possible mistakes.
194
Assignment #112: The Volume is Right! (continued)
Calculations: You will need to do the following theoretical calculations to be successful:
1. Write a balanced equation for the reaction of Mg with HCl. Recall that hydrogen gas is diatomic.
2. Determine how many MOLES OF GAS you will need in order to reach your target volume.
Hint: this is a Pivnert problem! Measure temperature of the room and estimate pressure in the room.
3. Determine how many MOLES of Mg you will need and how many MOLES of HCl you will need in
order to generate the correct quantity of H2. Hint: use the MOLE RATIOS from the balanced equation.
4. Convert MOLES of Mg to GRAMS of Mg.
5. Choose the concentration of acid you will use (i.e. 1-molar? 2-molar? 3-molar? 6-molar?). A
higher molarity will react faster, which may be desirable, or may cause problems.
6. Calculate the number of mL of acid you will need for the reaction. If you get a value of more
than 50 mL, you have made a bad calculation. Note: in order to calculate mL of acid needed, you will
have to use the molarity concept described below.
1-molar HCl = 1 mole HCl per liter
2-molar HCl = 2 moles HCl per liter etc.
7. Set up your practice run using the quantities of reactants you have calculated. Time your
reaction—in the contest, you will be allowed only 3-minutes to complete the reaction. Measure the
quantity of gas you have created and calculate a percent yield.
8. After completing your practice run, discuss the factors described below. You are expected to
make improvements to your procedures which address these factors. You are allowed to make
significant changes to your procedures if your practice run was a “dud”, but you are NOT allowed to
do a second practice run unless your instructor approves.
9. Factors to consider after your practice run. Discuss these as you plan your set up
for the actual contest. On your note page, make a sketch of your reaction set-up and clearly state
how you ran the reaction for the contest.
A. The jumbo graduated cylinder, when filled with water, is fairly heavy. If gravity pulls the big
cylinder down so that it crushes the plastic tube, it can block the flow of gases. Find a way to set
up your system so that the cylinder’s weight is supported by an iron ring.
B. A common cause for failure in this lab is that the hose carrying H2 gas pulls out from the
graduated cylinder. Devise a method to secure the hose in place so that all the gas arrives safely
in the graduated cylinder. Note: using human hands is not “legal”.
C. Another common cause for failure in this lab is spilling the reactants (Mg or HCl) as you are
rushing to get the reaction started. What methods can you employ to minimize this hazard?
D. Are you happy with your reaction rate? Perhaps you should change the concentration of acid
that you used in the practice run.
E. The magnesium metal is highly reactive and starts to fizz the moment it hits the acid. It is
likely that at the moment you mix the acid and magnesium, you will LOSE THE FIRST BURST of
bubbles because the stopper isn’t on yet. Perhaps you can find a way to minimize the initial loss of
gas so that you can collect close to 100% of the hydrogen that is produced by the reaction. NOTE:
You should be striving to find scientific methodologies that MAXIMIZE THE EFFICIENCY of your
reaction. Your goal is to make the right amount of gas. You are NOT allowed to add extra
magnesium to make up for deficiencies in your procedures.
F. The reaction of magnesium and HCl is EXOTHERMIC, heating up the reaction flask. You know
that hot gases can expand. It is possible that this thermal expansion can cause you to go past your
target volume. What can do to minimize the effects of thermal expansion?
G. It often happens that some of the magnesium that you carefully weighed out remains
UNREACTED in the flask. If this occurs, you will only be producing a FRACTION of the gas that you
intended to make. Can you devise a method to ensure that ALL the magnesium atoms will react?
195
Assignment #112 (continued): Note Page for The Volume is Right!
1. Show your calculations (neatly) that lead you from the assigned volume of gas to the mass of
magnesium and volume of acid required for your reaction:
2. Make a sketch of your experimental set-up for collecting gas. Also discuss your methods for
intelligently dealing with the factors described in step #9.
Problem
Your solution (write in PAST TENSE,
SPECIFICALLY STATING WHAT YOU DID!)
9A
Collection
cylinder can crush
delivery tube
9B
Hose could get
pulled out from
bottom of
cylinder
9C
Spilling reactants
is a common
source of failure
9D
Reaction rate may
be too fast or too
slow?
9E
Magnesium starts
to fizz before the
stopper is on
9F
Exothermic rxn
heats gases?
9G
Some Mg remains
unreacted
Sketch of set-up (include quantities of
reactants used)
196
Assignment #113: Gas Density and Simple Gas Laws
(good practice for next week’s test!)
Suppose you have two moles of sulfur dioxide gas (SO2). Sulfur dioxide is a common air
pollutant produced when burning coal.
1. Calculate the volume occupied by two moles of sulfur dioxide at STP.
2. Calculate the density of the two mole sample of sulfur dioxide at STP.
3. Calculate the volume occupied by 16-grams of sulfur dioxide at STP.
4. Calculate the density of the 16-gram sample of sulfur dioxide at STP.
5. Calculate the density of the 16-gram sample of sulfur dioxide at a temperature of 30 Celsius
(pressure still at 1-atmosphere).
6. Suppose a crazy chemist by the name of Lars Gutfernutten decides to make a hot air balloon
using sulfur dioxide inside the balloon instead of normal air. Knowing that the density of
normal air is 1.25 grams per liter, how hot must Lars make his sulfur dioxide in order for it to
reach a density less than that of normal air?
7. Check out a You-Tube video of an aluminum foil boat that can float in mid-air. I think you
will find this really cool!
Google “Magic Floating Boat” or type in the following web address
http://www.youtube.com/watch?v=XjCmwuGKR6g
197
Assignment #113 (continued): Gas Density and Simple Gas Laws
Charles’ Law: In an expandable container, volume is proportional to Kelvin Temperature
Gay-Lussac’s Law: In a non-expandable container, pressure is proportional to Kelvin
temperature.
Boyle’s Law: If the volume of a gas decreases (without changing temperature), the pressure
of the gas increases. If the volume of a gas increases, the pressure of the gas must decrease.
P x V = a constant value!
8. If a gas in a balloon occupies a volume of 450 mL at 28C, what volume will it occupy at
15C? This is an example of Charles’ Law.
9. A balloon with a volume of 1.2-liters at a pressure of 760 mm Hg is taken to a high
mountain, where the pressure is only 680 mm Hg. What volume will the balloon occupy in the
high mountain environment?
a) Which one of the three gas laws is applicable in this problem?
b) What is the value of P x V initially?
c) Use the P x V value calculated in part (b) (and some simple algebra) to determine the volume of
the balloon in the high mountain environment.
d) Provide an explanation (possibly with a sketch) explaining why the volume of the balloon
expands when taken up to the high mountain environment.
10. A sealed, room-temperature flask contains air at a pressure of 14.2 PSI. What pressure
(in PSI) will the air exert if the flask is placed in a boiling water bath? Hint: think about which gas
law you should apply here!!!!!
198
Assignment #114: Launching Rockets using Secret Fuel #1
The “secret fuel” is a mixture of hydrogen and oxygen
Overview: In the next lab block, we will be making hydrogen rockets using plastic bottles with volumes
between 1 and 4 Liters. We will fill the bottles with a mixture of hydrogen gas and oxygen gas in an
appropriate ratio. When ignited, the reaction will produce H2O gas at a very high temperature. The hot
molecules will generate a propulsive force (or the bottle will simply blow up).
Note: filling out these pages IS your write-up for this lab. Do your work neatly and
completely.
1. The first thing to determine is the appropriate ratio of hydrogen and oxygen to use in your rocket. You
need an explosive mixture to launch your rocket. A mixture too rich in hydrogen will burn quietly like a
Bunsen burner instead of igniting explosively. A mixture too rich in oxygen will explode, but weakly. A
mixture that is just right will produce maximum power for your rocket.
The unbalanced equation for the propulsion reaction is as follows:
H2 (g) +
O2 (g) 
H2O (g)
If you balance the equation shown above, you will know the correct MOLE RATIO of hydrogen to
oxygen needed to achieve the maximum propulsive force.
2. Use a 1-liter graduated cylinder to accurately measure the total volume of your bottle.
Using the mole ratio of H2 to O2 that you discovered from your balanced equation, you can start to figure
out how much hydrogen gas and how much oxygen gas you will need to fill your plastic bottle. In the
space below, sketch a picture of your bottle (label total volume) and draw a line showing what fraction
of the bottle you will fill with hydrogen and what fraction you will fill with oxygen. Then convert these
fractions into actual volumes of hydrogen and oxygen (in liters). Note: try to discuss how the MOLE ratio
compares to the VOLUME ratio that you have sketched in your picture.
3. Use the IDEAL GAS LAW and your picture above to calculate how many MOLES of hydrogen you will
need, and how many MOLES of oxygen you will need in your plastic bottle. Also give a one-sentence
explanation for why 22.4 L/mole is NOT a valid ratio to use for this experiment. Hint: what does STP
actually mean?
199
Assignment #114: Rocket Lab (continued)
From page one, you know how much hydrogen and oxygen you need for your rocket, but you will have to
GENERATE these gases by running chemical reactions. This page discusses production of oxygen gas.
4. OXYGEN gas can be generated from the decomposition of H2O2. The unbalanced equation is:
H2O2  H2O + O2
H = -190 kJ/mol
The reactant in this reaction, hydrogen peroxide, is thermodynamically unstable, and can rearrange its
atoms to form water and oxygen gas. This reaction is extremely slow at room temperature unless a
catalyst is added. We will use the salt potassium iodide (KI) as a catalyst to hasten this reaction. Note:
When running the actual reaction, a small scoop of the catalyst (about 0.7 grams ) will be “packaged” in weighing
paper and then added to the hydrogen peroxide solution. You will place the stopper on the reaction flask and then
shake to allow the catalyst to come out of its package. This method should enable you to collect 100% of the gases
created in this reaction.
a) Balance the equation shown above and use stoichiometry to calculate the MASS of hydrogen peroxide
needed to generate enough O2 for your rocket. Hint: your calculation starts with the number of moles of O2
you calculated in step #3.
b) The hydrogen peroxide that you will use to generate oxygen gas will be provided in liquid form. This
liquid is a solution of hydrogen peroxide diluted with water. The solution contains 10% hydrogen
peroxide (the other 90% being water). Use your number from part (a) to calculate how many grams of
this SOLUTION you will need to use. If you don’t know how to make the calculations required here, I suggest
you start by drawing a picture of what the 10% solution of hydrogen peroxide will look like. Then label the fraction
of the solution that you KNOW (the mass of H2O2 calculated in part (a)).
c) Assuming that the 10% peroxide solution has a density of 1.0 g/ml, convert the grams you calculated
in (b) into milliliters of solution.
d)** (complete after Tuesday’s lesson—in which your instructor will teach you some new ideas) Assume the sketch below
represents the energy profile for the UNCATALYZED decomposition of hydrogen peroxide.
i. In the sketch of the uncatalyzed reaction, place labels that designate positions of: H2O2, H2O + O2,
ACTIVATION ENERGY (Ea), and H VALUE for the reaction (H value is shown at top of this page).
ii. In the space to the right, sketch a profile for the catalyzed decomposition of hydrogen peroxide. Then
EXPLAIN HOW the catalyst helps the reaction proceed at room temperature.
Explanation for ii:
PE
PE
Uncatalyzed Reaction
Catalyzed Reaction
200
Assignment #114: Rocket Lab (continued)
This page discusses production of hydrogen gas.
5. Hydrogen can be generated in many ways. We will use the method shown below:
Ca (s) + H2O (l)  H2 (g) + Ca(OH)2 (s)
∆H = _________
a) Balance the equation shown above.
b) Calculate a H value for the reaction (in KJ/MOLE) knowing that the reaction is EXOTHERMIC and
produces 10.4 kilojoules of heat for each GRAM of calcium that reacts. Write the calculated value in the
space provided above.
b) Use stoichiometric calculations to determine the MASS of calcium and VOLUME of water needed to
produce enough hydrogen for your rocket. Be neat!!!! Do you know how much H2(g) you need?????
c) The amount of water you have calculated is the amount that will actually react with the
calcium you are weighing out. This is NOT a smart way to run this reaction. Circle two of
the choices below that express reasons why it is a good idea to add EXCESS water when you
run this reaction. Note: when you run the reaction, you will want to use a LARGE excess (as large as your
reaction flask permits. In the margin of this paper, make a SKETCH of what your flask should look like if you use a
large excess of water.
i. extra water will create extra hydrogen
ii. extra water will create more heat in the reaction flask
iii. extra water will keep the reaction flask cool
iv. extra water will reduce the amount of air that goes into the rocket
Sketch of Rxn flask
d) The calcium you will be using is a very reactive metal. Even while inside its jar, the outer surface of
the calcium will be oxidized to some extent (forming CaO). This reduces the amount of actual calcium in
the lumps that you will be using. How should you correct for this problem?
6. Two final questions (to be completed after the rockets are launched):
i. Discuss the role of the ignition spark in the combustion rxn that launches the rocket. Sketch an PE
energy profile in the space to the right and use this profile to discuss the term “activation energy”.
Combustion Rxn
ii. What is your explanation for why the bottle actually lifts off? Note: the ignition of the hydrogen/oxygen mixture
generates VERY HOT gas molecules with lots of kinetic energy. What do you think these hot molecules will do?.
201
Assignment #115: The Other Side of the Mountain
Activation Energies and Catalysis
In this activity, you will run a couple of Interactive Physics simulations to try to understand how
ENERGY influences whether a reaction is productive or not.
1. In the Chemistry Sims folder on the desktop, open the “Activation Energy and Catalysis” folder and
load the simulation entitled Sim #1—Activation Energy.
2. Run the simulation. After a 2-second delay, you should see the catapult fling a molecule towards the
energy barrier. Your GOAL in this simulation is to get the molecule to reach the “promised land” marked
RXN PRODUCTS.
3. In the space below, make a brief sketch of what you see in this simulation. Label the mountain using
the vocabulary terms shown on the computer screen.
4. After running the initial simulation, use the vertical sliding control to CHANGE THE VALUES FOR
CATAPULT ENERGY (TEMPERATURE). Use the table below to describe the results of the simulation using
various temperatures. Note: in the simulations, assume that the values for Catapult Energy represent
temperatures in degrees Celsius.
Temperature
(degrees C)
25
Description of Molecule’s Flight Path
Productive
Rxn?
No
35
45
50
5. Suppose the computer simulation represents the generic chemical equation shown below. The reaction
is known to be EXOTHERMIC.
X  Y+Z
∆H = -250 kJ/mol
A thoughtful student is puzzled how substance X could ever be kept in a bottle. She reasons that if X can
decompose to form Y and Z all on its own, it would be impossible to store substance X for any length of
time. In the space below, choose the energy profile that can EXPLAIN how substance X can survive in a
bottle for a long time (at least at low temperatures). Support your choice by discussing the concept of an
ENERGY BARRIER.
6. In a diagram of this sort, the HEIGHT OF THE MOUNTAIN is known as the “ACTIVATION ENERGY”
(abbreviated Ea) for the reaction. Explain how this term is a logical name.
Continued on next page!
202
Assignment #115: The Other Side of the Mountain (continued)
7. Load Sim #2—Catalysis. This should initially look very similar to Sim #1.
8. When you click “Run” in this simulation, the catapult will fire after a 4-second delay.
In the space below, DESCRIBE what you observed (a sketch may be helpful).
9. Experiment with several different temperatures in the simulation and summarize your findings by
completing the sentence below:
In the presence of a catalyst, a temperature of _____° C is required for
a productive reaction. Without a catalyst, at temperature of _______°C
is required for a productive reaction.
10. Which of the following statements do you feel more accurately describes the nature of a catalyst?
: A catalyst is an energy boost (like adding heat from a Bunsen burner)
: A catalyst is a molecule that makes it possible for a reaction to proceed WITHOUT
requiring an energy boost
11. Suppose you had 100 reactant molecules waiting to be converted into products. Explain how it is
possible for a single catalyst molecule to help all 100 reactant molecules turn into product molecules.
Hint: a metaphor of 100 pigs in a pen with a single gate may help you visualize. . .
12. Where should a catalyst be shown in the balanced chemical equation for a reaction? Should it go on
the reactant side? The product side?? In the middle, under the arrow??? Use the generic equation shown
below to show where you think a catalyst belongs. Explain your reasoning.
X
Y+Z
13. Either heat OR a catalyst can be used to “jumpstart” a reaction that has a large energy barrier.
Explain some ADVANTAGES OF USING A CATALYST instead of heat for initiating such a reaction.
203
Assignment #115: The Other Side of the Mountain (continued)
13. Load Sim#3—Diatomic Reaction with an Energy Barrier. Run the simulation at both 40 degrees and
80 degrees.
14. Assuming red circles represent hydrogen atoms, while blue circles represent chlorine atoms, write a
balanced equation for the reaction that occurs at 80 degrees.
15. Other than by reading the title, how would you know that this reaction has an energy barrier?
16. Provide an explanation based on chemical bonds for why an energy barrier exists in this reaction.
17. Some bond strengths are provided in the table below:
Bond
HH
ClCl
HCl
Strength of bond
430 kJ/mol
240 kJ/mol
440 kJ/mol
Use these bond energies to sketch an energy profile and find the H value for the reaction shown in this
simulation. Note: Bonds result in negative PE! The reaction produces two moles of HCl.
Potential
Energy
Rxn progress
18. The reaction of H2 and Cl2 can be triggered by high temperatures, but can also be triggered by blue
photons. Explain how blue photons can initiate this reaction. Note: Red or yellow photons would NOT
activate this reaction. Look back at The Photon Game to remind yourself of photon energies. . .
204
Assignment #116: Calculating your Carbon “Footprint”
A lesson on greenhouse gases
By the END of this worksheet, you should be able to completely fill in the table below:
Source of CO2
Human respiration
Monthly Consumption
60,000 Calories (2000/day)
Automobiles
_______ gallons of gasoline
Household Electricity
________ KWH
Household Heating
_________ therms
TOTAL
Pounds of CO2
produced each month
50 pounds
Pounds/mo.
N/A
CO2 FROM AUTOMOBILES
The equation shown below describes the combustion of gasoline (C8H18 = OCTANE, which is the
primary constituent of gasoline).
C8H18 (l) + 12.5 O2 (g)  8 CO2 (g) + 9 H2O (g)
1. A gallon of gasoline contains 2720 GRAMS of octane (C8H18).
a) Calculate the number of MOLES of octane contained in 1-gallon (2720 grams) of gasoline. Hint:
this is a simple gram to mole conversion--use the molar mass of C8H18.
b) Use the number of moles of octane in (a) to calculate the moles of CO2 produced when a gallon of
gasoline is burned. Hint: this is a mole-to-mole calculation! Use the coefficients in the balanced
equation!
c) Convert the moles of CO2 calculated in (b) into pounds of CO2 produced when 1-GALLON of
gasoline is burned. Hint: find grams of CO2 first—then convert to pounds.
Burning 1-gallon of
gasoline produces
______ pounds of CO2.
2. Use your estimated value for the number of gallons of gasoline your family automobiles burn in a
month to calculate how many POUNDS OF CO2 your automobiles produce each month. Enter this value in
the appropriate space of the table at the top of the page. Note: if you don’t have your personal data, you
can assume that you have an average two-car family that burns 100 gallons of gasoline per month.
Continued on the next page!
205
Assignment #116: Calculating your Carbon “Footprint” (continued)
CO2 PRODUCED TO GENERATE ELECTRICTY
On your electricity bill, the electrical energy you use is measured in units of KILOWATT-HOURS
(KWH). One KWH is equivalent to 3,600 kJ.
4. Convert your family’s KWH value on your electricity bill to KILOJOULES of electrical energy.
Note: If you don’t have your personal data, use the California average of 700 KWH/month.
5. A gas-fired power plant burns methane (CH4) to turn chemical energy into electrical energy:
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
H = -800 kJ/mole
a) Since the process of converting chemical energy to electrical energy is only about 40% efficient,
burning 1-MOLE of methane can produce only 300 kJ of electrical energy. Use this value to calculate
how many MOLES of methane must be burned (at a power plant) to supply your monthly electricity
demand. Note: your calculation should start with the number of kJ you found in step 4.
b) Use the balanced chemical equation to calculate how many GRAMS of CO2 are produced when the
moles of methane calculated in (5a) are burned at a power plant.
c) Convert your answer from (5b) to POUNDS of CO2. Enter this value in the appropriate space in the
data table at the top of page 1.
CO2 FROM HOUSEHOLD NATURAL GAS COMBUSTION
When you read a gas bill, you will see that you are charged in units of “therms”. One THERM is
equivalent to 130-MOLES of methane.
6. Use your monthly gas bill and the conversion factor provided above to calculate how many MOLES OF
METHANE your family burns in a month to supply your home heating needs. Avg. CA home = 40 therms/mo
7. Knowing that each MOLE OF METHANE that is burned produces 44 GRAMS of CO2, calculate how
many GRAMS of CO2 are produced each month to supply your household heating needs.
8. Convert your answer from #7 into POUNDS of CO2. Enter this value in the appropriate space in the
data table.
Total up your household’s monthly CO2 production in the data table. Your total mass of
CO2 produced is sometimes called a “Carbon Footprint” because it represents the mark
that you leave upon the earth when you burn fossil fuels (either directly or indirectly).
Reducing your footprint will result in less impact upon Earth’s ecosystems.
206
Assignment #117: Scientific Notation Introduction
Part 1: Examples
Number
Scientific Notation
Calculator Display
1,250
1.25 x 103
1.25E3
3,500,000
3.5 x 106
3.5E6
789,000,000
7.89 x 108
7.89E8
.0015
1.5 x 10-3
1.5E-3
.0000000266
2.66 x 10-8
2.66E-8
Part 2: Practice NO CALCULATOR!!!!!
Number
Scientific Notation
Calculator Display
4,200
9.14 x 107
6.1 x 10-5
8.42E5
3.8E-3
Part 3: Scientific Notation on a Calculator
To type the number 1.2 x 106 into your calculator:
1. Press the following buttons:
1
.
2
2. Find the EE key and press it (On a TI-83 graphing calculator, the EE key is the second
function on the comma key located above the 7).
On your screen, you should now see the display 1.2E. The “E” is asking you to input an
EXPONENT.
3. Type the number 6. The screen should now display 1.2E6. The calculator recognizes this
value as 1.2 x 106.
4. Use your calculator to multiply 1.2 x 106 by 3 x 109. Write your answer below, in both
calculator format (with the E in the middle) and in human format (scientific notation).
Calculator format
Human format (scientific notation)
207
Assignment #118: Mr. Toad and Moley
Introduction to Avogadro’s Number
Mr. Toad and Moley, while waiting for their tea on a Sunday afternoon in Toad Hall, had the
following conversation:
Moley: What's a word that describes a very large number of somethings?
Mr. Toad: A dozen.
Moley: How many somethings are in a dozen?
Mr. Toad: Twelve somethings, no more and no less.
Moley: And what if you have a dozen dozens?
Mr. Toad: Then you would have a gross. A gross contains 144 somethings, so that is a dozen
dozens.
Moley: And if you had 144 nothings?
Mr. Toad: Then you would have a gross of nothings. For a gross of anything, be it somethings
or nothings, would contain 144 things.
Moley: But what if you have a dozen dozen dozens?
Mr. Toad: Then you would have a dozen gross. Or is it a dozen grosses?
Moley: So there is no special word to describe a dozen dozen dozens?
Mr. Toad: That is correct. A dozen dozens is a gross, but a dozen dozen dozens has no special
name.
Moley: Then we shall make up a name! We shall call it a TOAD. A dozen dozen dozens is
now called a TOAD.
Mr. Toad: How splendid! Now when I order buttons for my waistcoats, I shall be able to order
a TOAD instead of a dozen gross. I shall get 1728 buttons in either case, for a dozen gross is 12
times 144, which equals 1728, and a TOAD is a dozen dozen dozen, which is also 1728.
Moley: And what if you had a truly incredible number of things, such as 600 million million
billion somethings?
Mr. Toad: Then you would have. . . . a MOLE!
Moley: A MOLE!!!
Mr. Toad: Yes, a MOLE. I hereby decree that whenever a person shall encounter 600 million
million billion of anything, he shall be justified in saying that he has just seen a MOLE of the
somethings that he has encountered.
Moley: I shall be careful to remember this the next time I see 600 million million billion
somethings, though it may be some time before I encounter such a large number of things!
Mr. Toad: Indeed, it may well be so. But when you do, you shall be justified in saying, "There
goes a MOLE of those things!"
And with that, the two friends sat down to enjoy their tea.
Continued on next page!
208
Assignment #118: Mr. Toad and Moley (continued)
Questions: (pages 179-183 in your textbook may be helpful if you are stumped)
1. According to Mr. Toad’s decree, there are 600 million million billion individual things in a
mole. Attempt to write out this number (in longhand). Then write the number using SCIENTIFIC
NOTATION. (Note: a million has 6 zeros, a billion has 9 zeros).
Longhand =
Scientific notation =
This value (the number of things in a mole) is called Avogadro’s number.
2. Although they did not realize it, both Mr. Toad and Moley were confronted with MOLES of
molecules in their cups of tea. A typical cup of tea contains about 10 MOLES of water
molecules!
a) How many WATER MOLECULES are in a typical cup of tea if it contains 10 MOLES
of H2O? Note: show a calculation using scientific notation that answers this question.
Consider using dimensional analysis in your calculation method.
b) How much would all these water molecules weigh? Hint: show a calculation involving molar
mass—this is an easy question that doesn’t involve Avogadro’s number!
c) Into his cup of tea, Moley stirs in 1/20th of a MOLE of sugar molecules (a heaping teaspoon).
i. How many sugar molecules has Moley added to his tea? Express in scientific notation and label your answer
appropriately!
ii. How much would 1/20th of a MOLE of sugar molecules weigh? (sugar = C6H12O6)
d) Which would contain more molecules: a MOLE of sugar molecules or a MOLE of water
molecules?
e) Which would weigh more: a MOLE of sugar molecules or a MOLE of water molecules?
3.* If you had a MOLE of water molecules, and you counted 1 molecule each second, how many years
would it take you to count the entire mole of water molecules? What if you had all the humans on earth
(7 x 109 humans) all helping you count the molecules (each human counting 1 molecule per second)?
209
Assignment #119: Pink-Be-Gone!
(LAB)
Objective: You will observe some interesting color changes using simple chemicals. Your goal is to
develop a SCIENTIFIC MODEL of how the color changes occur. In the process, you should learn something
about how acids and bases work. Note: This Lab is worth 5 pts.
Procedure:
1. The buffet table provides stock bottles labeled ACID and BASE. Fill a test tube 1/3 full of ACID. Fill a
second test tube 1/3 full of BASE. Label so that you know which is which. Put a pipet in each tube.
2. Pour some pink solution in a clean 250 mL beaker so that the beaker is about half-full. This solution
contains A POPULATION OF PINK MOLECULES DISSOLVED IN THE AQUEOUS SOLUTION. Place a stir bar
(magic bean) in the beaker and turn on the stirrer. Note: the chemical formula of the pink molecule will remain
a mystery.
3. While the solution is stirring, add 1 DROP of ACID to the pink solution. If no color change is observed,
add another drop. Record the number of drops needed to change the color.
4. Look at the figures shown below. Each presents a MODEL for how the acid may be acting to eliminate
the pink color. Scrutinize each model and choose the one that you think is the most likely explanation for
the elimination of the pink color. Note: in each of the models shown, the shaded figure represents the
pink molecule, while the unshaded figures represent colorless molecules.
Acid
Model A: The Acid is a destroyer,
cutting up the pink molecule into
smaller pieces which are not pink.
MODEL A
Pink molecule
Model B: The acid binds to the
pink molecule. The combined
molecule is no longer pink.
MODEL B
Acid
Pink molecule
MODEL C
Acid
Pink molecule
Model C: The acid
removes a small
portion of the pink
molecule; this
piece is essential
for making the
molecule appear
pink.
5. Once you have decided on a model for how the pink molecules have become colorless, add a SINGLE
drop of BASE to the beaker (while stirring). If no color change is observed, add another drop of BASE.
Repeat until a distinct color change is observed.
6. Try to change the color back and forth several times by adding a couple drops of ACID, then a couple
drops of BASE while stirring vigorously. End with the beaker being in a pink state.
210
Assignment #119: Pink-Be-Gone!
(continued)
7. Based on your observations, enhance your chemical model you chose in step 4 to explain how the
BASE is able to restore the pink color. Feel free to change your mind on which model you chose in step 4.
In other words, if your original choice no longer makes sense based on the new evidence you have
discovered, you should discard your original model and choose one that better fits the evidence. You are
also welcome to develop a new model that is not A, B, or C.
In the space below, sketch your new model that shows how a BASE acts to restore the pink color.
NOTE: YOU MUST INVENT A SYMBOL TO REPRESENT THE BASE!!!!
8. With the beaker in its pink state, add 1 mL of NAMMANA to the beaker (note: Nammana is a
mystery molecule!). Stir to mix the nammana into the solution.
9. Now add ACID (drop by drop) until the pink color has completely vanished. You should find that it
takes more than just a few drops to remove the pink color. Come up with a MODEL for what the nammana
molecules are doing to “protect” the pink molecules. The questions below are designed to help you think
of possibilities.
 Do the nammana molecules interact with the pink molecules or with the acid molecules?
 Do you think nammana is an acid? A base?? Or is it a different type of molecule that is neither
acid nor base???
 Do the nammana molecules form any bonds?
 Are the nammana molecules destroyed by the acid molecules?
 Are the nammana molecules inherently powerful, or do they exert their effects by having huge
numbers?
In the space below, sketch a MODEL depicting how NAMMANA affects the color changes of the pink
molecule in the presence of acid.
NOTE: YOU MUST INVENT A SYMBOL TO REPRESENT NAMMANA!
10. Use the model you have constructed in step #9 to make a prediction for how the molecules in the
beaker will respond if base is added (drop by drop) to the solution that is currently in your beaker.
Explain your prediction on a molecular level. Then perform an appropriate experiment to test your
hypothesis.
211
Assignment #120: Molecular Structure Practice Page
1. Draw a line structure (flat and square) that obeys the rule of HONC for the following
molecules (NO DOTS, please!). Then translate these flat square pictures into proper molecular
geometries. Use the model kit to check your answers!!!!
CH3Cl
CH3Cl
Flat and square picture
3-dimensional picture
N2H4
N2H4
Flat and square picture
3-dimensional picture
C2H2O2
C2H2O2
Flat and square picture
3-dimensional picture
3. Draw each of the following molecules with correct geometry and polarity:
a) CH3OH (methyl alcohol or methanol)
b) C2H2F2 (3 isomers exist!)
4. Sketch a picture of a water molecule forming a HYDROGEN BOND with an ammonia
molecule (NH3).
212
Assignment #121: Acids Rock!
Structures of molecules that act as acids
1. For each of the following common acids, do the following:
a) Draw a good molecular picture of the acid molecule. Think about the rule of HONC and the
five shapes of molecules.
b) Put polarities on the molecular structure.
c) Write a molecular equation showing how the acid can DISSOCIATE (or IONIZE) in an aqueous
environment. Place FORMAL NEGATIVE CHARGES where appropriate. Remember that only polar
hydrogens (+) can dissociate!
Question (a) has been done for you as an example.
a) Phenol (C6H5OH)
b) Formic acid (HCOOH) Note: This acid comes from ants—when an ant stings you, it is injecting some formic
acid into your body. The structure of the molecule is shown below.
c) Acetic acid (CH3COOH) Note: draw this molecule with a tetrahedral CH3 group on the left side followed by
a COOH group that looks very similar to the formic acid molecule shown above.
d) Carbonic acid (H2CO3) Note: make sure your carbon has four bonds and each oxygen has two!
Continued on next page!
213
Assignment #121: Acids Rock! (continued)
2. Some complicated acid structures are shown below. For each of these structures, identify the
acidic hydrogens and draw the structure that will be formed when the acid LOSES its acidic
hydrogens.
a) Acetyl salicylic acid (aka Aspirin)
b) Niacin (a B vitamin)
c) Citric acid (look for 3 acidic hydrogens)
214
Assignment #122: Bases Rule!
1. For each of the following chemical formulas:



Draw a proper structure for the molecule (pay attention to molecular SHAPE!)
Place formal negative charges where they belong.
Show where an H+ could attach to the molecule and draw the structure of the molecule that forms.
a) HCOO- (structure provided below)
b) NO2-
c) CH3COO-
d) NH3
e) CO3-2 (two formal negatives!)
f) HCO3- (The final structure, should possess one
(structure provided below)
double bond. Make sure your structure has only ONE
formal negative!)
2. A common misconception among chemistry students is that ALL substances must fall under
the category of acid or base. In fact, there are many molecules that are neither acids nor bases.
In the space below, try to come up with the structures of a few molecules that would be
incapable of donating a hydrogen and also incapable of accepting a hydrogen.
215
Assignment #123: Through the Looking Glass
Visualizing Acid-Base Reactions
1. For the following molecules and ions, draw out the molecule with appropriate shape and
polarity (include formal charges!) and show how each would react with an added HYDROXIDE ION.
Remember that hydroxide is a good base (i.e. the OH- is looking for an H+ to grab). If you think
there would be no reaction with hydroxide, state why not.
a) CH3COOH Hint: this problem is mostly done for you—all you need to figure out is what the second
product is.
b) C2H6
c) NH4
+
d) HCO3-
Continued on the next page!
216
Assignment #123: Through the Looking Glass (continued)
2. For each of the following molecules and ions, draw out the molecule with appropriate polarity
and show how each could react with an added molecule of HCl. If no reaction will occur, state
why not. In each case, draw the structure of the product (include + or - charges where appropriate)
a) NO2- (the picture below is shown as an example. . .)
b) CH3COO-
c) NH4+
d) HCO3- (this structure was shown on the previous page!)
3. Draw a picture of the reaction (if any) that you think would occur if you mixed the following
compounds: (note: draw good structures for these molecules!)
-2
a) CO3 + CH3COOH
b) NH4+ + CH3COOH
217
Assignment #124: The Love Song of J. Alfred Proton (LAB)
In the tube the H’s come and go, talking of Michelangelo.
This lab will focus on reactions of benzoic acid and phenolphthalein.
Benzoic acid
The two states of
Phenolphthalein
0. Put on appropriate safety gear.
1. Place approximately 0.3 grams of solid benzoic acid in a JUMBO-SIZED Pyrex test tube (note:
approximately 0.3 grams means anything between 0.25 and 0.35 grams). Add a single drop of
phenolphthalein solution to the test tube. Note: phenolphthalein is the pink molecule you worked with in
your “Pink-be-Gone” lab.
2. Add about 12 ml distilled water to the tube and agitate to try to dissolve the solid. When you
are confident that agitation will serve no further purpose, stop agitating.
a. Which of the hydrogens in the benzoic acid molecule would be capable of making friendship
bonds (hydrogen bonds) with water? Explain briefly.
3. Set up a Bunsen burner, and with GOGGLES FIRMLY IN PLACE, heat your test tube to a gentle
boil. Continue heating until you have a nice, clear solution. Be very careful here--it is quite
easy to have the test tube boil over in a sudden rush--try to avoid this by heating gently, but
also keep yourself in a position where you won't be burned if it does boil over.
4. Once you have the clear solution, set the test tube in a DRY beaker to cool. Allow the tube
to cool WITHOUT DISRUPTION for 5 to 10 minutes. While waiting, consider the questions below:
a) Are the phenolphthalein molecules in your tube in the state with a hat or in the state without a hat?
How do you know?
b) In your test tube, is there an acid-base reaction (a proton transfer) occurring between benzoic acid
molecules and phenolphthalein molecules? Explain.
c) Can there be an acid-base reaction between benzoic acid molecules and water molecules?
Explain.
218
Assignment #124: The Love Song of J. Alfred Proton (CONTINUED)
5. After several minutes of cooling, your tube should have formed some pretty crystals. These
crystals are a form of pure benzoic acid.
a) The solid powder you originally put into your tube consisted of benzoic acid molecules BONDED
depicted below.
TO EACH OTHER as
Explain why the benzoic acid molecules seemed to DISAPPEAR when they were HEATED with
the water. Note: you should understand that the molecules were not irreversibly destroyed.
b) Give a molecular explanation for how and why the molecules of benzoic acid revert to a solid
form as the solution cools.
6. When you are confident that the crystallization is complete (and the temperature has returned
to near room temp), obtain a SINGLE SMALL PELLET of solid NaOH (NOTE: DO NOT TOUCH THE
PELLET!). Add this pellet to the tube and observe what happens. Allow the NaOH to act "on its
own" for a couple minutes while you discuss the questions below:
a) In the space below, sketch a molecular picture of the reaction that will occur when HYDROXIDE
ION (the shark!) meets BENZOIC ACID. Hint: the sodium is just a spectator ion.
b) Will the reaction of hydroxide with benzoic acid molecules have any effect on the ability of
benzoic acid molecules to make friendship bonds with each other? In other words, could “benzoic
bases” (technically called benzoate ions) still bond with each other to form a solid? Explain.
219
Assignment #124: The Love Song of J. Alfred Proton (CONTINUED)
7. Gently swirl your tube by hand, and then let the tube rest. I am hoping you will observe that
there is a “battle” of colors in the tube (pink vs. colorless). In particular, look for zones (or
territories) within your test tube where a particular color dominates.
a) Consider possible reactions between HYDROXIDE ions and PHENOLPHTHALEIN molecules. Decide
which one of the picture equations shown below describes a possible reaction and show the expected
products (i.e. complete the picture equation). Then discuss whether there is any evidence
demonstrating that this reaction has occurred in your tube.
OH- +
OH- +
b) During the battle phase, a reaction can occur between BENZOATE ions and PHENOLPHTHALEIN
molecules. Write a molecular picture equation (use the model of the bald man with/without a hat) for
this reaction. Then ponder whether there is any evidence that this reaction is occurring in the tube.
+
c) During the battle phase, the pink “zone” has a tendency to retreat toward the bottom of the tube.
During this color transition, the phenolphthalein molecules in the top half of the tube are being
converted into their colorless form. Write a molecular picture equation to explain how the PINK
molecules are converted to COLORLESS molecules during the battle. Hint: this reaction also results in
the destruction of crystals!!!!
8. Vortex your mixture in the test tube until everything is completely dissolved. If your solution
is not pink at this point, ask your instructor for some aqueous NaOH and add drops of base until
your solution is completely pink.
9. Your final task is to restore your molecules to the nice, crystalline state that you had in step 5.
Your instructor has prepared a “buffet table” of chemical choices for your use in this step.
Please use smaller test tubes to try out a few different choices.
A well-chosen chemical at this stage will participate in THREE acid-base reactions within the test tube.
Try to identify all three reactions that are occurring in this phase of the lab. . .
10. When cleaning up, pour the contents of your test tubes into the waste beaker on the buffet
table. Place the clean test tubes upside down on your test tube rack.
Write-up instructions: For this lab, you may turn in your question pages for 7.5 points. To
earn more than this, you must do a paragraph-based write-up (with pictures) for the lab. A page
of write-up guidelines is provided on the next page.
220
Assignment #124 (continued): Write-Up Help for Benzoic Acid Lab
(The Love Song of J. Alfred Proton)
1. Start your write-up with an introduction. Below are suggestions for suitable topic sentences
(with supporting statements). You may copy (or modify) one of these suggestions (fleshing it
out with your own details), or you may write your own introduction in the same spirit as the
examples provided.
Ex #1: This lab was all about the dissociation of acids. When an acid dissociates, it ___________.
We found that _____, ____ and ______ (you choose how many different factors!) affected the
dissociation of acids. The dissociation of acids produced visible changes in our test tube such as
_________
Ex #2: This lab was primarily about bases. Bases are defined as __________________. In this lab,
we had many bases in our test tube such as ____________, and often they were in competition
(competing for ?????). The competition produced visible results involving __________________
Ex #3: Acid-base reactions are sometimes described as proton transfer reactions. In this lab, we saw
many examples of proton transfers between _______________________. The proton transfer
reactions were (predictable? random?? surprising??? difficult to understand????). Proton transfers
enabled molecules to switch between _______________________, which resulted in visible changes
such as _____________.
2. Your next section should be devoted to a description of the reactions observed during the
heating/cooling experiments. You should state what you did and what you saw (a short
paragraph). Your next paragraph(s) must EXPLAIN your results on a MOLECULAR level.
Your explanatory paragraph(s) for this section should describe what happened to the molecules
in the test tube (phenolphthalein, benzoic acid, water) during the heating and cooling
experiments. You should include at least one MOLECULAR PICTURE. In your write up,
PLEASE DO NOT COPY THE QUESTIONS—let your PARAGRAPHS discuss the important
ideas involved in the heating/cooling experiments.
3. Your final section should be the longest part of the write-up. It should cover the reactions
that resulted from the introduction of NaOH and your subsequent attempts to restore the crystals.
In this section, you must EXPLAIN THE MOLECULAR CHANGES that made the COLOR
CHANGE (from colorless to pink and back again) and that made the CRYSTALS appear and
disappear in the test tube. You should write chemical equations (including molecular pictures)
for the molecules benzoic acid, phenolphthalein (with hat? without hat??), OH-, benzoic base
(benzoate), and any other “actors” you think are important. Several separate equations may be
better than one gigantic equation.
If you are thoughtful, you should be able to cover all the important reactions with a couple of
succinct paragraphs (with several chemical equations to support what you write in the
paragraphs). If you are not so thoughtful, this section may ramble on and on over a couple of
pages.
I hope that these guidelines help you make a write-up that you are proud of (and that will earn
you a good grade). I know that it will require some serious time and effort to make a good writeup—I’m hoping you will put in that time and effort.
221
Assignment #125: Should I Stay or Should I Go?
Acid Base simulations using IP
Find the Acid-Base Folder within the Chemistry Sims folder (located on the desktop). Open the folder to
locate the sims listed below.
Sim #1: Hydrochloric Acid in Aqueous Solution
Load this simulation and run it to see the behavior of the molecules. The squares in
this simulation represent random molecular motion that would be encountered at room
temperature in an aqueous solution.
Hydrochloric acid molecule
a) The pink circle in this simulation represents an H+ ion. The diamond shape represents _____________
b) Describe what happens in the simulation. Try to use the “d” word in your description.
c) Complete the chemical equation shown below describing the behavior seen in the simulation. Hint:
the H+ ion has zero electrons!
d) How skillful is the chloride ion at recovering its lost proton? Provide specific evidence that supports
your statement.
e) Does the dissociation of HCl appear to be a REVERSIBLE or IRREVERSIBLE reaction? Explain briefly.
f) Although this simulation labels the diamond shape as "chloride", the behavior would be
identical if another strong acid, such as nitric acid (HNO3), were observed. In the space below,
write a chemical equation for the irreversible dissociation of nitric acid.
f) Imagine a simulation that showed 4 HCl molecules on the screen (with an appropriate number of
squares to produce random molecular motion). Which of the following would actually describe the
contents of the “beaker”? Explain your reasoning. Note: after you have made your hypothesis, run Simulation
1.6 to see whether it supports your hypothesis.
a) 4 protons bouncing around freely and 4 chloride ions bouncing around freely
b) a couple of protons bouncing around free and a couple attached to chloride ions
c) all the protons attached to chloride ions, none bouncing around freely
g) What PERCENTAGE of the HCl molecules are dissociated in this simulation?
222
Assignment #125: Should I Stay or Should I Go? (continued)
Sim #2: Acetic Acid in Aqueous Solution (Load this simulation, but don’t run it yet!)
a) Acetic acid is an example of a WEAK acid. Before running the simulation, make a
prediction for how a weak acid’s behavior will differ from the strong acid’s behavior
you observed in Sim #1.
Acetic acid molecule
Run the simulation and observe the molecular behaviors. Once again, the 4 squares represent random
molecular motion that would be encountered at room temperature.
b) Describe what you observe in the simulation. In particular contrast the behaviors seen in this
simulation to the behaviors seen in the strong acid simulation.
c) The dissociation of acetic acid can be sketched as a picture equation.
i. Complete the chemical equation by sketching the products of the reaction.
ii. Explain why a DOUBLE-ARROW is an appropriate way to describe the dissociation of a weak acid.
d) In the simulation, is the ACETATE ION acting as an acid? A base?? Neither??? Both???? Explain.
Acetate ion
e) Now imagine a simulation that showed a population of FOUR acetic acid molecules on the screen (with
an appropriate number of squares to produce random molecular motion). Which of the following would
actually describe the contents of the “beaker”? Explain your reasoning.
a) 4 protons bouncing around freely, and 4 acetate ions bouncing around freely
b) a couple of protons bouncing around free and a couple attached to the acetate ions
c) all the protons attached to acetate ions, none bouncing around freely
f) A chemical equilibrium exists whenever a population of molecules is divided into two states that can
interconvert readily. For example, in a human population, an equilibrium can exist between married and
single people. People are constantly getting married, but people are also constantly getting divorced, so
the population never reaches a situation where all are married or all are divorced.
Run the simulation entitled “Sim 2.1--Bryant”. Define the two states for the molecules that are interconverting
and explain how this is an example of a CHEMICAL EQUILIBRIUM.
223
Assignment #125: Should I Stay or Should I Go? (continued)
Sim #3: Hydrochloric acid meets Water Molecules
Run the simulation and observe the behaviors.
a) In this simulation, what role does the H2O molecule play when it meets the HCl
molecule? Hint: your answer should include a four-letter word starting with the letter
“b”. . .
b) When acids dissociate in water, HYDRONIUM ions (H3O+) are formed. In the space below, sketch the
structure of a hydronium ion.
c) Write a chemical equation that describes the reaction that occurs as the chloride ion hands off its H+ to
a single water molecule. Note: hydronium ion should be a product in your chemical equation. . .
d) The transfer of H+ from HCl to water is an IRREVERSIBLE process. Once an HCl molecule meets a
water molecule, Cl- and H3O+ ions will be formed.
What species are actually contained in a bottle of 2-molar HCl?
H3O+ = _________ molar
Cl- = _________ molar
HCl = _________ molar
Hint: one of these values is ZERO! If
you can figure out which is the zero,
you may be able to figure out what the
other numbers must be. . .
Note: the hydronium ion (H3O+) is often written as H+ (aq). You should think of these
symbols as “synonyms”
e) Which is a stronger base: a Cl- ion or an H2O molecule? Explain your reasoning.
224
Assignment #125: Should I Stay or Should I Go? (continued)
Sim #4: Hydroxide and Acetate Compete
Before running the simulation, answer the following two questions:
a) In the box below, sketch a picture of the hydroxide ion seen on the screen. What does the shape of
the hydroxide ion suggest about its ability to grab protons?
Hydroxide ion (OH-)
b) What do you think will happen to the proton once you start the simulation? Explain briefly.
Now run the simulation and observe the behaviors of the molecules.
c) Describe the behaviors observed and attempt to explain the chemistry behind the behaviors. In your
explanation, try to use the terms “strong base” and “weak base”. Then try to translate your word description
into a chemical equation (or maybe two chemical equations).
d) How would the outcome of the simulation be altered if there were two protons available? Make a
hypothesis and then test your hypothesis by editing the simulation appropriately.
Sims #5,6,7, etc.
Attempt to create your own “experimental” sims in which you investigate any or all of the
following. Note: you can also create questions of your own to investigate!
a)
b)
c)
d)
Can a strong base ever be induced to “cough up” the proton it has grabbed?
Would temperature affect the degree of dissociation of a weak acid?
What would you get if you mixed together a strong acid, a weak acid, and a strong base?
How strong an acid is a hydronium ion?
Show your instructor anything you create that you feel is worthy of notice.
225
Assignment #126: Follow-up Questions For the Acid-Base Sims:
1. Why is a strong acid called “strong”? Provide evidence from the simulations that supports
your answer:
a)
b)
c)
d)
It has a strong bond
It is very good at donating H+
It is very good at regaining its H+
Two of the above (specify which two!)
2. Why is a weak acid called “weak”? Provide evidence from the simulations that supports your answer.
a)
b)
c)
d)
e)
It has a weak bond
It is reluctant to donate H+
It is likely to regain its H+
Two of the above (specify which two!)
All of the above
3. Hydroxide ion is a strong base. Describe the behavior of a strong base (based on what you observed
in the simulations).
4. Label the following ions as: “STRONG BASE”, “WEAK BASE”, “NOT A BASE AT ALL”
Acetate ion
Chloride ion
Hydroxide ion
5. In aqueous solution, strong acids are always 100% dissociated, while weak acids have a much
smaller percent dissociation.
a) Explain why a population of weak acid molecules must have a percent dissociation less than 100%.
b) Explain what it means for an acid to have “10% dissociation”.
226
Assignment # 127: The Nature of Chemical Equilibrium
A + B  AB
Imagine a reaction schematized by the equation shown above. Although this equation appears
simple, if the possibility of a chemical equilibrium exists, there are a number of possible
outcomes.
Scenario A: Complete reaction of A & B (100% conversion to AB).
Start with 100 molecules of A and 100 molecules of B in an enclosed container
End with: ____ molecules of AB; ____ molecules of A, ____ molecules of B
Number of Molecules
Graph:
= [A], [B]
= [AB]
time
Interpretation: This reaction is Irreversible!
Scenario B: Partial reaction of A & B (70% conversion to AB).
Start with 100 molecules of A and 100 molecules of B in an enclosed container
End with: ____ molecules of AB; ____ molecules of A, ____ molecules of B
Number of Molecules
Graph:
= [A], [B]
= [AB]
time
Interpretation:
Continued on next page!
227
Assignment # 127: The Nature of Chemical Equilibrium (continued)
Scenario C: Partial reaction of A & B (15% conversion to AB).
Start with 100 molecules of A and 100 molecules of B in an enclosed container
End with: ____ molecules of AB; ____ molecules of A, ____ molecules of B
Number of Molecules
Graph:
= [A], [B]
= [AB]
time
Interpretation:
Scenario D: No reaction of A & B???
Start with 100 molecules of A and 100 molecules of B in an enclosed container
End with: 100 molecules of A and 100 molecules of B, zero molecules of AB
Number of Molecules
Graph:
= [A], [B]
= [AB]
time
Interpretation:
1.
2.
228
Assignment #128: Introduction to Ka values
A chart of acid dissociation constants
This chart lists some common weak acids along with their Ka values. The Ka
value is an equilibrium Konstant for acid dissociation, measuring the ratio of
dissociated ions to associated molecules present at equilibrium when the acid is
dissolved in water. The smaller the Ka value, the less likely the acid is to donate
H+ to H2O (or any other base), meaning the acid is more likely to retain its H+.
Acid Name and Structure
Hydrofluoric acid
Ka value
7.2 x 10-4
HF
Nitrous Acid HNO2
4.0 x 10-4
Lactic acid CH3CHOHCOOH
1.4 x 10-4
Benzoic acid C6H5COOH
6.4 x 10-5
Acetic acid CH3COOH
1.8 x 10-5
Ammonium ion NH4
Weak acid dissociation equilibrium
in water
4.3 x 10
+
Dissociated ions on top
(numerator)
Associated molecules on
bottom (denominator)
HX (aq) + H2O  H3O+ (aq) + X- (aq)
-7
Carbonic acid H2CO3
[ H 3O  ][ X  ]
Ka 
[ HX ]
5.6 x 10-10
1.8 x 10-16
Water
1. Which number is bigger: 4 x 10-4 or 6.4 x 10-5? Note: write out the actual values below. . .
4 x 10-4 = .0004
6.4 x 10-5 =
2. According to the table, nitrous acid (Ka = 4.0 x 10-4) is a STRONGER acid than acetic acid
(Ka = 1.8 x 10-5). This concept can be illustrated with arrows as shown below. Explain which of these
weak acids would have a higher percent dissociation when equilibrium is established.
HNO2 (aq) + H2O (l)
CH3COOH (aq) + H2O (l)
H3O+ (aq) + NO2- (aq)
H3O+ (aq) + CH3COO- (aq)
3. Based on the Ka values provided in the table, which of the statements shown below is true if 0.1 moles
of hydrofluoric acid (2.0 grams) are dissolved in 1-liter of distilled water?
H3O+ ions will outnumber F- ions
HF molecules will outnumber F- ions
4. In comparison to the Ka values shown on the chart, HCl, a true STRONG acid, has a Ka value of greater
than 1,000,000. This should suggest to you that even though hydrofluoric acid is “top of the chart”, it is
still a WEAK acid.
Add appropriate arrows to the middle of the following equations to describe how the behavior of
HCl differs from the behavior of HF.
HCl molecule
HCl + H2O
H3O+ + Cl-
HF + H2O
H3O+ + FHF molecule
229
Assignment #129: Cover your bases!
A study of relative Acid strength and Base strength
(Have your Ka chart in front of you when you do this worksheet)
The concept of conjugate bases:
All acids are capable of donating an H+. We can therefore generalize the structure of an acid
as HX, where X could be something as simple as a single atom (e.g. X = Cl-), or as complex as
benzoate (X = C6H5COO-).
When an acid gives away its H+, the rest of the molecule is left behind (usually as a negative
ion). This "leftover" is called the conjugate base of the original acid. For example, in the
following scenario, X- is the conjugate base of HX.
HX  H+ + X1) What is the conjugate base of HCl? (Hint: this is supposed to be a very easy question)
2) Draw the structure of BENZOATE, the conjugate base of benzoic acid.
3) Knowing that HCl is STRONG acid, which do you think is a stronger base: chloride or benzoate?
Explain. Hint: remember the simulations. . .
4) Find HF and NH4+ on your Ka chart. Based on the information presented in the Ka chart, which do you
think is a stronger base: F- (fluoride) or NH3 (ammonia)? Explain your reasoning. Hint: the arrows
shown in the equations below should help you make the right decision.
HF (aq)
NH4+ (aq)
H+ (aq) + F- (aq)
H+ (aq) + NH3 (aq)
5) Suppose you place 100 H+ ions (from a strong acid), 100 F- ions, and 100 NH3 molecules in an
aqueous solution (plenty of H2O molecules present). Select the outcome described below that you think
most accurately describes what species would be present once the bases have sorted out the
COMPETITION for H+ ions in the solution. Explain your reasoning.
Choice A
100 HF molecules
100 NH3 molecules
Choice B
50 H3O+ ions
25 HF molecules
25 NH4+ ions
75 F- ions
75 NH3 molecules
Choice C
2 H3O+ ions
10 HF molecules
88 NH4+ ions
90 F- ions
12 NH3 molecules
Continued on the next page!
230
Assignment #129: Cover your bases! (continued)
Reversibility in Acid-Base Reactions
Consider the following reaction:
CH3COOH + NH3  CH3COO- + NH4+
6. Draw pictures (with good geometry) that show the structures of the reactants and products in the
space above.
7. On the left hand side of the equation, LABEL which molecule is the acid and which is the base. Then
do the same for the right side of the equation.
8. Explain how this reaction is REVERSIBLE. In other words, explain how the molecules on the right side
of the equation could react together to revert to the original reactants in the equation. Note: a reaction that
can go in both directions will establish a balance between products and reactants that is called an EQUILIBRIUM.
9. Which of the following best describes the competition occurring in this equilibrium? Explain your
reasoning.
a) there are two bases competing for a single proton (H+)
b) there are two protons competing for a single base
10. While beginning students often believe that the equilibrium mixture shown in the chemical equation
should contain MOSTLY NEUTRAL MOLECULES (CH3COOH and NH3), the truth is that the reaction
mixture will contain MOSTLY IONS (NH4+ and CH3COO-). Provide an explanation (based on Ka values)
for why there will be more ions than neutral molecules in the equilibrium mixture.
11. Suppose that you throw a bunch of HYDROXIDE IONS into the equilibrium mixture of NH3,
+
CH3COOH, NH4 and CH3COO . What chemical species will you actually end up with??
Hint: remember that hydroxide ions behave like sharks. . .
231
Assignment #130: Breaking Up is Hard to Do
Intro to dissociation of ions
For each of the substances shown below, you will be asked to:
a) complete the chemical equation showing how the substance can DISSOCIATE to produce ions
in solution
b) consider which dissociation reactions would be readily reversible, requiring a double arrow
c) determine whether the solution would be highly conductive (bright light), weakly conductive
(dim light), or non-conductive (no light at all). Note: The first problem has been done for you as an
example.
1. Sodium acetate (a salt) = NaC2H3O2 (s)
NaC2H3O2 (s)
a)
 Na+ (aq) + C2H3O2- (aq)
b) Sodium acetate is a fully soluble salt. When a salt dissolves, its ions dissociate. In
this case, all the solid salt is expected to dissolve, so there is
NOT
a second arrow.
c) Conductivity is based on concentration of ions. Since 100% of the sodium acetate
has turned into ions, there should be many ions, resulting in a
solution.
HIGHLY CONDUCTIVE
2. Benzoic acid (from the lab!) = C6H5COOH (s)
a) C6H5COOH (s)

C6H5COO- + _____
b) Benzoic acid is a weak acid. This means that only some of the original benzoic acid
molecules will be dissociated at any given time (i.e. an equilibrium will form). Therefore,
there SHOULD be a second arrow in the equation. Draw in the second arrow!
c) Since only a small percentage of the benzoic acid molecules will be dissociated at
equilibrium, the solution’s conductivity will be ____________________________.
3. Potassium chloride (a salt) = KCl (s)
+
+
KCl (s) 
(aq) +
(aq)
b) Is the rxn reversible? No
+
+
a) Chemical eqn for dissociation:
+
c) What level of conductivity? Why??
4. Nitric acid (a strong acid) = HNO3 (aq)
a) Chemical eqn:
b) Reversible?
c) Conductivity?
5. Hydrofluoric acid (a weak acid)
HF (aq)

a) Chemical Eqn:
b) Reversible?
c) Conductivity?
Continued on the next page!
232
Assignment #130: Breaking Up is Hard to Do (continued)
6. When a gram of each of the following salts is mixed into 100 mL of water and tested with a
conductivity meter, the results vary. Note: Use your periodic table to determine solubilities of these
salts!
Salt A = CuSO4
Solubility =
Salt B = Ca(OH)2
Solubility =
Salt C = PbCl2
Solubility =
a) Each of the solid salts starts as an ionic lattice. One of these solutions described above will produce a
bright light, one a dim light, and one will produce no light at all. Explain why these results vary (ionic
pictures appreciated!).
+
+
+
+
+
Ionic lattice
b) When at room temperature, the mixture of lead chloride in water produces no light. However, when
heated, the mixture of lead chloride in water will produce a fairly bright glow. How can you explain
this change in conductivity?
7. If glucose (C6H12O6 (s)) is mixed with water, the sugar will dissolve readily to create a sugarwater solution, but a conductivity meter will show no conductivity. Which of the following is a
reasonable explanation for why the sugar solution lacks conductivity?
a) The sugar molecules are insoluble
b) The sugar molecules break up into ions that are non-conductive
c) The sugar molecule is incapable of dissociating in water
233
Assignment #131: Good Titrations (LAB)
Overview:
In this lab, you will use the method of acid-base titration to determine the molar mass of
an unknown weak acid. The steps you will take to accomplish this goal are summarized below:
1. Run a titration experiment to find the precise molarity of a base solution with unknown
concentration.
2. Use your known base solution to titrate an unknown ACID and use the titration data to find the
molecular weight of the unknown acid
Write-up instructions: For this lab, I would like each individual to turn in a NOTE PAGE that summarizes
the work that you did to accomplish the tasks presented to you. Your note page should include
 a diagram of the reaction set-up (perhaps pictures of results, as well)
 brief descriptions of what you observed
 well-labeled numbers
 3 significant digits of precision on every numerical value
Part One: Determining the molarity of an unknown base solution
Your instructor has prepared three solutions of NaOH (a strong base) for your use. You will use either
solution X, solution Y, or solution Z (your instructor will assign you a letter). All of the solutions should
have a concentration between 0.15 and 0.35-moles per liter. Your goal in part one is to determine the
molarity of your particular solution.
You will use potassium hydrogen phthalate (formula C8H5O4K, abbreviated KHP), a solid weak acid,
to determine the molarity of your solution of base. This process is called “standardizing” the solution of
base. To standardize your base, you must run a titration using a KNOWN acid (in this case, the KHP).
By measuring how many milliliters of your base solution is needed to neutralize the known acid, you will
be able to determine the TRUE MOLARITY of your base solution.
To standardize your base, follow the steps listed below:
0. Equip yourself with proper safety gear.
1. Weigh out precisely 5.00 millimoles of solid KHP (molar mass = 204 g/mol) into a beaker. (Note:
you should know what “milli” means, as in milliliters or milligrams... 5.00 millimoles = _____ moles)
2. Add about 50 mL of distilled water to the KHP and stir with a magic bean until all the solid is
dissolved (note that the exact quantity of water is unimportant, but it IS important that you completely
dissolve the KHP).
3. Add 1 or 2 drops of colorless phenolphthalein to your solution of KHP.
4. Obtain 55 mL of your assigned NaOH solution in a graduated cylinder. Use a funnel to pour the
NaOH into your buret.
5. Open the valve to let a few mL of base run out the bottom of the buret into a WASTE vessel
(beaker or test tube). This should remove any air space in the tip of your buret.
6. Record the starting level of the NaOH in the buret—try to read to the TENTH of a mL (i.e. started
at 5.7 mL)
7. Place the beaker with the KHP (and magic bean) under the buret and begin stirring at a vigorous
rate.
8. Now your goal is to run a titration to determine HOW MANY mL of your base solution are needed
to neutralize the .00500 MOLES of KHP. You will know the KHP has been neutralized when the
phenolphthalein in the beaker turns PINK. If you do your titration well, you will be able to turn off the
buret at the EXACT moment when the beaker turns pink (light pink, not dark pink). In order to do this
properly, you will have to be going very slowly when you near the endpoint of the titration (i.e. think
“Drip Drip Drip").
9. Record the final level of NaOH in the buret and do subtraction to determine how many mL of
base were used during the titration to neutralize all the KHP.
Continued on next page!
234
Assignment #131: Good Titrations (CONTINUED)
10. The last step is to calculate the MOLARITY of your NaOH solution.
a) The key to titration is that ONE molecule of NaOH neutralizes ONE molecule of KHP. Since there
were .00500 moles of KHP in the beaker, you must have added _______ moles of NaOH to
neutralize all the KHP. Note: .00500 moles has 3 significant digits!
b) To calculate the MOLARITY, you need to set up a PROPORTION using the mL from step #9 and
the moles from step #10a to determine how many MOLES of NaOH there are PER LITER (1000 mL) of
your solution. Note: express your answer to three significant digits.
11. Write your value for the MOLARITY of solution X, Y or Z on the board. As more data appears,
consider whether your results appear to be valid.
12. You may dispose of your pink solution by pouring it down the drain.
13. If you have at least 40 minutes left in the period, repeat your titration experiment starting with a
fresh batch of KHP and the same unknown base (X, Y, or Z) you used in the first titration. If your
results are reproducible, you can proceed with a high degree of confidence. If the results are not
reproducible, you have a BIG problem. Note: if you are pressed for time, you may skip this
reproducibility step.
Part Two: Determining the molar mass of an unknown acid
Your next task will be to do a titration of an UNKNOWN acid. The goal of your titration is to
identify the unknown acid as one of the molecules shown on the next page. You should be able
to identify the unknown acid by determining its MOLAR MASS.
You will run the titration by weighing out a known quantity of an unknown acid and placing it
in a beaker. You will then use solution X, Y, or Z (you know the molarity now!) to run a
titration on the unknown acid. Your titration will tell you how many MOLES of unknown acid
you put into your beaker. By knowing the number of GRAMS and the number of MOLES placed in
your beaker, you can determine the MOLAR MASS of the unknown acid.
To do the titration experiment with the unknown acid, CHOOSE ONE of the unknown acids
listed below:
Acid C (standard difficulty level): weigh out between 0.95 and 1.15 grams (record mass
precisely). Dissolve the solid acid in a suitable amount of distilled water and then titrate the
acid using your standardized solution of NaOH. Remember to put some PHENOLPHTHALEIN
into your titration beaker to act as an indicator.
Acid G (challenge level): weigh out between 0.80 and 1.05 grams (record mass precisely).
1. Acid G is not very soluble in water. However, it will become more soluble as you titrate
with base. You should add about 50 mL of distilled water to the sample of Acid G you
have weighed out. When you stir this, you should expect an opaque suspension due to the
limited solubility of Acid G.
2. Instead of phenolphthalein, use 5-10 drops of bromothymol blue as your indicator.
When you add the bromothymol blue to the suspension of Acid G, the mixture should look
like lemonade.
3. The endpoint of this titration will be difficult to judge. You are trying to get to the
point where the indicator is BLUE (not green).
4. Due to the low solubility of Acid G, you may titrate to a blue endpoint, but then see the
indicator slowly change color back to a green or yellow color. You’ll have to figure out
what to do if this occurs.
235
Assignment #131: Good Titrations (CONTINUED)
Calculation steps for Part Two: (write all calculations on your NOTE PAGE!)
The titration can tell you how many moles of acid were dissolved in the titration beaker. If
your sample of unknown acid contains MANY MOLES, it will require MANY mL of base to
neutralize it. On the other hand, if the sample of unknown acid contains only a FEW MOLES, it
will require only a FEW mL of base to neutralize it.
a) Record the number of milliliters of NaOH needed to neutralize the unknown acid in your
titration experiment.
b) From part one, you know the MOLARITY value of your NaOH solution. Since you
know how many moles of base are in a liter, you should be able to determine the moles of
base you used in the actual titration.
Use algebra to solve for this
On the left side of the
proportion, you know how
many moles of NaOH are in
each liter of solution!
unknown number of moles of NaOH!
On the right side of the proportion,
you know how many mL of NaOH
solution you used in the titration!
c) The acid molecules you are using are monoprotic, which means that each acid molecule
has a single proton to donate to a base. This means that the chemical equation for the
reaction is very simple, as shown below:
HX + OH-  X- + H2O
Since the mole ratio between acid and base in this equation is 1:1, the moles of base used in
the titration must be EQUAL to the moles of acid originally present.
d) At this point, you should know the number of moles of acid originally present (because
you titrated it) and the grams of acid originally present (because you weighed it). Use these
two values to set up a PROPORTION to calculate the grams in one mole of the unknown acid
(i.e. its molar mass!). Note: use 3 significant digits in your calculations!
e) Try to use your calculated molar mass value to identify your unknown acid as one of
those shown on the next page. If there is some doubt about which acid is your unknown,
express that doubt on your note page and suggest possible reasons why your experiment was
not definitive.
236
Assignment #131: Good Titrations (CONTINUED)
Possible identities of mystery acids:
237
Assignment #132: I Can Do Logarithms!
THIS PAGE IS TO BE DONE WITHOUT A CALCULATOR!!!!!!!!!!!
Use the skills taught in the “Practical Logarithms” Lesson (found on Haiku) to
solve the following problems:
Given:
Log(2) = 0.3
Log(3) = 0.5
1. Find the log of 8 (Hint: 8 = 2 x 2 x 2)
For all questions, show your method of calculation instead of just writing the answer!!!!!
2. Find the log of 18
3. Find the log of 2 x 103
4. Find the log of 3 x 10-5
5. Find the log of 4 x 105
6. Find the log of 5 x 10-12
7. Find the log of 60 x 1015
238
Assignment #133: Powers of Ten
Use a calculator only when expressly instructed to do so (i.e. on question 7)!
Exponents and Logarithms:
1.
103 = 1,000
104 = ______
2. Express the number 1,000,000 as a power of 10.
3. Knowing that 103 is 1,000 and 104 is 10,000, choose the number below that is a reasonable
3.7
ESTIMATE for the value of 10
50
4.
500
5000
50000
The logarithm of 105 = 5
The logarithm of 106 = 6
The logarithm of 107 = ____
5. What is the logarithm of 1,000?
6. Which of the following is a reasonable ESTIMATE for the logarithm of 2 x 106 (2,000,000)?
(circle one of the following and explain briefly):
5.2
5.8
6.3
7.4
9.8
-6
7. Use your calculator to take the LOGARITHM of 3 x 10 . On your calculator, this should look
like:
log(3E-6) =
8. Based on your answer to #7, is the EQUATION shown below TRUE or FALSE? Explain.
3 10
10
.
The logarithmic nature of the pH scale
-6
If the concentration of H+ ions in a solution is 10 M, the pH of the solution is 6. This example
illustrates the relationship between H+ molarity and pH value, which can be summarized by the two
equations shown here:
[H+] molarity = 10-pH
pH = -log[H+]
10. Suppose you have a solution with a pH of 8.2. Write a proper EQUATION in the space below showing
how the [H+] concentration in this solution could be calculated. Then use your calculator to evaluate this
exponential equation to express the [H+] molarity in scientific notation.
Continued on the next page!
239
Assignment #133: Powers of Ten (continued)
Use a calculator only when necessary. . .
KEY EQUATIONS FOR pH:
[H+] molarity = 10-pH
pH = -log[H+]
11. If the concentration of H+ ions in an aqueous solution is 1 x 10-4 molar, what is the pH of
this solution? Explain.
Your explanation:
H+ = 1 x10-4 mol/L
pH = _______
12. If the pH of a solution is 8, what is the concentration of H+ in the solution? Write your
answer both as a power of ten (exponential notation) and in longhand (with a bunch of zeroes).
Hint: concentration means molarity.
Your explanation:
H+ = _______ _____ OR
Label here!
13. Suppose you have 3 solutions, as described below:
X) [H+] = 3.1 x 10-5 M
Y) [H+] = 1.0 x 10-6 M
Z) [H+] = 7.2 x 10-9 M
a) Without making any calculations, explain which of these solutions would have the lowest
pH. Note: lowest pH means most acidic.
b) Which solution would have the HIGHEST PH? Explain without the use of any calculations.
c) Use a calculator to find the pH of solution Z. Show your calculation method in the space
below.
14. Suppose you have a solution labeled 0.065 M HCl. In this solution, all the HCl molecules
will be dissociated, so that the concentration of H+ ions = 0.065-molar. What will the pH of this
solution be? Note: show your method of calculation.
15. Suppose you have a solution labeled 2.5 x 10-4 M NaCl. What will the pH of this solution
be? Hint: think before you calculate!!!!
240
Assignment #134: Phun with pH
A pH Chart
The meaning of pH
I think you have all heard of the idea of pH. pH can be used to determine whether a solution is acidic,
basic, or neutral. The "H" in pH stands for H+ ions, which should make sense to you since H+ IONS
+
+
COME FROM ACIDS. Only DISSOCIATED H ions (H3O ions) “count” when calculating pH values.
 MANY H3O+ ions in a solution = an ACIDIC solution (pH less than 7).


FEW H3O+ ions = a BASIC solution (pH greater than 7).
A NEUTRAL solution = equal concentrations of H+ and OH- ions (pH = 7).
The mathematical relationship of H+ concentrations to pH values is shown in the table below:
+
+
[H3O ] molarity
[H3O ] molarity
(normal #'s)
(exponential notation)
pH value
[OH-] molarity
(exponential notation)
.1 M
10-1 M
1
10-13 M
.01 M
10-2 M
2
10-12 M
.001 M
10-3 M
3
10-11 M
.0001 M
10-4 M
4
10-10 M
.00001 M
10-5 M
5
10-9 M
.000001 M
10-6 M
6
10-8 M
.0000001 M
10-7 M
7
10-7 M
etc.
10-8 M
8
10-6 M
10-9 M
9
10-5 M
10-10 M
10
10-4 M
10-11 M
11
10-3 M
10-12 M
12
10-2 M
10-13 M
13
10-1 M
10-14 M
14
1. Which has a higher concentration of H3O+ ions: a solution with pH 6 or a solution with pH 8?
2. When the pH value changes by one unit (e.g. pH 4 to pH 3), how does the H+ concentration change?
Express your answer in the following way:
“The H+ concentration changes by a multiplicative factor of _____”
3. The equation below expresses the mathematical relationship between column #2 (H+ molarity
expressed in scientific notation) and column #3 (pH value). Try to complete the second equation so that it
also is mathematically valid. Hint: the equation involves a logarithm. . .
10
pH =
-4
4. Use an equation from #3 to calculate the pH of a solution containing 2 x 10 M dissociated H+ ions.
241
Assignment #135: Introduction to percent dissociation
(strong vs. weak acids)
To be completed with Video Instruction!
Use your calculator only when instructed to do so!
1. Percent dissociation and pH for strong acid solutions:
a) Suppose you have ______ molecules of a strong acid in aqueous solution. If ______ %
of these molecules dissociate, how many H+ ions will be present in the solution?
b) The percent dissociation described in the problem above is redundant, because a strong acid always
dissociates c
_ m p _ _ t _ _ _ , meaning _____ % dissociation.
c) Suppose you have a solution labeled ___-molar HCl. What is the molarity of H+ ions in this
solution?
d) Use a calculator to find the pH of the solution described in 1c. Show your calculation method in the
space below:
2. Percent dissociation and pH for weak acid solutions:
a) Suppose you have _______ molecules of a weak acid in an aqueous solution. If ____ % of these
weak acid molecules are dissociated at equilibrium, how many H+ ions will be present in the solution at
equilibrium?
b) Suppose you have a _____-molar solution of a weak acid in an aqueous solution. If ____ % of the
weak acid molecules are dissociated at equilibrium, what will the molarity of H+ ions be at equilibrium?
c) Use a calculator to find the pH of the solution described in 2b. Show your calculation method in the
space below:
d) Suppose you have a _____-molar solution of aqueous lactic acid. If the H+ molarity at equilibrium is
_____-molar, what percent of the lactic acid molecules are dissociated at equilibrium?
%
e) Using a calculator, determine the pH of the solution described in 2d. Explain your method in the
space below:
242
Assignment #136: Playing the Percentages
Practice problems involving percent dissociation and pH
Use a calculator only when necessary. . .
1. Suppose you have a solution labeled 0.1-MOLAR HF. If 10% of the hydrofluoric acid
molecules are dissociated at equilibrium, what will the pH be? Explain. Note: Do this problem
WITHOUT
a calculator!!!
[HF] = 0.1 M
[H+] molarity = 10 percent of 0.1 M = ___________
[H+] molarity (expressed in exponential form) = 10
- __
molar
pH (the pH is the exponent!) = _______
2. Suppose you have a 0.067 M solution of lactic acid. If 3% of the lactic acid molecules are
dissociated when equilibrium is established, what will the pH of this solution be? Please show
your calculation steps (similar in format to problem #1) and label every number! Express your
final pH value to one decimal place.
3. Suppose you have a solution labeled 0.040 M HCl. What will the pH of this solution be?
Explain. Hint: HCl is a STRONG ACID, and therefore has 100% dissociation! No calculator required?
4. Suppose you have a solution of a weak acid. The solution’s label says 0.5-molar
CH3COOH. Upon measuring the pH of this solution using a pH meter, you find that the
solution’s pH = 2.5.
a) Explain why the [H+] concentration in this solution must be less than 0.5 M.
Note: please provide a conceptual answer without calculations.
b) Use the equation below (and a calculator) to calculate the [H+] molarity in the solution.
10
c) Calculate the percent dissociation of the acetic acid molecules in this solution.
243
Assignment #137: How do you spell KaT?
Calculating pH using Ka expressions
Formic acid (HCOOH) is a weak acid with a Ka value of approximately 2 x 10-4.
1. Write out a chemical equation (with double arrows) for the dissociation of formic acid.
Formic acid
2. The Ka expression for this dissociation is shown below. Note: you don’t need to do anything here,
just accept the information provided.
Ka = 2 10
3. Suppose you have a solution which contains 2-molar formate ion (HCOO-) and 1-molar HCOOH.
a. Using the “empty” Ka expression shown below, plug in NUMBERS for the concentrations of HCOOand HCOOH. Then use algebra to solve for the [H+] concentration and clearly label this value (i.e.
show very clearly “[H+] = ____ M”).
Formate ion molarity goes in this bracket!
2 10
b. Find the pH of the solution by taking the LOGARITHM of the H+ concentration. Label this value
clearly.
4. Now, using steps similar to those employed in question #3, calculate the pH of a solution containing
1 M HCOO- and 3 M HCOOH. Compare your pH to what you calculated in question 3b and comment
on why the pH value of this solution is lower than that of the previous solution.
5. HOCl (hypochlorous acid) is a weak acid with a Ka of 3.5x10-8.
a) Write a chemical equation for the equilibrium established when HOCl dissociates in aqueous solution.
b) Use a Ka expression to calculate the pH of a solution containing 1 M HOCl and 1 M OCl-.
a
c) Calculate the pH of a solution that contains 1-molar HOCl WITHOUT any added OCl-.
Hint: Use the Ka expression shown above. In your algebra, if [H+] = X, then [OCl-] will ALSO equal X (for every
plus there is a minus). Your equation should therefore end up with an X2 function.
Continued on next page!
244
Chemistry Assignment #137: How do you Spell KaT (continued)
6. Consider the two beakers shown below. Each represents an aqueous solution containing
dissolved molecules and ions. Assume both beakers contain the same volume.
a) Fill in the sentences below to accurately describe the contents of each beaker:
Beaker A initially had 4 total acid molecules, of which ____% are dissociated.
Beaker B initially had ___ total acid molecules, of which ____% are dissociated.
b) Explain why the pH of Beaker B is lower than the pH of beaker A. Note: lower pH means a pH
closer to zero
c) Which beaker shows a situation that could be described using a Ka value? Explain.
7. Acetic acid is a WEAK acid, while nitric acid is a STRONG acid. Explain how a mole of acetic
acid would behave differently from a mole of HNO3 if both were dissolved in a liter of water.
Hint: Reversible? Equilibrium?? Percent dissociation??
8. Suppose you have a solution of acetic acid and a solution of nitric acid, each with a pH of 3.0.
a) The pH value measures the concentration of H+ ions in the solution. Which of the following
correctly expresses the H+ molarity in a solution with pH 3.0?
[H+] = 3.0-molar
[H+] = 1/3-molar
[H+] = 10-3 molar
b) Use the Kw equation shown below to calculate the molarity of [OH-] in a solution that has a pH
of 3.0.
Kw = [H+][OH-] = 10-14
c) Normally, a strong acid solution is highly conductive, while a weak acid solution is only slightly
conductive. However, the factor that truly determines conductivity is the CONCENTRATION OF IONS in
solution. Based on the concentration of ions, which solution would be more conductive: the acetic
acid solution (pH 3.0) or the nitric acid solution (pH 3.0)? Explain your reasoning.
245
Assignment #138: The Color of X (LAB)
Equilibrium is all around us. . .
Write-up instructions: Fill out this packet with neat, complete answers. Provide DETAILED
answers including chemical equations and molecular pictures wherever possible.
Procedure:
1. Locate the test tube at your station containing a sample of AMMONIUM CHLORIDE and the test tube
containing a sample of AMMONIUM CARBONATE.
a. Fill in the correct formula for ammonium carbonate. Hint: for every plus there is a minus!
Ammonium chloride = NH4Cl
Ammonium carbonate = ________________
2. Smell the contents of the tubes. One of the salts should have a sharp odor, while the other is odorless.
a. Which salt has a distinct odor?
b. The gas you smell is AMMONIA. Which of the descriptions given below best describes how
AMMONIUM ION can become AMMONIA GAS?
 An ammonium ion must gain a proton (H+) from an acid to become ammonia gas.
 An ammonium ion must lose a proton (H+) to a base to become ammonia gas.
3. The following questions are designed to guide you to an understanding of why one of your salts reeks
of ammonia while the other has no odor.
a. Sketch a molecular picture showing an ammonium ion next to a chloride ion (simulating what you
have in the ammonium chloride tube). Then discuss whether an ACID-BASE REACTION can occur between
the ammonium and chloride ions. Hint cards available.
b. Sketch a molecular picture showing an ammonium ion next to a carbonate ion (simulating what
you have in the ammonium carbonate tube). Then discuss whether an ACID-BASE REACTION can occur
between the ammonium and carbonate ions. Hint cards available.
c. Explain (with the use of sentences and chemical equations) why one of your tubes reeks while the
other tube is odorless.
246
Assignment #138: The Color of X (continued)
4. Test the gases inside each tube by folding a piece of pH paper and hanging it over the lip of the test
tube. You should see the pH paper change color in one of the tubes, but not in the other tube.
a. The pH paper is coated with some weak acid molecules (HX) that are ORANGE in color when their H’s
are attached. When these weak acid molecules lose H+, they change color. Complete the chemical
equation below and EXPLAIN THE CHEMISTRY that makes the color change.
HX
+
Orange color

+ X_________ color
b. If you leave the pH paper out on your bench top for a while, you should see that it returns to its
original orange color. This can only occur if X- is becoming HX again. Explain how/where the X- is
able to gain a proton to become HX. Note: hint cards are available.
5. Use the jar next to the scale to weigh out enough ammonium chloride to make 100 ml of a 0.10 M
solution. Then dissolve the salt in distilled water. Note: 0.10 M means 0.10 MOLES PER LITER!
a. Show calculations for the quantity of ammonium chloride you weighed out to make this solution.
b. Make a prediction for what you think the pH of this solution will be. Choose one of the answers listed
below and explain your reasoning. Note: discuss the ammonium ion separate from the chloride ion.
pH between 1 and 3 (highly acidic)
pH between 3 and 7 (slightly acidic)
pH almost exactly 7 (neutral)
pH between 7 and 9 (slightly basic)
6. Use a digital pH meter to test the pH of your ammonium chloride solution.
a. Record the pH value reported by the pH meter. Use this value to calculate the [H3O+] molarity (in
scientific notation) and write a chemical equation that demonstrates how these H3O+ ions are produced.
pH measurement = ________
Chemical equation that demonstrates the origin of H3O+
ions in the solution
Calculation for [H3O+] molarity:
b. Test the conductivity of your solution using the light bulb device (located in the fume hood). Then
explain which ions in the solution provide the level of conductivity you observe. Hint: your pH
measurement should show that H+ ions are in very low concentration.
247
Assignment #138: The Color of X (continued)
7. Your next step is to compare a solution of ammonium carbonate to the solution of ammonium
chloride you made in step #5.
a. Using the pictures shown below, predict whether the pH of the ammonium carbonate solution will be
LOWER, HIGHER, or the SAME as the pH of the ammonium chloride solution that you made in step #4.
Explain your reasoning.
Hint: In which beaker will the H+ remain free for a longer time?
I predict the pH of the ammonium carbonate solution will be ___________ than the pH of
the ammonium chloride solution because:
b. Weigh out enough ammonium carbonate to make 100 ml of a 0.10-molar solution. Then add water
and dissolve the salt. In the space below, show your calculations for the mass of (NH4)2CO3 used.
8. Measure the pH of this solution using a digital pH meter. Based on this pH value, calculate the
molarity of [H+] in the beaker. Note: if you don’t have time to complete this step, assume that the pH of
the solution is 8.7.
9. Write the names and formulas of at least 6 chemical species (molecules or ions) that are swimming
around in the beaker of ammonium carbonate. Hint: consider all the possible EQUILIBRIA that exist
within the beaker. Some of these species are resonance-stabilized—sketch these structures accordingly.
10. Which of the following is the best description of a solution of ammonium carbonate? Explain.
a) The solution is a base
b) The solution is an acid
c) The solution is a complex mixture of acids and bases
248
Assignment #139: Review of pH Calculations
1. If the pH of a solution is 5.3, calculate the H+ concentration (molarity) in the solution.
Express your answer in scientific notation with proper chemical labels!
2. Find the pH of a 0.038-molar solution of HNO3. Hint: HNO3 is a STRONG ACID!
3.
HF  H+ + FHF is a weak acid that establishes an equilibrium in aqueous solution. The Ka value for HF
is 7.2 x 10-4.
Calculate the pH of a solution containing 2 M HF and 0.5 M F-.
a
4. If a 0.25-M solution of acetic acid has a pH of 2.4, what percent of the acid is dissociated?
249
Assignment #140: The Equilibrium Island
An introduction to the concept of a dynamic equilibrium
The goal for the day is for you to explore how an equilibrium can be established in a population. In
this exercise, you will be dealing with populations of humans, but all the ideas can be carried forward to
deal with populations of molecules.
Imagine that there is a small semi-tropical island out in the Pacific, and onto this island, we put one
thousand single men and one thousand single women (assume all are heterosexual). There is a single
office on the island that performs marriages (no blood test required), and a single office that conducts nofault divorces. Both offices are capable of handling up to 600 couples per day.
We will assume that the people on this island are fairly impulsive, and make major decisions (such as
whether to get married or divorced) on a whim. Basically, if a single gal meets a single guy who seems
compatible, they'll run off to the marriage office in a jif, whereas a married couple that has a couple of
arguments will be headed for the divorce office. There is a law that you can't get married and divorced on
the same day (it's known as the "One Day Cool Off" law). Both offices are closed between 11:00 pm and
7:00 am.
QUESTIONS:
1. Let us assume that the Islanders are pretty friendly people, so that 60% of the single people will get
married on a given day. Another way of looking at it is that if you are single, you have a 60% chance of
getting married that day.
a) How many weddings will take place on the first day?
b) How many weddings will take place on the second day? Hint: it’s not the same answer as (a)
because there are fewer single people out there to get married! Only 60% of the SINGLE people will get
married. . .
2. On the first day, there will be no divorces, but starting on the second day, couples will be breaking up.
Let's assume that 20% of the couples will break up on a given day. That is, if you started the day
married, you will have a 20% chance of ending the day single.
a) How many DIVORCES will there be on the second day? Hint: this number will be 20% of the
COUPLES. That means you have to calculate 20% of 600. . .
b) At the end of the second day, there will be 720 COUPLES on the island. Explain (with the use of
mathematics) why there will be 720 couples at the end of the second day. Note: some of these couples
will be "brand new" while others will be "leftovers" from day one.
Leftover couples from Day 1 weddings +
New couples from Day 2 weddings =
Total couples at the
end of Day 2
Turn to the next page!
250
Assignment #140: The Equilibrium Island (continued)
3. Complete the table shown below. Starting on day 4, you will get decimal values. You should
use these decimal values up until the last line (“Equilibrium”) in which you should round to
the nearest whole number.
Note: some numbers have been placed in the table by your instructor. Don’t try to change these
numbers—instead use them as check points to assist you in getting the right answers for the other blanks
in the table.
Day
Number of
COUPLES
at the start
of the day
Number of
SINGLE
women (or
single
men) at
the start of
the day
1
0
1000
2
600
400
Number of
additional
couples
formed on
that day
(i.e. number
of weddings)
60% OF
SINGLES
GET
MARRIED
Number of
couples that
break up
(i.e. number
of divorces)
20% OF
COUPLES
BREAK UP
Number of
couples at
the end of the
day
Arrows
S
C
S
C
S
C
S
C
5
S
C
6
S
C
7
S
C
S
C
3
4
use decimal
values here!
0
744
Equilibrium
(round the
value for # of
couples to a
WHOLE no.)
4. As you have seen in the table, after 7 days, the number of couples on the island will reach a STEADY
(for example 750 couples today, 750 couples tomorrow, 750 couples the day after,
etc.). Explain why the number of couples reaches a "plateau" that remains stable day after day. Note:
DAY-TO-DAY VALUE
this is asking you to define how an EQUILIBRIUM works.
251
Assignment #140: The Equilibrium Island (continued)
Number of Couples
5. Using your numbers from the data table, complete the line graph shown below.
1000
900
800
700
600
500
400
300
200
100
0
Compare this
graph to the one
you drew on
Assignment
#127!
0
2
4
6
8
10
Days
6. After the plateau has been reached, what is the RATIO of married men to single men on the island?
Can you relate this ratio to the 60% marriage rate and 20% divorce rate?
Assume that changes in questions 7-10 are done INDEPENDENTLY (NOT SEQUENTIALLY).
7. Suppose a ship carrying 500 lonely men runs aground on the island. After a couple weeks, how will
the following numbers have changed compared to before the shipwreck? (your answer may simply say
increase or decrease – don’t try to calculate mathematically, just rely on intuition!)
a) the number of single men on the island
b) the number of single women on the island
c) the time it takes a single woman to find a mate
d) the number of couples on the island
8. In summer, the island gets very hot, and so do people's tempers. This makes people less likely to be
in the married state. Assuming that the marriage rate drops to 30%, while the divorce rate climbs to
50%, show how the number of couples will adjust to form a new equilibrium during the summer months.
Note: In addition to using the table below, try to find a way to determine the equilibrium state algebraically.
Couples
1
750
2
3
4
5
Equilibrium
Algebraic solution:
Singles
250
Weddings
Divorces
End of day
(30% of singles)
(50% breakup)
(# of couples)
252
Assignment #140: The Equilibrium Island (continued)
9. Suppose the marriage office staff goes on strike, so that the office can only handle 5 weddings per day,
but the divorce office remains open as usual. How many COUPLES will be present on the island after a
few weeks when a NEW equilibrium is established? Assume that the 20% probability of divorce remains valid.
Recall that equilibrium will be reached when the number of people getting married on a given day equals the
number of people getting divorced on that day.
Day
1
Couples
Singles
750
Weddings
Divorces
End of day
(5 per day)
(20% of
couples split
up)
(# of couples)
250
2
3
4
5
Equilibrium
(extrapolation
10. Lastly, suppose that a band of Brad Pitt look-alike aliens swoop down onto the island every night for
a week, stealing away nearly all the SINGLE women available each night. What will happen to the
number of couples on the island during this week of terror? (Assume that once carried off by the Brad Pitt
look-alikes, the women never come back)
253
Assignment #141a Demonstrations of Equilibrium Shifts
An Introduction to Le Chatelier’s Principle
If a chemical system in equilibrium is subjected to altered
conditions, a new equilibrium will be established. The shift in
equilibrium opposes the introduced perturbations (alterations)
and acts to moderate these changes.
Example 1: Graduated Cylinders and Colored Water
Experiment #1:
Perturbation =
Equilibrium shift =
Experiment #2:
Perturbation =
Equilibrium shift =
Example 2: NO2 Demonstration tubes


Two sealed tubes, each containing NO2 (an orange/brown gas) + N2O4 (an
invisible gas)
Three temperatures: ice cold, room temp, hot
Results when tube containing equilibrium mixture of NO2 and N2O4 is submerged in hot water:
Gas sample becomes ____________, indicating _______ NO2 (g)
Results observed when tube with NO2 and N2O4 is submerged in cold water:
Gas sample becomes ____________, indicating _______ NO2 (g)
Chemical explanation involving a shifting equilibrium!!!!!!
1. Original
2. HOT
3. cold
N2O4 (g)
NO2 (g) + NO2 (g)
4 things in the
equilibrium:
 NO2
 NO2
 N2O4
 Heat
254
Assignment #141b: Introduction to Le Chatelier’s Principle
Thinking about how an equilibrium will shift as conditions are altered.
The following chemical equation describes a simple equilibrium system.
NO2 (g) + NO2 (g)  N2O4 (g)
Le Chatelier’s Principle
 If a chemical equilibrium is disturbed by changing the conditions, the
equilibrium will shift to counteract the change.
Suppose you have an equilibrium mixture of NO2 molecules and N2O4 molecules in a
compressible syringe as described below:
= NO2 molecule (brown)
= N2O4 molecule (colorless)
6 NO2 and 2 N2O4 molecules
If the syringe is compressed, the pressure in the syringe will rise (according to Boyle’s Law).
However, following this initial rise in pressure, a shift in the equilibrium system occurs that
reduces the pressure, as illustrated by the graph shown below.
B
pressure
C
Equilibrium shift
A
time
A chemical equilibrium shift is usually described as “to the right” or “to the left”. In the
equilibrium shown below, “to the right” means formation of additional N2O4, while “to the left”
means formation of additional NO2.
NO2 (g) + NO2 (g)  N2O4 (g)
Left side of equation
Right side of equation
1. In this chemical equilibrium, which of the following shifts decreases the pressure in the
syringe? Explain your reasoning.


A shift to the right, turning two NO2 molecules into one N2O4 molecule
A shift to the left, turning one N2O4 molecule into two NO2 molecules
2. Complete the sentence below to provide an explanation for how Le Chatelier’s Principle
could be used to predict this shift.
Initially, the system was at equilibrium (point __ on the graph). Then the
system was stressed by increasing the pressure of the sample. In order to
____________ this stress, the number of gas molecules _____________
by shifting the equilibrium to the ________ side of the equation.
255
Assignment #142: Applying Le Chatelier’s Principle
Summary of Le Chatelier’s Principle:
Change in Conditions (i.e. the “stress”)
Equilibrium Shift (relief of stress)
1. Increase in gas pressure
Equation shifts towards side with fewer gas
molecules (reduces pressure)
Equation shifts in the endothermic direction
(absorbs some heat)
Equation shifts towards the side with the “lost”
molecule (restoring the missing molecules)
Equation shifts to the side away from the
added molecule (reducing the excess quantity)
2. Increase in temperature
3. A molecule is removed from the equilibrium
4. The concentration of a molecule is
increased (i.e. you add some extra)
Practice questions:
1.
N2 (g) + 3 H2 (g)  2 NH3 (g)
The equilibrium shown above will shift to the right if the pressure is increased (i.e. if the system
is compressed in a syringe). Draw this equation with appropriate sized arrows demonstrating
this equilibrium shift and provide an explanation for why the arrows will shift.
Equilibrium arrows for
reaction mixture
compressed in a syringe:
2.
CoCl242(aq)
NO
2 (g)  CoCl2 (aq) + 2 Cl (aq)
H = -16 kJ/mole
When an equilibrium mixture of pink CoCl2 is heated in a solution containing excess Cl- ions,
the mixture becomes a dark blue color (indicating an increased concentration of CoCl42- in the
solution). This evidence indicates that the equilibrium:
a) shifts to the right when heated
Explanation:
By shifting in the
b) shifts to the left when heated
____ thermic direction, the
CoCl4
c) doesn’t shift in either direction
http://youtu.be/BGfYf8OQzuk
(demonstration of color change)
PE
CoCl2
equilibrium relieves the
stress of the added heat
3.
HF (aq)  H+ (aq) + F- (aq)
An equilibrium solution of 0.1 Molar HF has a pH of 2. If calcium ions (Ca2+) are added to
this equilibrium mixture, CaF2 precipitates, and the pH of the solution drops to pH 1. This
means that the addition of calcium ions causes the equilibrium to:
a) shift to the right (creating additional H+ ions)
b) shift to the left (creating fewer H+ ions)
Explanation: Hint: precipitation removes ions from solution.
256
Assignment #142: Applying Le Chatelier’s Principle (continued)
Thinking about ways to shift an equilibrium
4. Recently, in the Advanced Chemical Research class at LBHS, students were attempting to make the
organic molecule known as an ester according to the chemical equation shown below (in chemistry,
“organic” means a carbon-based molecule). This reaction is somewhat miraculous in that it takes butyric
acid that smells like vomit and turns it into an ester that smells like pineapple.
Bad smell
(vomit)
Good smell (pineapple)
A typical reaction starts with 0.050 moles of vomit-like butyric acid.
a) The double arrow in the middle of this equation shows that the reaction is reversible, and will
establish an equilibrium. What would the reaction mixture smell like if equilibrium is
established with 60% of the molecules on the right side of the equation and 40% on the left side?
b) The mole ratio from the balanced equation requires only 0.050 moles of ethanol to react with
0.050 moles of butyric acid. However, the ACR students making this ester used extra ethanol (0.65
moles) in their reaction mixture. Explain why adding a large EXCESS of ethanol would shift the
equilibrium in a favorable direction.
c) The students making the ester also added little pellets called molecular sieves to the reaction mixture.
The molecular sieves ABSORBED THE WATER that formed during the reaction. Explain why absorbing
water would be helpful in shifting the equilibrium in a favorable direction.
257
Assignment #143: How Buff is your Buffer?
A buffer is a solution that resists changes in pH. A good buffer is very strong. . .
HF (hydrofluoric acid) is a weak acid. Suppose you wish to make a BUFFER using HF and its
conjugate base, F-.
1. Write out a CHEMICAL EQUATION to show the reversible dissociation of HF.
2. Complete the Ka EXPRESSION (with chemical species in the brackets) that describes this equilibrium.
a 3. The Ka value for HF is 7.2 x 10-4. Substitute this value into the Ka equation you wrote in part 2 and
calculate the [H+] molarity of a buffer solution that contains 1-molar HF and 1-molar F-. Then
calculate the pH of this solution. Note: LABEL all important numbers!
Calculation method:
[H+] = ________ M
pH = ________
4. Now calculate the pH of a buffer solution containing 1 M HF and 3 M F-.
5. Which of the buffer solutions described above in #3 and #4 has a LOWER equilibrium
concentration of [H+] ions? How can you EXPLAIN this decrease in [H+] concentration?
Note: the picture shown in the margin may help you visualize the chemistry.
6. Suppose some STRONG ACID (e.g. HNO3) is added to the buffer solution depicted here
a) Complete the chemical equation for the reaction that would occur when the strong acid HNO3 is
added to the buffer. Hint: F- is a base!!!!
HNO3 (aq) +
F- (aq) 
b) Describe the shifts in equilibrium concentrations that will occur after some HNO3 is added to the
buffer. For each of the species listed below, state whether the equilibrium concentration will
increase, decrease, or remain the same. Explain briefly.
[F-] will ___________________ because some of the F- will bond with the added H+
[HF] will ___________________ because
[H+] will ____________________ because
c) Typically, a buffer will hold the pH fairly constant until a certain point, at which time the pH
will suddenly change drastically. In the case of adding nitric acid to the buffer described in (d), what
chemical event will result in a sudden drop in pH? Hint: something in the buffer becomes depleted. What
would that something be?
258
Assignment #144: Building a buffer
(LAB)
Overview: Today you will build a buffer! You will be given a specific pH value to shoot for. You will
then choose the proper chemicals and measure out appropriate quantities so that your buffer’s pH will
match the desired value. As a final step, you will put in an indicator and test the strength of your buffer
by TITRATING with a strong acid or base.
Each student needs: Calculator, Periodic Table, Scratch Paper,
Blue Equation Sheet, Safety Gear
Materials: The following chemicals will be provided on the buffet table.
Weak Acid: glacial acetic acid (pure CH3COOH) Ka = 2 x 10-5
Conjugate Base: solid sodium acetate NaCH3COO3 H2O Note: this substance is a
crystalline solid—the waters in the formula are part of the crystal and add to the molar mass.
Weak Acid: Tris hydrochloride (C4H12NO3Cl) Ka = 6.1 x 10-9
Conjugate Base: Tris base (C4H11NO3)
Weak Acid: sodium bicarbonate (NaHCO3): Ka = 1.4 x 10-10
Conjugate Base: sodium carbonate (Na2CO3H2O) Note: this substance is a crystalline
solid—the water in the formula is part of the crystal and adds to the molar mass.
Procedures: Each lab group will start by signing up on the board for a specific pH.
These are the pH values that will be available:
4.1
8.5
4.5
9.6
5.2
10.2
7.6
10.4
7.9
Try to perform each of the following steps thoughtfully. There will be many calculations to make—the
best way to avoid mistakes is to try to reach CONSENSUS within your group.
1. You will need to choose the proper weak acid/conjugate base system to use. The best way to do this is
to take the logarithm of the three Ka values shown above. This is called finding the pKa of each acid.
The pKa that is closest to your buffer’s desired pH is the correct acid to use.
2. After you have chosen the correct acid/conjugate base system, you will need to find the proper ratio of
acid to base needed to give you the specific pH you desire. To do this, you should:
a) Set up a general Ka expression (using the symbols [H+], [A-], and [HA]) and set it equal to the
Ka value of the weak acid you have chosen in step #1.
b) In the Ka expression, plug in values of “1” for the molarity of your weak acid and the molarity of
your conjugate base . Then solve for [H+] and find the pH.
c) The number you have calculated in ii. is the pH that you will get IF you use 1 M weak acid and 1 M
conjugate base. You will see that the pH you have calculated is not quite the pH you desire. You will
now need to ALTER THE CONCENTRATIONS of weak acid and/or conjugate base until you get the pH
that you want. This is best done in a guess and check fashion (unless you have excellent math skills
and can find a more elegant method). For example, what pH will you get if you try 1 M acid and
10 M base? Is this getting closer or farther from your desired pH? Keep guessing (intelligently) until
you find a ratio of acid to base that gives you a pH very close to your desired buffer pH.
259
Assignment #144: Building a buffer (continued)
3. Your work in step 2 should give you a proper MOLE RATIO needed to make your buffer solution. Now
you need to convert moles to grams so that you can weigh out the right amounts needed to make your
buffer. Note: don’t actually go and get anything out of the bottles yet. If you are thoughtful, you should realize
that the quantities you have calculated are huge!
4. You will need to scale down the grams you have calculated in step 3 so that they become reasonable
values. Ideally, you should try to get your masses to be between 0.6 and 3.0 grams of acid and 0.60 and
3.0 grams of conjugate base. When scaling down, it is critical that you keep acid and conjugate base in
the SAME RATIO that you calculated in step 3. (for example, if your ratio in #3 was 100 grams acid to 40
grams base, this is the same RATIO as 10 grams acid to 4 grams base)
5. If your values for both acid and conjugate base are reasonable masses, you may weigh out the
appropriate quantities into a beaker. To complete your buffer solution, add about 100 mL distilled water
and stir to dissolve all contents. Note: if you are using acetic acid, you will need to weigh out LIQUID glacial
acetic acid in the fume hood. Try to weigh your acetic acid directly into the beaker you will be using to make the
buffer and then add water immediately to dilute the pungent smell.
6. Once you have made your buffer solution, test it with a pH meter. There will be two pH meters set
up on the buffet table, one for high pH values and one for low pH values. Read the labels on the buffet
table so that you use the proper meter. If your pH is within 0.1 pH units of the desired pH, consider
yourself successful. If not, you will have to try again (look for mistakes in your original calculations or in
your hands-on measurements).
7. After confirming that you have made a proper buffer solution, you will challenge your buffer with acid
and base to determine how strong a buffer you have made. You will do this by performing a titration
using strong acid or base. A data collection table is provided on the next page.
a) Set up a buret at your lab station. If you made a buffer that is higher than pH 8.1, you should
titrate with acid (0.5 or 1-molar HCl—ask your instructor for guidance). If you made a buffer that is
lower than pH 8.1, you should titrate with base (0.5 or 1-molar NaOH—ask your instructor).
b) Fill your buret with the appropriate solution (as described above).
c) Place your buffer in a beaker below the buret. Add some universal indicator to the buffer while
mixing with a stir bar (magic bean) to make a vibrant color in the beaker.
d) You will now add 5-mL portions of acid or base to your buffer while stirring. After each 5 mL
portion of acid or base, record the color of the indicator and the pH of the solution using a digital pH
meter (try to not take up too much time using the meter—stir for just a few seconds to get a reading).
e) Continue adding 5 mL portions of acid or base until you see a dramatic pH change (i.e. at least 2
pH units away from your original pH value. Ponder what your titration results tell you about the
strength of your buffer.
f) Make a graph of pH vs. mL of acid/base added. Label your axes appropriately and be sure your
graph has a consistent scale. You may use a computer program or graph paper to help you make a
quality graph. Note: your graph should be a line graph, not a bar graph. See sample on next page.
Write-up: Make a CONCISE write-up that covers the following topics WITHOUT COPYING words from
the instruction page. Consider using pictures where appropriate.
 introduce the lab
 describe the decisions you made (with reasoning)
 show important calculation methods
 describe the results you obtained in your experiments
 describe the methods you used in your titration experiment
 include a nice graph of pH vs. mL of acid/base added in the titration experiment
 discuss the significance of your titration experiment (i.e. did you make a good buffer?)
260
Assignment #144: Building a buffer (continued)
Table and Graph of Titration Data
Use this page to collect titration data. This note page will not be turned in, but you should
include this data in your write-up.
Solution placed in buret = _____________________
Milliliters of Solution added
from Buret
Color of Universal Indicator
pH Reading
(from the meter!)
0 mL
5 mL
Your graph should be formatted something like the example shown below:
If using Excel to make your graph, make a SCATTER PLOT, NOT A LINE GRAPH. INSTRUCTIONAL
VIDEO POSTED ON HAIKU!
8
7
6
5
pH 4
3
2
1
0
0
5
10
15
20
25
milliliters of __‐Molar KOH added
30
35
261
Assignment #145: Acid-Base summary
1. STRUCTURES OF ACIDS AND BASES
Draw structures with correct GEOMETRY for the following and classify each as an ACID, a BASE, or both
acid and base. Include formal charges where appropriate. Hint: Review assignments #121 and #122 if
you’ve forgotten how to do this.
a) CH3COOH
b) HCO3-
c) NH4+
d) CO3-2
2. ACID-BASE PROPERTIES
Many negative ions are bases, capable of grabbing H+ ions, but some negative ions are not interested in
grabbing H+ ions. Name the LEAST GRABBY negative ion and the MOST GRABBY negative ion that you can
think of. Review Assignments #125 & 126.
3. CHEMICAL EQUATIONS FOR ACID-BASE REACTIONS
Write a chemical equation that describes what happens when the following chemicals are mixed. If there
is an equilibrium established, show the double arrows. Review Assignments #123,124,128,129
a) HNO3 dissolved in water
Note: you don’t need to write H2O in this equation, as long as you use (aq) appropriately
b) CH3COOH + OH-
c) NH3 + HF
4. DYNAMIC EQUILIBRIUM Review assignment #129,130,138,140
When you add sodium chloride to water, it dissolves. However, if you add a massive amount of sodium
chloride, not all of it dissolves. Instead, the solution becomes saturated, and some of the solid salt will
remain on the bottom of the beaker. Why does the salt stop dissolving at this point? Hint: it DOESN’T stop
dissolving—there is just something “undoing” the dissolving process, creating an EQUILIBRIUM.
Continued on the next page!
262
Assignment #145: Acid-Base summary (continued)
5. pH MATHEMATICS
a) How do you find the pH of a solution of STRONG ACID? Provide a sample calculation to support
your answer. Review assignments #133,134.
b) How do you calculate the pH of a weak acid solution when you are given the percent dissociation?
Prove your skills by calculating the pH of a 0.44 M solution of a weak acid that has ____ % dissociation
(you fill in the blank). Review assignment #135,136.
c) How do you calculate the pH of a buffer solution? Support your answer by calculating the pH of a
solution that contains 0.1 M CH3COOH and 0.3 M NaCH3COO. Hint: the Na+ is a spectator. Review
assignments #137,143,144
6. TITRATION
Suppose a 10-mL sample of vinegar (containing a drop of phenolphthalein) is titrated with a 0.215-molar
solution of NaOH. If the titration requires 45.3 mL of the NaOH to turn the phenolphthalein pink, how
many MOLES of acid were originally present in the 10-mL of vinegar? Review assignment #131.
7. CONDUCTIVITY
Give examples of solutions that are highly conductive, slightly conductive, and non-conductive. Note: try
to think of SEVERAL possibilities for each category. On your test you’ll have to explain what makes a solution
conductive or not. . . Review assignments #130, 138.
8. SHIFTING EQUILIBRIA
If you have pure H2O in a beaker, it undergoes the equilibrium reaction shown below:
H2O  H+ + OH+
In pure water, [H ] concentration = [OH-] concentration.
If some ammonia (NH3) is added to a solution of pure water, predict what will happen to the
concentrations of [H+] and [OH-]. Explain your reasoning. Note: you should see this as a “Brad Pitt”
example of shifting the equilibrium. Review assignments #140-142.
[H+] will:
a) go up
b) go down
c) remain the same
[OH-] will:
a) go up
b) go down
c) remain the same
263
Assignment #146: Less than Zero (LAB)
Overview: Your objective in this lab is to cool an aqueous solution to below zero Celsius. All groups
will start by running a scripted reaction as a “trial run”. Each group will then make subsequent runs
utilizing novel ideas for improvement. It is in these "improved" runs that you may be able to lower the
temperature to less than zero. There will be no write-up for this lab, but each individual will be expected
to turn in an official note page (provided on the third page of this lab) at the end of the hour.
Part I (scripted reaction):
1. Boot up a computer. Pre-weigh a 250 mL beaker. Mass of beaker = _________ grams
2. Obtain a computer-interfaced thermometer (including USB link) and plug it into the computer. When
prompted, choose “Launch Data Studio”. You should see a graph appear that has temperature on the yaxis and time on the x-axis.
3. Double click the “DIGITS” option on the lower left of the Data Studio screen to provide a box that
easily allows you to read the temperature (with one or two decimal places).
4. Click the RUN button and verify that your thermometer is working properly. Then click STOP to
terminate this experiment.
5. Weigh out between 2.80 and 3.20 grams of solid baking soda (NaHCO3) on weighing paper.
6. Use a graduated cylinder to measure out 25 mL of 2 M HCl. Pour the acid into your pre-weighed
250-mL beaker. Click RUN to measure the initial temperature of the acid.
7. While the thermometer is still running (recording data on the computer screen), carefully add the preweighed baking soda to the beaker (much fizzing will occur). Stir with the thermometer and record data
until the reaction is complete.
8. Click “Scale to Fit”. Right-click on the computer graph and choose STATISTICS to show the minimum
temperature obtained.
Calculation time:
The reaction that occurs in the beaker is:
NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g) H = +28 kJ/mol
a) Use the H value for the reaction given above to calculate the number of joules that would be
absorbed by the reaction using the mass of baking soda (NaHCO3) you weighed out in step #5.
b) Convert the joules in (a) to calories.
c) Divide your calories from (b) by the number of GRAMS OF SOLUTION you have in your beaker. This
will give you a theoretical value for how much your temperature should have dropped during the
reaction. Hint: to find grams of solution, weigh the beaker and subtract the weight of the empty beaker.
d) Compare your theoretical temperature drop from (c) to the experimental temperature drop you
recorded with the thermometer. Ponder why these numbers are not the same.
e) Use the MOLARITY concept to calculate the moles of HCl present in 25 mL of 2-molar acid.
Compare to the moles of baking soda you used in the reaction. Note: the balanced equation shows you
that the proper mole ratio is one-to-one. Is this what you actually used in this reaction?
264
Assignment #146: Less than Zero (continued)
Part II
Your objective now is to modify the scripted reaction so that you will achieve a colder
temperature than you got in Part I. You will be allowed to try 2 EXPERIMENTS to get the
temperature to drop as low as possible. Your grade on this lab will be based on how cold a
temperature you achieve, as shown in the following table:
Lowest temperature
Less than 20C
Less than 14C
Less than 12 C
Less than 10C
Less than 6C
Less than 1C
Best in the Class
Grade (out of 10 points)
7
7.5
8
8.5
9
9.5
10
Note: the values shown in the table are base values and may be adjusted up or down
depending on the quality of your note page (provided on the next page).
The questions shown below are designed to have you consider possible avenues for
modification. These are NOT the only things that you could change. I believe the best strategy
will be to carefully discuss the parameters you would like to change (and why) prior to
performing actual experiments. You must learn from your mistakes if you are to be successful in
this lab.
Does the container affect the results of the experiment?
Ceramic, metal and Styrofoam vessels are available.
Is the ratio of baking soda to acid that you used in the first reaction the "right" ratio?
Hint: think about the MOLES you calculated in question (e). . .
Would there be an advantage to adding some extra water to the reaction?
Is there an advantage to scaling up or scaling down the reaction?
You may want to use more moles or fewer moles in your reaction mixture
Is the volume of acid important? If so, do you want more or less volume in your
reaction?
Would there be an advantage to using a different molarity of acid?
1 M, 2 M, 3 M, and 6 M HCl are available
265
#146 (continued): Less Than Zero Lab note page: (turned in for points!)
Part Two:
Unscripted Experiment #1:
Type of reaction vessel used: ________________________________________
Mass of
baking soda
Moles of
baking soda
Volume and
molarity of acid
Moles of
acid
Lowest
Temperature
Theoretical T
Describe the logic behind your experimental procedures. That is, what changes did you
make to the original procedure? Why??
Unscripted Experiment #2:
Type of reaction vessel used: ________________________________________
Mass of
baking soda
Moles of
baking soda
Volume and
molarity of acid
Moles of
acid
Lowest
Temperature
Describe the reasoning behind the modifications you made in this experiment.
Theoretical T
266
Assignment #147: Note-Taker for Nuclear Chemistry
1. The NUCLEUS contains p _ _ _ _ _ _ and n _ _ _ _ _ _ _
+
n
2. The s __ __ __ __ __ n __ __ __ __ __ __ FORCE holds the nucleus together!
n
3. ATOMIC NUMBER = Number of _______________
Protons
repel
4. ATOMIC MASS = Number of _______________ + _______________
5. Each element has various __ __ __ __ __ __ __ __ (nuclei with different masses)
a) hydrogen:
1
6
b) carbon:
0 5 6. UNSTABLE isotopes are r __ d __ __ __ __ t __ __ __. In the above examples, 11C and
14
C are unstable isotopes of carbon. 12C and 13C are stable isotopes of carbon.
7. Radioactive isotopes undergo DECAY processes
a) alpha decay = nucleus splits and shoots out an alpha particle (symbol = _____)
(alpha particle = ____________ nucleus = ___ protons and ___ neutrons)
b) beta decay = nucleus transmutes (a n __ __ __ __ __ __ becomes a proton) and
shoots out a high speed e __ __ __ __ __ __ __.
c) gamma decay = nucleus relaxes from an EXCITED STATE and shoots out a very
high energy p h __ __ __ n
8. Nuclear decay processes can be written as chemical equations
a) examples of alpha decay:
→ → ____ +
b) examples of beta decay:
→ 32
P → ____ +
9. The types of radiation differ in their ability to PENETRATE through matter.
a) _______ radiation can usually be stopped by a piece of paper.
b) ______ radiation can be stopped by a thin lead shield or about ½ inch of Plexiglas.
c) ________ radiation can be stopped by about 8 inches of solid lead
10. Einstein’s famous equation E = mc2 states that matter ( m __ __ __ ) can be
converted into e __ __ __ __ __ and vice-versa.
+
267
Assignment #148: Alpha Beta
Unstable isotopes and radioactive decay patterns
1. Give the number of PROTONS and NEUTRONS in the following isotopes. For each unstable isotope,
state whether the atom has TOO MANY neutrons or TOO FEW neutrons (compared to a stable isotope).
(stable)
a)
b)
(unstable)
9 protons
__ protons
__ neutrons
19 neutrons
c)
22
Na (unstable)
d)
35
Cl (stable)
Unstable b/c too
many neutrons
2. Show nuclear equations for the following radioactive decays. Note: do just one step for each, but then
make a guess as to whether the daughter isotope formed would be stable or still radioactive.
a)
240
94
Pu undergoing  decay. (Pu = plutonium). Hint: the symbol for an alpha particle is 24 He .
→ Pt is
platinum
b)
c)
190
78
28
13
Pt undergoing  decay
Al undergoing  decay. Hint: the symbol for a beta particle is
0
1
e
3. Radon is a radioactive gas that seeps out of the ground. Elevated levels of radon in a residence have
been associated with increased cancer risk. Radon levels are particularly high in underground caves, such
as Carlsbad Caverns. Radon undergoes alpha decay, as shown in the equation below.
222
Rn 
218
Po +
4
He
a) In the equation above, it looks as if the mass of the products is EQUAL to the mass of the reactants.
However, when the precise masses of the isotopes are used, there is a measurable mass difference
between the products and reactants. Use the precise masses provided below to determine how much mass
has been DESTROYED in the nuclear reaction.
Isotope
222
Rn
Precise
mass (g/mol)
222.0175
218
Po
218.0089
4
He
4.0026
b) Why do you suppose radon levels are so high in underground caves (about 100 times more
concentrated than the outside air)? Hint: A radon-222 atom has a half-life of only 3.8 days, so it doesn’t live
very long.
268
Assignment #149: All My Children
The family tree of Uranium
The table below (taken from Zumdahl, p. 1004), shows the decay patterns and half-lives of the
nuclides produced from 238U.
269
H. Chem Supplemental Assignment #149: All My Children (continued)
Follow your teacher’s instructions for filling in the table below.
240
238
236
234
232
230
228
226
224
Atomic
mass
222
220
218
216
214
212
210
208
206
204
81
82
83
84
85
86
87
88
89
90
91
92
93
Atomic number
Suppose a sample of lead-210 emits 40,000 beta particles per minute. Calculate the time required for this
sample to decay to a safe level of only 160 counts per minute. Note: the half-life of 210Pb = 20.4 years!
E4E—use algebra and/or geometry to approximate the total number of -particles emitted during this
length of time.
270
Assignment #150: Mr. Fusion
The most powerful nuclear transformations known to man
Fusion = mashing together small nuclei to create larger nuclei
Fission = splitting large nuclei into smaller nuclei
The following is an equation for a typical FUSION reaction:
P
2
n
H
nucleus
2
H + 3H  4He + 1n
H = -1.7x109 kJ/mole
In this reaction, 1.7 x 109 kJ of heat appears as each mole of helium is formed. This heat can (in
theory) be used to run an electrical power plant.
1. An electrical power plant can also burn a chemical fuel, such as coal, to provide the heat needed to
generate electricity. Coal can be thought of as pure carbon, which burns as shown here:
C + O2  CO2
H = -393 kJ/mole
Note: This H value tells you that for every mole of carbon burned (12 grams), 393 kJ of potential
energy is converted to heat.
Calculate how much coal you would have to burn to equal the 1.7 x 109 kilojoules of heat produced by the
fusion reaction shown above.
2. As seen in the nuclear fusion equation shown above, only a few grams of fusion fuel can produce a
prodigious amount of heat energy, which can then be turned into electrical energy. A typical US
household requires 2.6 x 107 kJ of electrical energy per year. Assuming a 33% efficiency in converting
heat to electricity, verify that the combined mass of 2H and 3H required to provide a year’s worth of
electricity for a typical US household is about ¼ -gram! Note: the alternative could be burning
2,382,000 grams of coal. . .
2
3. In Einstein’s famous equation: E = mc , energy is measured in joules and mass in kilograms. In
the fusion reaction shown above, 1.7 x 1012 joules of energy are produced, with a corresponding loss of
mass. Use Einstein’s equation to calculate the mass (in kilograms & grams) lost in the fusion reaction.
4. Do you see any negative aspects to the fusion reaction that might make it undesirable as a source of
energy for human use?
5. A FISSION reaction splits large nuclei (such as uranium or plutonium nuclei). A typical fission
reaction is shown below:
n + 235U 
142
Ba +
91
Kr + 3 n
H  -2 x 1010 kJ/mole
While this nuclear reaction produces a tremendous quantity of usable energy, it has a major problem
because it creates “radioactive waste”. Try to explain why barium-142 and krypton-91 are properly
described as “radioactive waste”. Note: the fusion reaction shown at the top of the page creates stable
helium nuclei, which are harmless for the environment. No radioactive waste produced in the fusion rxn!
271
Assignment #151: Cloud Chambers
Overview: The goal of the day is for you to observe the particles that are shot out from a
radioactive nucleus.
Background: Radioactivity is a property of unstable nuclei (“nuclei” is the plural of the word
“nucleus”). Stable nuclei can exist unchanged for billions of years, but unstable nuclei will
change over time, emitting particles that shoot out from the nucleus with great energy. The
particles that are shot out can be detected by a Geiger counter. Alternatively, the particles can be
visualized using a device called a cloud chamber.
Types of radioactive particles: There are three major types of radioactivity, and they are
labeled using letters from the Greek alphabet.
 = alpha radiation.
Alpha particles are relatively heavy particles composed of two protons and two
neutrons. They were used by Ernest Rutherford in his experiment that discovered the nucleus.
 = beta radiation.
Beta particles are relatively light particles. The most common type of beta
particle corresponds to an electron. Strangely enough, in beta radiation, an electron is shot out from
the NUCLEUS, which is made of protons and neutrons. In other words, the unstable nucleus creates an
electron out of protons and neutrons!!!
 = gamma radiation.
Gamma “rays” are high energy PHOTONS. They possess no mass and have no
real substance. However, they do carry tremendous amounts of energy. A gamma photon is created
when an unstable nucleus in an EXCITED STATE drops down to a lower energy level.
In metaphoric terms, you can think of these types of radioactivity in the following way:



Alpha particles can be thought of as cannonballs (big and heavy, packing a big “punch”)
Beta particles are like bullets—small and speedy, they can often penetrate through living tissue
without causing fatal damage
Gamma rays are like sci-fi laser weapons. They can travel through solid objects (like x-rays do)
and they are almost impossible to stop (because they have no real substance)
Running the Experiments:
You will have a small plastic chamber that will be saturated with alcohol vapors. This
chamber must be cooled with dry ice so that the alcohol vapors will want to condense into liquid.
Interestingly, radioactive particles will catalyze the condensation of vapor to liquid. Therefore,
when a radioactive particle travels through the chamber, it will create a trail of condensed vapors
(similar in concept to the trail that a jet plane leaves behind as it travels through the atmosphere).
Your chamber should already be assembled with a radioactive lantern mantle stuffed into a
hole in the side of the chamber. Although it is radioactive, this lantern mantle is harmless
(unless you were to eat it).
To start the experiment, place your cloud chamber atop the piece of dry ice that is provided
for your station. Periodically shine your flashlight into the chamber (either from the top or from
the rectangular “window” in the side of the chamber. Within a few minutes, you should see
tracks forming inside the chamber. Each track is caused by the passage of one of the types of
radiation described above.
Ideally, you will observe different types of tracks—some corresponding to alpha particles,
some corresponding to beta particles, and some to gamma.
Sketch your observed results on the back of this page. If you did see different types of
tracks, make a guess as to which track corresponds to each particle.
272
Assignment #152: The Alien Blood Lab
Overview:
In the classic 1979 science fiction movie "Alien", the crew of the ship Nostromo bring an
alien creature on board their spaceship. This alien possesses blood that can eat through metal
almost instantly (with much fizzing and smoke produced). The alien eventually kills everybody
on the ship except Ripley (played by Sigourney Weaver) and a cat (played by ????).
In today’s experiments, you will try to make your own “Alien Blood”. You will start with the
instructions given below. After performing this initial experiment, you will experiment to create
the most effective Alien Blood possible.
Use caution when doing your experiments. If things are going well, you probably will be
making large clouds of noxious gases, which you should avoid inhaling. Work on a small scale
while you do your experiments--avoid wasting materials!
Initial procedures:
1. Cut a piece of aluminum foil (about 10 cm x 10 cm) and wrap it over the top of a 150 mL beaker.
Depress the middle of the foil so that it forms a small bowl.
2. Place the beaker atop a paper towel to absorb spills that may occur.
3. Measure out 4 ml of 6 M hydrochloric acid and pour it into your aluminum "bowl".
4. Measure the time needed for the acid to eat through the aluminum and fall into the beaker below.
Use of a cell phone timer is encouraged.
5. Describe the results of this reaction and write a chemical equation that explains your
observations.
6. Place foil waste in your trash box and pour liquid waste into the big beaker on the buffet table.
Free-form experiments:
Your ultimate goals are to develop a highly effective Alien Blood and a means for protecting
a ship from an Alien Blood “attack”. Some ideas worth investigating are suggested below. Your
investigations can be performed in any order you choose and may explore concepts not
suggested in the list. Creativity is encouraged!
I. Is there a salt that would work similar to (or better than) the HCl in eating through the foil? Note:
recommended mass of salt is around 1-gram (or less). Ask your instructor if you want to use more.
II. Would temperature have any influence on the “Alien Blood’s” effectiveness? Use a hot water
bath to warm your “blood”. Bunsen burners are NOT allowed in today’s lab. Avoid dipping a
METAL thermometer in Alien Blood! (glass thermometers are OK to dip)
III. Would a mixture of a salt + acid work better than either component on its own? Note: scientists
use the term “synergy” to describe a combination that is more effective than its components.
IV. Can you find a way to PROTECT the good ship Nostromo? (i.e., Is there some kind of alien blood
"extinguisher" that you could dump onto an alien blood spill that would halt its destructive powers?
Or is there a way to "pre-treat" the metal so that it becomes resistant to attack??)
To investigate these questions, you may have access to anything reasonable from the
stockroom. A number of items have been placed on the buffet table for your use—please look
there first before requesting chemicals from your instructor. Keep notes on all the experiments
you try, even those that end up with disappointing results.
For your write-up, please write paragraphs that communicate your ideas (hypotheses?), your
experimental methods, the results of your experiments, and your interpretation of these results.
Include chemical equations and pictures where appropriate. Ideally, your paper will describe key
discoveries you have made.
273
Assignment #153: I’ve Got Your Number
Assigning Oxidation numbers
1. Assign oxidation numbers to the following elements or ions
a) Mg (s)
_____
Hint: apply rule #1
b) Mg+2 (aq)
_____
Hint: apply rule #2
2. Assign oxidation numbers to each atom in the following compounds:
a) NaCl
Na _____
Cl _____
This is an ionic compound! Apply rule #2!
b) N2H4
H _____
N _____
Apply rule #3 AND rule #5a
WRITE IN OXIDATION
NUMBERS FOR
INDIVIDUAL ATOMS,
NOT GROUPS OF ATOMS!
(e.g. H = +1, NOT +4)
c) PbO2
Pb ____
O _____
d) HClO4
H _____
Cl _____
O _____
Use rule #5a
H _____
S _____
O _____
Use rule 5b here!
e) HSO4
-
3. Consider the ions SO42- and SO32-.


Apply rule #4 AND rule #5a
(SO42- is called sulfate, SO32- is called sulfite)
When an element is OXIDIZED, its OXIDATION NUMBER INCREASES.
When an element is REDUCED, its OXIDATION NUMBER DECREASES.
2-
2-
When SO4 is converted to SO3 , is the sulfur being OXIDIZED or REDUCED? Explain.
274
Assignment #154: LEO Says GER
(loss of electrons = oxidation; gain of electrons = reduction)
Oxidation-Reduction reactions are often called “Redox” reactions
For each of the following reactions:
a) Assign oxidation numbers to EACH ATOM in the reaction on both the LEFT and RIGHT hand sides of
the equation. The pictures are designed to help you do this well.
b) Determine whether the reaction is a REDOX reaction (oxidation-reduction reaction). If it is not a
redox reaction, what type of reaction is it? (e.g. acid-base, precipitation).
c) If it is a redox reaction, identify the element that is OXIDIZED (oxidation # increases) and the
element that is REDUCED (oxidation # decreases). Support your answer by specifically stating the
oxidation number changes that you observe.
1.
Fe+2 (aq) +
Al (s) 
Fe (s) +
Al+3 (aq)
Example!
2.
3.
4.
HOCl
+
Br-
Cl-

+
Br2
+
H2O
MnO 4 - (aq) + HCOOH (aq) + H + (aq)  Mn 2 + (aq) + CO 2 (g) + H 2 O
BrO 3 - (aq) +
H + (aq)
+
I - (aq)

I 2 (s) +
-
Br (aq) +
Draw your
own pix!
5.
BaCl2 (aq) +
H2SO4 (aq) 
BaSO4 (s) +
HCl (aq)
H2O
275
Assignment #155: Redox Mini-Lab
Overview: In this mini-lab, you will run a redox reaction using a copper penny and nitric acid.
You will interpret the results using your understanding of oxidation numbers.
Procedures:
1. You will need a PERIODIC TABLE and your PINK PAGE of Oxidation Number Rules.
2. Put on safety glasses and proper shoes!
3. Obtain a “NEW” penny from the buffet table (date 1983 or newer).
4. Place your penny in an Erlenmeyer flask.
5. Answer the questions shown below:
A) Copper is a metal that is RESISTANT TO OXIDATION. Name two elements that are even MORE resistant to
oxidation than copper. Hint: look at the ACTIVITY SERIES on your periodic table.
B) Explain why H+ is incapable of oxidizing Cu. Note: discuss “strong” and/or “weak” in your answer.
6. Measure out 15 mL of 6-molar nitric acid (HNO3) from the buffet table. Move your group to
the FUME HOOD and add the acid to the penny in the flask (toxic gases are produced by this
reaction). Observe your flask until you see a vigorous reaction. When your reaction begins to
slow, LEAVE THE FLASK IN THE HOOD and return to your table to answer the questions below:
Questions: Note: don’t write any chemical equations until the very end!!!!!
1) The copper penny is an example of COPPER METAL. What is the oxidation number of the copper
atoms in the penny?
2) After reacting with nitric acid, a blue solution containing copper ions is created. What is the
OXIDATION NUMBER of the COPPER IONS within this BLUE solution?
3) In the reaction with nitric acid, is the copper being OXIDIZED or REDUCED? Use the CHANGE IN
explain your answer.
COPPER’S OXIDATION NUMBER to
4) Write out the oxidation numbers for each element in nitric acid (HNO3).
5) When copper is oxidized, an element in the nitric acid must be reduced. Explain why it is the
in HNO3 (and not hydrogen or oxygen) that is REDUCED in this reaction. Note: if you aren’t
sure about this question, move ahead to questions 6-8 and then return to #5.
NITROGEN
Continued on next page!
276
Assignment #155: Redox Mini-Lab (continued)
6) You have learned that many elements can have variable oxidation numbers. For example, carbon can
have an oxidation number of any value between –4 and +4. This range holds true for any element in the
group IV family. Sulfur, and its fellow elements in group VI, can be in oxidation states ranging from
–2 to +6. What would you expect the range of oxidation states to be for nitrogen, which is in group V?
Hint; you should see that the range of oxidation numbers in each of the cases discusses is a span of
EIGHT. This relates to the OCTET RULE for electrons.
7) Write the oxidation numbers for each element in ammonia (NH3).
8) Based on your oxidation numbers from question #7, fill in the sentence below with appropriate words
to make a meaningful statement about ammonia’s possible role in a REDOX reaction.
Since the nitrogen in ammonia is in a -3 oxidation state (as low as nitrogen
can possibly go), it cannot be _ _ _ _ c e d. This means that ammonia is
incapable of _ _ _ d _ _ i n g an element such as copper.
9) Which one of the formulas shown below is a plausible identity for the GAS that is produced in the
reaction of nitric acid and copper? Note: use process of elimination—reasons for elimination are
provided in the text box shown below!
N2O5 (g)
Oxidation # of nitrogen =
NO3 (g)
Oxidation # of nitrogen =
NO2 (g)
Oxidation # of nitrogen =
Possible reasons for elimination
(match each reason to one of the compounds
shown, leaving you with only one surviving
choice!)
(A) This substance doesn’t exist because
nitrogen’s oxidation number can never be
this high!
(B) This gas makes up most of our
atmosphere (has no color)!
(C) This gas smells bad, but isn’t toxic.
N2 (g)
Oxidation # of nitrogen =
NH3 (g)
Oxidation # of nitrogen =
(D) Since the nitrogen in HNO3 is reduced
in the reaction (starts at +5, ends at
something lower than +5), this oxidation #
is too high to be the product gas.
10. Attempt to write a balanced chemical equation for the reaction you witnessed in today’s minilab.
277
Assignment #156: Summary of Oxidation and Reduction
Oxidation
 When an element is oxidized, its oxidation number goes UP.
 Oxidation is defined as ________ of electrons. (LEO)
Reduction
 When an element is reduced, its oxidation number goes DOWN.
L
E
O
Says
G
E
R
 Reduction is defined as ________ of electrons. (GER)
Redox
 It is impossible to have oxidation without reduction (or vice-versa).
 The term “Redox” means both reduction and oxidation occurring simultaneously
in a chemical reaction.
Examples:
1. Use oxidation numbers to answer the following questions.
a) If NO3- is converted to N2, has the nitrogen been oxidized or reduced?
b) If H2CO3 is converted to CO2, has the carbon been oxidized or reduced?
c) If HCl is converted to Cl2, has the chlorine been oxidized or reduced?
2. One of the equations below represents a redox reaction. The other is impossible because it
shows reduction occurring without oxidation. Identify which is which.
a)
HClO4 + H+  HCl +
b)
HClO4 +
Sn2+ +
H+ 
H2O
Sn4+ +
H2O + Cl-
GER!
278
Assignment #157: Wolves and Goats
In this class, we will use a metaphor of “wolves” and “goats” to describe what happens in a
REDOX reaction.
A redox reaction always contains:
 one chemical species that provides electrons
 one chemical species that takes electrons
Based on your knowledge of wolves and goats (in the real world), make a guess as to which of
the descriptions given above matches the “wolf” and which description matches the “goat”
Wolf = the chemical species that ___________________ electrons
Goat = the chemical species that ___________________ electrons
In a redox reaction, the _________ attacks the goat and steals its
___________________. In this process, the wolf’s hunger
diminishes, which is represented chemically by a decrease in its
oxidation number. The wolf gets reduced. The goat gets oxidized.
Balancing a redox reaction.
Redox reactions can be very difficult to balance. The wolf and goat metaphor can be a helpful
tool when trying to balance a redox reaction.
Consider the following riddle:
Suppose you have a “wolf” that needs ___ electrons to satisfy its hunger.
If each “goat” can provide only 2 electrons, HOW MANY GOATS must a wolf
kill to satisfy its hunger? No waste of electrons is allowed.
2e
I need
__ e-
-
Solution to the riddle:
Applying the wolf and goat metaphor to a redox reaction:
Consider the following unbalanced chemical equation:
+6
__ CrO42- + __ Fe2+ + __ H+  __ Cr3+ + __ Fe3+ + __ H2O
a) Identify which element is reduced: _____
This element is gaining electrons, and is therefore the WOLF.
b) Identify which element is oxidized: _____
This element is ____________ electrons and is therefore the GOAT.
c) Each Cr (wolf) changes from +6 to ___ meaning it needs ____ e- to satisfy its hunger.
d) Each Fe (goat) changes from ___ to ___ meaning it provides ___ e-.
e) In order to “feed” the wolf, this reaction will require ___ goats for every wolf. This means
that the coefficient for Fe2+ (goat) is ___, while the coefficient for CrO42- (wolf) is ___.
f) The rest of the equation is balanced using standard techniques for placing coefficients.
I need
__ e-
279
Assignment #158: Feed The Wolf
Using oxidation numbers to BALANCE redox reactions
For each of the reactions below:
a) Determine which elements are changing their oxidation state (i.e. which element is reduced and
which is oxidized).
b) Label the element that is oxidized as the “goat” and determine how many electrons this goat
provides.
c) Label the element that is reduced as the “wolf” and determine how many electrons this wolf is
gobbling up.
d) Use COEFFICIENTS for WOLF and GOAT to balance the number of ELECTRONS transferred. Then
use intuition and trial and error to balance the “accessory” molecules that do not directly participate in
the electron transfer. In the end, ALL ELEMENTS and CHARGE should be balanced.
1. 3 Fe+2 (aq) + __ Al (s)  __ Fe (s) + __ Al+3 (aq)
Al is the goat in this reaction (Al → Al3+). Each Al goat provides 3 eFe2+ is the wolf in this reaction (Fe2+ → Fe). Each Fe2+ wolf consumes __ eTherefore, need ___ goats to feed ___ wolves!
-
2. __ BrO3- (aq) + __ H+ (aq) + __ I- (aq)  __ I2 (s) + __ Br (aq) + __ H2O
In this reaction, Br is
the wolf, as it
changes from +5 to
___, gobbling up ___
electrons.
In this reaction, I is
the goat, as it
changes from -1 to
___, providing ___
electron.
3. __ As2O3 (s) + __ NO3- (aq) + __ H+ (aq) → __ H3AsO4 (aq) + __ NO (g) + __ H2O (l)
4. __ CrO3 (aq) + __ H+ (aq) + __ CH3OH (aq)  __ Cr2+ (aq) + __ CO2 (g) + __ H2O
280
Assignment #159: Color By Number (Lab)
Permanganate reaction with oxalic acid
Write-Up: Everyone will turn in the question page of the lab (on Monday) for
7.5 points. If you want a better grade than this, you will also need to make a
paragraph-based write-up describing the experiments you designed in step #8.
Your paragraphs should be thoughtful, explaining your ideas instead of just
stating what you did.
Procedure: (Bring a calculator, periodic table, and pink page to your table)
1. You will need 1 clean beaker and 1 clean flask for this lab. The flask will be used to create a
solution of oxalic acid solution. The beaker will be used to run a redox reaction.
2. Weigh out between 0.62 and 0.74 grams of solid oxalic acid (H2C2O4) into a flask.
Then dissolve this solute in enough water to make the concentration equal to
0.20-molar. Hint: your calculated volume should be between 30 and 45 mL.
3. Using a graduated cylinder, measure out 10 mL of your oxalic acid solution into the
REACTION BEAKER (save the remaining 20-35 mL of oxalic acid solution for future experiments).
Add a SMALL MAGIC BEAN to the REACTION BEAKER atop the magnetic stirring plate.
4. Add 10 ml of 3-molar sulfuric acid (H2SO4) to the reaction beaker. Sulfuric acid is a 150 mL beaker
strong acid that is very effective at burning holes in clothing. Because the H2SO4 molecule can
form many friendship bonds (hydrogen bonds), the acid evaporates very slowly, so a drop that
gets on your clothes will remain there for hours, slowly eating away the fabric. Note: don’t
expect to see any reaction between the sulfuric ACID and the oxalic ACID.
5. To start the redox reaction, add 10 ml of 0.015-molar potassium permanganate solution
(KMnO4) to your reaction beaker and stir with the magic bean for a few seconds. After the
solutions are mixed, STOP stirring. Watch the beaker as it goes through several color changes
and RECORD THE TIME it takes for the reaction to go to completion (start timing as soon as you
mix the purple KMnO4 into the beaker). Expect the reaction to take a FEW MINUTES to run to
completion. The reaction is done when the solution is colorless.
6.Look carefully at the solution once the reaction is done to see subtle clues as to the products of
the reaction (in particular, look for some TINY BUBBLES that may accumulate on the magic bean).
7. AFTER THE REACTION HAS FINISHED, answer the questions on the next page. You should be
able to figure out all the answers if you apply the concepts you have learned this week!
8. Once you understand the chemistry of the reaction, try to perform experiments of your own
design to accomplish the following tasks: Note: there is a waste beaker on the buffet table!
i. eliminate the "lag time" in the beginning of the reaction (when it is purple and just sits there)
ii. arrest the progress of the reaction at one of the intermediate colors so that you can keep the
color permanently (or at least for a long time)
Note: When faced with a challenge of this sort, a chemist would consider three general ways of
changing a reaction’s rate:
a) Alter the TEMPERATURE of the reaction
b) Alter the CONCENTRATIONS of reactants (your instructor has solutions of varying molarities
available for your use!)
c) Include a CATALYST in the reaction (this is the most interesting idea to try if you have a clue as
to what the catalyst might be!!!!)
281
Assignment #159 (continued) Color By Number Lab Questions:
A. What is the OXIDATION NUMBER of the Mn atom in KMnO4? Hint: K is a +1 ion.
B. The manganese atom ends this reaction as a free-swimming Mn2+ ion. Based on your experimental
results, what is the COLOR of manganese in a +2 oxidation state?
C. Explain how the REDUCTION of manganese can produce the many colors seen during the reaction.
Hint: consider the TITLE of this lab!!!
D. The structure of the oxalic acid molecule is shown below. Assign an oxidation number to each atom
in the structure.
E. When manganese is reduced (gaining electrons), something must be oxidized (losing electrons). In
this case, the element that is oxidized is in the oxalic acid molecule. Based on your understanding of
oxidation numbers, WHICH ELEMENT in the oxalic acid is being oxidized? Hint: Your choices are limited to
H, C, and O. Only one of these is a “variable” that can change its number. . .
F. Determine the OXIDATION NUMBER OF CARBON in each of the molecules shown below. Then EXPLAIN
why only CO2 is a plausible identity for the molecule produced when oxalic acid is OXIDIZED.
H2CO
CO2
HCOOH
G. Using the idea of wolves and goats, fill in the appropriate coefficients to balance the chemical
equation for the reaction of permanganate with oxalic acid:
___ MnO4- + ___ H2C2O4
+ ___ H+
 ___ Mn2+ + ____ CO2 + ____ H2O
Mn (wolf) goes from +7 to +2, gobbling up ___ e-.
C (goat) goes from ___ to ___, providing ___ e-.
H+ ions are
supplied by H2SO4
(strong acid)!
Each oxalic acid
molecule contains
TWO carbon atoms!
Need ___ carbons to “feed” each Mn.
H. You may have noticed that the reaction in this lab starts very slowly, but appears to speed up after a
minute or two. The reason for this acceleration in rate is that the reaction is “autocatalytic”. Use the
suggestions below to come up with a hypothesis for how the catalyst works its “magic”. Note: you are
encouraged to test your hypothesis by performing appropriate experiments in procedure #8.
a) The catalyst is one of the reactant molecules
b) The catalyst is one of the product molecules
c) The catalyst is a substance that does not appear in the chemical equation
Explain your choice in the space below.
Note: after completing this question page, return to the previous page to complete step #8.
282
Assignment #160: Two halves make a whole
Learning how to use redox half-reactions
1. On your ½-rxn chart, find the half-reaction showing Fe (s) being oxidized to Fe2+ (aq).
a) Complete the half-reaction in the space below:
Fe (s)

b) In this half-reaction, iron’s oxidation number changes from zero to +2. Therefore, two electrons
are shown in the half-reaction. If iron’s oxidation number changed from zero to +3, there would be
____ electrons shown in the half-reaction.
c) Compare the half reaction for iron’s oxidation to the half-reaction for the oxidation of silver metal.
Explain why the silver half-reaction only shown ONE electron (instead of two or three).
2. Look at the half-reaction for the reduction of NO3- to NO.
a) Copy the half-rxn in the space below:
b) Why are there 3 electrons involved in this half reaction? Hint: calculate nitrogen’s oxidation number
in the NO3- ion and in the NO molecule.
c) In the reduction half-reaction, the nitrate ion (NO3-) loses two oxygen atoms. What molecules are
formed from these “unwanted” oxygen atoms?
d) Which of the following substances would be a good choice for providing the H+ ions needed to
run this half-reaction? Explain your reasoning. Hint: remember your acid-base chemistry!
i. CH4
ii. H2SO4
iii. H2O
3. Attempt to combine the two half-reactions you wrote in 1a) and 2a) to make a "whole"
reaction. When you combine the two half-reactions, you need to use multipliers to balance the
number of electrons that are given off and received. This is similar to the “Wolf and Goat”
concept. When you balance the electron transfer, use whole numbers instead of fractions.
Continued on next page!
283
Assignment #160: Two halves make a whole (continued)
4. Find the half-reaction for the reduction of Cr2O7-2 on your yellow page (this ion is called
dichromate).
a) Write out the half-reaction in the space below:
b) Why are there 6 electrons involved in this half-reaction? Hint: there are two Cr atoms here. Each
Cr starts in a + ___ oxidation state and ends in a + ___ oxidation state. . .
c) Why are 14 H+'s needed in this half-reaction?
d) Write out the half-reaction for the oxidation of iodide (I- ion) and combine it with the halfreaction for the reduction of dichromate (Cr2O72-) to make a balanced redox reaction.
e) Based on the balanced chemical equation you wrote above, answer the following questions
regarding observable changes that occur during the reaction.
Would any bubbles be produced? Explain.
Would any precipitates form? Explain.
Would any color changes be expected? Explain.
5. Bromate ion (BrO3-) is an oxidizing agent that is not on your chart. When bromate gets
reduced, the central bromine atom turns into Br-. Attempt to invent a half-reaction that
describes this reduction. Note: follow the pattern exhibited by the half-reactions that are on the chart.
You should be able to balance electrons (by looking at oxidation # change), oxygens (using H2O), and
hydrogens (using H+). Think along the same lines as suggested in questions 4b & c.
284
Assignment #161: The Kid’s Got Potential
1. Permanganate ion (MnO4-), is commonly used in redox reactions. It reacts differently
depending on whether it is used in the presence or absence of ACID (H+ ions).
a) Using your yellow page, copy the half-reaction for permanganate being reduced to Mn
2+
in
an ACIDIC environment. Include the ELECTRODE POTENTIAL!
Acidic
MnO4-
b) Complete the balancing of the following half-reaction for permanganate being reduced to
MnO2 in a BASIC environment (no acid present).
1 MnO4- + 2 H2O +
___ e- 
___ MnO2 + ___ OH-
E = + 0.59 volts
2. If CHLORIDE ion (Cl-) (a goat) is mixed with PERMANGANATE ion (a wolf) in a BASIC
environment, NO reaction will occur. EXPLAIN. Hint: look at electrode potentials! The half
reaction for the oxidation of Cl- is on your yellow page. The half-reaction for the reduction of
permanganate in basic solution is shown in 1b.
3. If ACID is added to a mixture of MnO4- and Cl-, a reaction DOES occur. WRITE THE BALANCED
EQUATION for this reaction and EXPLAIN why the addition of acid enables permanganate to
oxidize chloride ion. Hint: you should be combining TWO HALF REACTIONS here!
4. Describe two changes that would be OBSERVABLE as the reaction written in #4 proceeds.
5. Determine the oxidation number of sulfur in the sulfate ion shown below:
6. Complete the half-reaction for the reduction of SULFATE ion (SO4-2) to SULFITE ion (SO3-2).
__ SO42- (aq) + __ H+ (aq) + __ e-  SO32- (aq) + __ H2O
E = +0.20 V
7. Now write a correctly balanced half-reaction (including E value) for SULFITE ion (SO3-2)
being OXIDIZED to form SULFATE ion (SO4-2). Hint: This should be a trivial task if you take a look at what
you did in problem #6!!!!! tsuJ kniht sdrawkcab!!!
8. If SULFITE ion is mixed with permanganate ion in a BASIC environment, would a reaction
occur? How about if sulfite were mixed with permanganate in an ACIDIC environment?? EXPLAIN.
Basic
MnO4-
285
Assignment #162: Redox Practice Prior to E4E
Skill #1: Combine appropriate half-reactions from the yellow page to determine the overall
BALANCED EQUATION (with ELECTRODE POTENTIAL!) for the reaction of copper metal and nitric
acid. Hint: nitric acid’s half reaction starts with NO3- (aq). . .
Skill #2: Describe at least two observable changes that would occur as the reaction written in
question #1 proceeds.
Skill #3: DISSECT each of the redox equations shown below into TWO HALF REACTIONS and
describe why there can be NO REACTION in either of these situations:
a)
Fe2+ (aq) + 2 Ag (s)  2 Ag+ (aq) + Fe (s)
Oxidation ½ reaction:
Reduction ½ reaction:
No reaction because:
b)
MnO2 (s) + 4 H+ (aq) + Sn2+ (aq)  Mn2+ (aq) + 2 H2O (l) + Sn (s)
Skill #4: Complete the half-reaction shown below:
__ CrO4-2 (aq) + __ H+
(aq)
+ __ e-  __ Cr3+
(aq)
+ __ H2O
(l)
286
Assignment #163: A Shadow of Doubt?
(LAB)
Overview: In this lab, you will perform redox reactions with a variety of common anions
(ANION = A NEGATIVELY CHARGED ION). Each reaction can be understood using half-reactions to
identify the products of oxidation and reduction. At the start of the lab, you will NOT KNOW
the identities of the various ions, but you may be able to DEDUCE them based on the experimental
evidence you collect. However, it is quite likely that (as the title of the lab implies) you may be
faced with some doubt as to which anion is which.
Write-up instructions: Please make a multi-paragraph write-up that does NOT copy any
phrases or sentences from the instruction page. Be sure to write balanced chemical equations for
all reactions that occur (that means 6 total equations!) and explain WHY things worked the way
they did. Include pictures that show the COLORS present in the test tubes, and explain what
molecules/ions create these colors. Present your evidence in as convincing a manner as possible.
If you are left with a shadow of doubt, please explain the reasons for your doubt.
Procedures: As always, start by equipping yourself with proper safety gear!
1. To do this lab, each individual will need a half-reaction chart and some scratch paper.
2. At your lab station, you should find 4 clean test tubes in the test tube rack.
3. The buffet table has jars containing the 4 salt solutions you will be using in this lab. Each
bottle is filled with a solution of a sodium or potassium salt containing a particular anion. It
is the anion that is of importance in today's experiments. The salts are labeled A, B, C, & D,
and the four anions (in random order) are CHLORIDE, BROMIDE, IODIDE, and SULFITE.
4. Use your half-rxn chart to find an OXIDATION HALF-REACTION for chloride, bromide and
iodide. Copy these onto your note page. Also copy the following half-reaction for the
oxidation of sulfite ion onto your note page:
SO32- (aq) + H2O (l)  SO42- (aq) + 2 H+ (aq) + 2 e-
E° = -0.20 Volts
5. Use the pipets provided in the stock jars to accurately measure 1 mL of Salt A
into a labeled test tube. Then measure out 1 mL of Salt B into a second tube.
Continue with salts C and D in your two remaining test tubes. Note: try to make
each member of your group responsible for getting a particular salt.
6. Use a graduated pipet to accurately add 1 mL of 0.015-molar potassium
permanganate (KMnO4) solution to each of your four test tubes. Note: There is a
test tube containing 0.015 M KMnO4 at each lab table.
7. Look for signs of reaction. Describe (sketch?) the changes that you see. You should have
that have reacted (and two tubes that have not reacted). At this point, you have
added KMnO4 without any acid (H+). In the absence of acid, permanganate undergoes the
following half-reaction (note the electrode potential).
TWO tubes
MnO4- (aq) + 2 H2O (l) + 3 e-  MnO2 (s) + 4 OH- (aq)
E = +.59 volts
Use this half-reaction to determine why you have TWO tubes that have reacted and TWO tubes
that have NOT reacted. Write BALANCED EQUATIONS for the TWO reactions that have
occurred.
8. Thoughtfully consider the chemical equations you have written, looking for evidence of
reaction products in your test tubes. You may find it helpful to test the pH of a drop of liquid from
each test tube!
287
Assignment #163: A Shadow of Doubt? (continued)
9. In the presence of acid, MnO4- undergoes the following half-reaction, with a potential of
+1.50 volts:
MnO4- (aq) + 8 H+ (aq) + 5 e-  Mn2+ (aq) + 4 H2O (l)
Question: If the reaction mixture is acidified to increase permanganate’s potential to +1.50 volts,
which of the anions (Cl-, Br-, I-, SO32-) should become oxidized?
10. Write FOUR balanced redox equations for the reaction of ACIDIFIED MnO4- with all the anions
in this lab (include electrode potentials). Hint: this is not much work if you realize the
similarities between the oxidation half-reactions!
11. Based on your BALANCED EQUATIONS and the hints provided below, make predictions of
what you should expect to see (and smell) in your 4 test tubes.
Cl-, Br-, I-, SO32-: these are the ions you started with--you already know that all of these ions are
colorless with no particular odors.
Mn2+: you saw this ion last week in your Color by Number lab!
Chlorine (Cl2) from the Greek chloros, meaning “yellow-green”: Chlorine gas dissolves well in water to
produce a pale yellow-green color. As you know, chlorine has a powerful odor.
Bromine (Br2) from the Greek bromos, meaning “stench”: Orange-red when concentrated, yellow when
dilute. Some swimming pools utilize bromine instead of chlorine as a disinfectant.
Iodine (I2) from the Greek iodes, meaning “violet”: Yellow, orange, brown or red when in aqueous
solution. Violet when dissolved in non-polar solvent. Also can produce violet vapors.
Sulfate (SO42-) The color of sulfate ion is evident in the bottle of 3-M sulfuric acid.
12. Use a plastic pipet to add 1 mL of 3-molar H2SO4 to all four tubes. Agitate and look for
color changes. Please borrow a bottle of sulfuric acid from the buffet table to perform this step.
13. Sketch the appearance of your test tubes at this stage. If you still have a purple color at this
stage, it means that the PERMANGANATE has NOT yet been REDUCED.
14. Carefully smell the contents of each tube. Wafting recommended!
15. Compare the results observed with the results you expected based on your balanced
equations. You should have some discrepancies between expected and observed results, which
will lead to doubt, but you should be able to identify some of the anions.
16. Remove 1 mL of solution from your purple test tube and transfer to a clean test tube. This
tube should remain heptane-free and will be used in step #19.
17. To gather further evidence, you will use heptane as a non-polar organic solvent to “extract”
the non-polar chemicals present in your test tubes. Heptane is a hydrocarbon with the formula
C7H16 that will form a layer on top of the water.
Use a pipet to add about 1 mL of heptane to each of your 4 original tubes and vortex
vigorously. Heptane (although flammable) is rather inert in most situations. It is NOT reacting
with anything in this experiment. Its only purpose here is to act as a solvent. The heptane will
extract any non-polar molecules produced in the reactions. Look for signs of these non-polar
molecules in the heptanes layers
18. Identify the molecules that are producing colors in your test tubes. Try to label the contents
of each tube in BOTH the TOP AND BOTTOM LAYERS (i.e. what non-polar molecules are in the
heptane layer? What ions and molecules are in the aqueous layer??).
288
Assignment #163: A Shadow of Doubt? (continued)
19. At this point, you still have one tube that has undergone very little reaction. Therefore, it
hasn’t progressed to make products that would give it a unique color or smell. In order to speed
the rate of this reaction, you should put 60 mL of tap water in a beaker, microwave the water for
1 minute, and immerse the tube containing the purple solution that you reserved in step #16 in
the hot water bath. Look for signs of reaction. A smell test may help identify products that are
forming.
20. When you are finished, pour the contents of all your tubes into the waste beaker on the
buffet table. Rinse your tubes with water and set them upside-down in your test tube rack.
289
Sketch Page for Assignment #164: How to Build a Battery
This assignment consists of a DIAGRAM (shown below) and step-by-step instructions (on following page). By following all the instructions on the
following page, you should learn what components are required for making a battery using oxidizing and reducing agents.
A
B
290
Assignment #164: How to Build a Battery
In the diagram provided, there are TWO beakers. One beaker will be designed to RECEIVE
electrons, while the other beaker will be designed to PROVIDE electrons. First focus on beaker A
1. We will arbitrarily choose beaker A as the beaker to RECEIVE electrons. This beaker must contain a
chemical substance that is hungry for electrons (an OXIDIZING AGENT, or “wolf”). Using the choices
given below, choose the oxidizing agent with the more favorable potential and place it in beaker A.
Ag+(aq)
Ni2+(aq)
E° = +0.80 volts
E° = -0.25 volts
2. Beneath beaker A, write the HALF-REACTION for the reduction that will occur when beaker A receives
electrons. Include the ELECTRODE POTENTIAL.
3. The oxidizing agent you chose above is a positively charged ion. Since every plus charge needs a
minus, you will need to add an anion to beaker A. Choose an anion from your solubility chart that will
NOT make a precipitate and sketch its presence in beaker A.
4. Add some water to beaker A (sketch a wavy line in the beaker).
5. In order for the electrons to reach the oxidizing agent in the aqueous solution, a conductive bar (called
an ELECTRODE) is required. In beaker A, sketch a metal bar that will extend from the bottom of the
beaker to above the beaker’s rim. Then decide what metal this bar could be made of so that it will NOT
react with the ions in the solution (THIS METAL MUST RESIST OXIDATION).
You will now focus on beaker B. Since beaker A is set up to RECEIVE electrons, beaker B
should be set up to PROVIDE electrons. A substance that provides electrons is called a reducing
agent (a goat).
6. From the choices given below, choose the best REDUCING AGENT (i.e. the best electron provider).
Zn (s)
Sn (s)
7. The reducing agent you chose above is a solid metal. In this battery, the reducing agent will act as
the ELECTRODE for beaker B. Sketch a metal bar in beaker B and label it appropriately.
8. Beaker B will need to be filled with a CONDUCTIVE SOLUTION. Typically, a salt is dissolved in water
to make a conductive solution (though sometimes acids are used to make a conductive solution). The
safest choice is to make the conductive solution using a SALT OF THE METAL USED AS THE ELECTRODE.
That is, if your electrode is made of COPPER METAL, you should dissolve a COPPER SALT to make the
conductive solution. Sketch an appropriate salt solution in beaker B that will provide conductivity
surrounding the metal electrode.
9. Beneath beaker B, write the HALF-REACTION for the oxidation that will occur as the metal electrode in
beaker B gives up electrons. Include the ELECTRODE POTENTIAL.
Your battery is nearly complete. To make the battery function, two ELECTRICAL CONNECTIONS
are needed so that there is a complete circuit.
10. In order to get electrons to flow (the entire purpose of a battery), you will need to have a WIRE that
connects the TWO ELECTRODES in your battery. Sketch a wire in your diagram that connects the two
electrodes and show the direction of electron flow through the wire. Hint: the “goat” (reducing agent)
provides electrons, the “wolf” (oxidizing agent) accepts electrons.
11. The final connection needed to complete the circuit is called a SALT BRIDGE. This concept will be
introduced in tomorrow’s lesson.
12. Determine the OVERALL BALANCED REDOX EQUATION by combining the half-reactions appropriately.
13. Your battery is now complete!
291
Assignment #165: Shock Therapy (intro to electricity)
Electrochemistry is the interface between Electricity and Chemistry.
 The chemistry is always REDOX chemistry
 Electricity can be described/measured using many different terms:
1. Electrical Potential, measured in __________.
2. Current, measured in _________. Current measures the flow of
electrons through a circuit.
3. Resistance, measured in ________. Resistance decreases electron flow.
4. Power, measured in Watts.
There are two types of current: AC and DC
AC stands for _________________ ______________. The electrons will
first move in one direction then move back in the other direction. In the
U.S., electrical outlets use AC that switches direction 60 times per second.
DC stands for ____________ _______________. The electrons are
always moving in the same direction. All batteries are DC devices.
The effects of Current on a miniature
light bulb (Light emitting diode =LED)
Current flowing
through bulb
Result
Resistors added
220 ohms (220 )
6800 ohms (6.8 k)
The effects of Current on a human being:
Voltage
used
Human subject(s)
Notes on Resistance:
Current flowing
through human(s)
Results
292
Assignment #166: Signor Volta
Questions about galvanic cells (batteries)
1. In the space below, sketch a picture of a two-beaker setup that will produce voltage (i.e. a picture of a
battery). Use the following chemicals to create your battery:
1.0-molar CuSO4 (Hint: the copper sulfate is used as a source of Cu2+ ions. . .)
1.0-molar ZnSO4
a copper bar
a zinc bar
some wire
a salt bridge
2. Fill in the following information in the picture you drew above:
The half reaction occurring in each beaker Note: one half-reaction must be an oxidation, the
other must be a reduction!
Direction of electron flow (through the wire—electrons can’t swim)
Anode, cathode (anode = electrode at which oxidation occurs, cathode = electrode at which
reduction occurs)
Voltage produced by the battery
3. As electrons flow in the battery you sketched above, one of the electrodes will shrink, while the other
will grow. Which electrode shrinks? Which electrode grows? Explain. Hint: look at the half reactions.
An electrode can only “grow” if additional solid forms on the electrode.
4. All batteries eventually “die”. This happens because one or more key chemicals become depleted.
For the battery you drew in question #1, what chemicals would be likely to run out and cause the battery
to die? Hint: one of the chemicals that could run out is a solid—the other is an aqueous ion.
293
Assignment #167: Let There Be Light!
Building batteries in the lab
In the beginning, God created the heavens and the earth, and the earth
was without form and void. . .And God said, “Let there be Light”. And
there was light. Genesis 1:1-1:3
Overview: Your goal in this lab is to make a powerful battery, capable of lighting up some
miniature light bulbs (LED’s). You will all start by making a standard copper/zinc cell. Then
you will try to make a BETTER battery using a particular “special ingredient”.
Part One: For the zinc/copper cell, you should use:
1. Two 250 mL beakers, each containing 75 mL of distilled water
2. A strip of zinc metal (for use as an electrode)
3. A strip of copper metal (for use as an electrode)
4. Between 1.2 and 1.6 grams of solid copper sulfate
5. Between 1.2 and 1.6 grams of solid zinc sulfate
6. A salt bridge (filled with .25 M Na2SO4).
Use the materials described above to construct a battery in the manner you have seen in your
homework assignments.
In order to determine how well your battery works, you will use a digital voltmeter to measure
the voltage and current output of your battery.
a) Measure VOLTAGE by turning the dial on your voltmeter to the 20 V setting (indicating DC
voltage). Use an alligator clip to connect the black lead of the voltmeter to the ANODE of your
battery. Use a second alligator clip to connect the red lead of the voltmeter to the CATHODE of your
battery.
Your voltage should measure close to 1.10 volts (the theoretical potential of the redox reaction). If
your voltage is not close to 1.10 volts (or if it jumps around wildly), you probably have a bad
electrical connection. Check to make sure your alligator clips are secure. You may also want to
shine up your metal electrodes using a Scotchbrite scrubbing pad (available at any of the sinks).
b) To measure MILLIAMPS, first remove the plugs on the meter. Turn the dial to the 20m setting in
the A
section of the meter (indicating DC amps). Reconnect the leads to the voltmeter (black to
anode and red to cathode). You should get a current reading less than 1 milliamp.
Record your voltage and current readings and show them to your instructor when he/she passes
by your table.
Remove the salt bridge from your battery set-up while measuring the voltage (or current). Your
voltage/current should drop to zero. Then reinstate the salt bridge to complete your circuit—thus
restoring your voltage/current. This demonstration should emphasize the importance of the t salt
bridge.
Make a rough sketch of your battery set-up and identify the direction of electron flow
through the wires. Include the two relevant half-reactions (with potentials) in your sketch.
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Assignment #167: Let There Be Light! (continued)
Part Two: You will now try to make a better battery. In order to do this, you will need new
chemical species so that you can run more powerful half-reactions.
1. For the better battery, REPLACE YOUR SALT BRIDGE with a POROUS CUP to maximize electrical
current (milliamps). The porous cups provided on the buffet table are designed to fit inside a 400
mL beaker. This set-up will allow you to have TWO COMPARTMENTS within a single beaker.
Note: a porous cup allows MIGRATION OF IONS between the two compartments. Since the ions have a LOT
of surface area through which they can migrate, a porous cup provides less resistance than a salt bridge.
2. Your instructor will give you a CARD which specifies a “special” chemical that you will use to
improve your existing set-up. These chemicals will be available on the buffet table.
3. You will use your special chemical to create a new half-cell, which you will combine with
one of your original half-cells to make a superior battery. You must decide which of your
original half-cells (the zinc or copper half-cell) you will keep for use in your better battery.
4. If you are using a powerful “wolf”, you will need to employ an inert GRAPHITE ELECTRODE
(non-oxidizable). Graphite electrodes are available on the buffet table.
5. Design your improved battery on paper before you try to construct it in real-life. Remember
that you need a wolf (oxidizing agent) and a goat (reducing agent) in separate compartments in
order to make a functional battery.
6. After constructing your “new and improved” battery, MEASURE THE VOLTAGE and MILLIAMPS
produced by the cell (USE THE 200m SETTING IN THE A SECTION TO ACCURATELY MEASURE
MILLIAMPS). You should achieve a voltage  1.5 volts and a current  10 mA. Note: voltage and
current data should be included in the scientific diagram that you turn in for a grade.
7. Let There Be Light! If your voltage and current readings exceed the lower limits
described in step #6, you should attempt to light an LED. Disconnect the voltmeter and use
alligator clips to hook up your battery to the red LED provided at your lab station. When using
an LED, the longer lead (marked with a + sign) should be attached to the cathode of your
battery. Note: it is expected that every group will light a red LED. To get the other LEDs to light, you
should combine volts with another group as outlined below.
8. If you have time, connect your battery to your neighbors’ battery to create a
“super” battery of very high voltage. In order to do this, you will have to make
intelligent connections (a physicist would say that you have to connect your cells
in SERIES). Measure the VOLTAGE and CURRENT of your super battery. Then try
to light some of the high voltage LEDs.
9. When cleaning up, you should pour your copper sulfate solution into the
CuSO4 RECYCLING beaker. Permanganate goes in the WASTE beaker on buffet
table. All other solutions that you used in the lab can go down the drain.
Batteries
in series
Write-up instructions: Draw a careful SCIENTIFIC DIAGRAM of the BEST SINGLE battery that you
constructed during the lab period. Label everything that is important, and try to show HOW THE
CHEMISTRY WORKS. You may want to have “windows” in your diagram where you zoom in to
show the details of the chemistry that is occurring at each electrode.
Please do NOT try to draw a picture of a “super battery” (a combination of your cell with another
group’s cell). Show only a single cell in your diagram.
Minimum size for the scientific diagram = 8.5 x 11”
Maximum size = 11 x 17”
295
Assignment #168: Introduction to Electrolytic Cells
The achievements of Sir Humphry Davy
1. While gold metal has been known since ancient times, sodium metal could only be produced
after the discovery of electricity. In 1807, Sir Humphry Davy was the first human to create
sodium metal by passing an electric current through molten sodium hydroxide.
a) Using your yellow page, write out the reduction half-reaction that produce sodium metal. Include its
E° potential.
b) Consider the unbalanced half-reaction shown below that describes the oxidation that would have
occurred in Sir Humphry Davy’s electrolytic cell. Attempt to balance this half-reaction.
___ OH-  ___ H2O + ___ O2 (g) + ___ e-
c) Provide an explanation for why sodium metal was impossible to obtain before the advent of electricity.
d) Sketch a picture of what you imagine Sir Humphry Davy’s electrolytic cell looked like. Show the
flow of electrons in your picture and determine which half-reaction occurs at each electrode.
2. Magnesium metal is produced through the electrolysis of magnesium chloride (as discussed in
Monday’s class). Lithium metal is produced through the electrolysis of lithium chloride.
a) Write appropriate half-reactions (with potentials) for the reduction of magnesium ions and the
reduction of lithium ions.
b) The passage of one mole of electrons into molten lithium chloride will produce one mole of lithium
metal, whereas the passage of on mole of electrons into molten magnesium chloride will produce only
half a mole of magnesium metal. Explain.
296
Assignment #169: Self-guiding analysis of Electrolysis
In ELECTROLYSIS, an external source of electrical energy is used to do interesting chemistry
1. Consider the picture below of a battery hooked up to two inert graphite electrodes that are
immersed in a beaker containing a solution of copper iodide (CuI2).
Side A = Reduction
Side B = Oxidation
a) Look at the two ends of the battery. Decide whether ELECTRONS will flow out from the positive or
negative pole on the battery. Then draw arrows showing the flow of electrons out one of the wires and
back through the other.
b) Now look at electrode A. Your picture should show electrons flowing from the battery towards
electrode A. This means that electrode A is acting like a “vendor” of electrons (imagine it calling out to
the solution, “Electrons here! Get your electrons here!”)
Decide whether it is the Cu+2 ions or the I- ions in the solution that will swim over to take the
electrons that electrode A is handing out. Then write a half-reaction for the chemistry that will occur at
this electrode. Write the half-reaction on the actual picture drawn above. . .
c) Your picture should show ELECTRONS flowing INTO the battery FROM ELECTRODE B. This means that
an ion in the solution (or electrode B itself) must be LOSING ELECTRONS.
Decide whether it is the Cu+2 or the I- ions that will be losing electrons in this case. Then write a
half-reaction for the chemistry that will occur at this electrode. Note: in this case, you know that electrode B
cannot itself be the chemical that gives up electrons because it is made of graphite (an inert electrode). An INERT
electrode is UNREACTIVE-- it will not gain or lose electrons.
d) Identify which electrode is the ANODE and which is the CATHODE by referring to the site of oxidation
and the site of reduction.
e) In the space below, combine the two half-reactions that you wrote for parts b & c to make an overall
balanced redox equation with electrode potential. Explain why a battery is needed to make this overall
reaction happen.
e) This electrolysis reaction PRODUCES TWO USEFUL ELEMENTS. Try to fill in the blanks shown below
to properly identify these two elements:
____ = highly conductive metal used in electronic circuits. Also used to make water pipes that
are resistant to oxidation.
____ = commonly used as a disinfectant. Also used in halogen light bulbs.
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Assignment #169: Self-guiding analysis of Electrolysis (continued)
2. Now consider the set-up diagrammed below
a) Sketch the direction of ELECTRON FLOW in the external circuit (i.e. the flow of electrons through the
wires). Remember that the + pole of the battery attracts electrons, while the – pole repels electrons.
b) The brass ring is the cathode in the diagram. The CATHODE is always the site of REDUCTION. In the
diagram, write the half reaction that will occur on the surface of the brass ring. Hint: Look for the most
favorable reduction potential!!!
c) The silver electrode is the anode in the diagram. The ANODE is always the site of OXIDATION. Decide
what half-reaction will occur at the silver electrode and write it in the diagram. Hint: The electron donor
will NOT be something in the solution. Consider the METAL ELECTRODE ITSELF as an electron donor!
d) Combine the two half reactions from (b) and (c) to get an overall balanced redox equation.
e) If you look at your answer to question (d), you should see that there isn’t much of an overall reaction
going on here. However, this electrolysis set-up serves a very useful purpose. The set-up shown in the
diagram is designed to create a silver-plated ring. Explain how the chemical half-reactions will plate the ring with
a layer of silver metal.
f) Suppose you wanted to make a gold-plated ring. What would you change in the set-up to enable it to
plate an object with gold instead of silver?
298
Assignment #170: Redox and Electrochem Concept Review
1. Determine the oxidation numbers for the following elements:
a) Cr in CrO4-2
b) C in CH3OH
2. When an element is oxidized, its oxidation number goes up. When an element is reduced, its
oxidation number goes down. Which of the following classifies as a REDUCTION of the underlined
element? Explain.
a) Br-  Br2
b) CH4  CO2
c) SO4-2  SO3
d) PbO2  PbCl2
3. Combine appropriate half reactions to make a balanced equation for the oxidation of copper metal by
acidified nitrate ion (Note: acidified nitrate ion = nitric acid).
4. The battery set-up shown below has several flaws. One of the flaws is that both half-reactions shown
are oxidations. Fix all the problems in the diagram so that the set-up will function as a usable battery.
Note: start by inverting one of the half-reactions so that you have a reduction ½ rxn.
Zn (s)  Zn2+ + 2e- E = +.76 V
Pb (s)  Pb2+ + 2e- E = +.13 V
5. Sketch an appropriate experimental set-up for the ELECTROLYSIS of a solution of nickel chloride using
a graphite anode and a copper cathode. Include a power source and sketch the direction of electron
flow. Show the half-reactions for the chemistry you would expect to occur at each electrode and explain
what useful products could be derived from this set-up.
299
Assignment #171: Electrolysis Mini-Lab
Procedure A: Electrolysis of a stannous chloride solution
1. Obtain a 6 V battery, petri dish, and two pieces of nichrome wire.
2. Bend the nichrome wires so that they lie in parallel lines in the petri dish, with a small amount of wire
overhanging the side of the dish (see board for diagram). Tape the part of the wires outside the dish to the
lab bench so that you can connect alligator clips to the wires without having them move about.
3. Pour about 20 mL of 0.20-molar stannous chloride solution (SnCl2) into the petri dish.
4. Make a prediction for what chemistry will occur when a battery is connected to the two nichrome
wires. Note that the stannous chloride solution contains aqueous Sn+2 ions, aqueous Cl- ions, aqueous
HCl, and lots of H2O molecules.
5. Hook up the battery and allow the electrolysis reaction to run for a short time (stop when the growth
reaches half-way to the other electrode). When you’ve seen a reasonable amount of reaction, disconnect
the wires from the battery. DO NOT disturb the dish when you do this. SKETCH your experimental set up
and DESCRIBE your observations in the space below.
6. Write a half-reaction that describes the chemical nature of the “growth” that occurred in step #5. Since
the electrode that exhibits growth is the CATHODE, your half-reaction should be a REDUCTION.
7. If you see some color (typically yellow or green) appearing around the other wire (the anode), this
indicates the wire itself is being oxidized to produce metal ions in the solution (nickel and chromium ions
from the nichrome wire can produce various colors in the solution).
8. Now make a prediction for what will occur in the dish if you reconnect the wires to the battery with
reversed polarity (that is, the wire that was attached to the + now gets attached to the -, and vice-versa).
9. As gently as possible, connect the nichrome wires to the battery in a reversed orientation. Record
results in the space below, and write a half-reaction for EACH electrode.
10. Carefully remove the wires from the petri dish and wipe the wires clean.
11. Dump the stannous chloride solution into the TIN WASTE beaker specified by your instructor. Rinse
out the petri dish and prepare for Procedure B (on the next page).
Continued on the next page!!!
300
Assignment #171: Electrolysis Mini-Lab (continued)
Procedure B: Electrolysis of potassium iodide
1. Obtain a graphite electrode and a piece of nichrome wire (reuse one of the wires from procedure A).
2. Place the petri dish on a WHITE BACKGROUND and pour about 20 mL of 0.1 M potassium iodide
solution into the dish.
3. Add a single drop of phenolphthalein to the dish and carefully mix it into the solution.
4. Hook up your graphite electrode to the positive pole of your battery. Then connect the piece of
nichrome wire to the negative pole of your battery.
5. Immerse the electrodes on opposite sides of the petri dish. When you do this, rest your graphite at an
angle on the edge of the dish. AVOID AGITATING the solution—if the solution gets stirred up, it will be
more difficult to see the colors produced.
6. Sketch the electrolysis set up, showing the direction of electron flow through the wires and the colors
that appear at each electrode.
7. Write a HALF-REACTION for the chemistry occurring at the GRAPHITE ELECTRODE. Then explain why
a color appears at this electrode.
8. Write a HALF-REACTION for the chemistry occurring at the NICHROME WIRE ELECTRODE (consider
water as a reactant!). Then explain why a color appears at this electrode. Hint: you should remember that
phenolphthalein, in its colorless state, has an attached H+ and that it turns pink when its “hat” is removed. In order
to steal the hat, you might need a SHARK. . .
9. Determine the overall balanced equation (with electrode potential!) for the reaction occurring in the
petri dish.
10. Identify which electrode is the ANODE and which electrode is the CATHODE in this setup. Explain
your reasoning.
11. Try writing a message on paper using the graphite electrode after it been in the electrolysis bath for
some time. Explain the “magic” of this writing implement. Hint: you may recall from biology that
iodine (I2) reacts with starch to form a blue-black color. The chemical structure of paper is similar to the
chemical structure of starch.
301
Assignment #172: Electrolysis of a Nickel
(LAB)
A United States 5-cent piece (better known as a nickel) is an ALLOY made up of two common
metals melted together to create a homogenous mixture (no layers). The metals are chosen to be
durable & resistant to oxidation, yet not terribly expensive. As you might guess, nickel (Ni) is one
of the metals used in making a 5-cent piece. The second metal used in the 5-cent piece is a
“mystery metal” that you should be able to identify by the time you finish this lab.
Write-up instructions: Your write up is limited to ONE SINGLE-SIDED PAGE. Use this one page to
convey your understanding of the reactions that occurred during the electrolysis. Use PICTURES to help
describe the procedure and results. DO NOT waste space rephrasing the instruction page. Focus your
time and effort on explaining the CHEMISTRY—the REACTIONS that occur and WHY they occur. Consider
writing a thoughtful 2-sentence introduction to the lab (required for an “A” paper).
0. Equip yourself with safety gear, a periodic table, half-rxn chart, note page, and calculator.
1. Your instructor will provide you with a nickel (5-cent coin) and a carbon fiber rod to use as electrodes.
Weigh the nickel and record its mass.
2. At your lab table you will find two alligator clip patch cords, a 6-volt battery, and a cardboard lid.
3. Pass the light colored alligator clip through the cardboard lid and clip onto the 5-cent coin as shown in
the instructional video.
4. Use a cylinder to measure 25 mL of 6 M HCl in the 250 mL beaker. The depth of the liquid should be
about 1 cm. Place a piece of white paper under your beaker to enable you to see colors more easily.
5. Place the cardboard lid on the beaker so that the coin is half-immersed in the acid. Wait at
least two minutes while you look for signs of a reaction—read steps 6 & 7 while you are waiting.
6. Put the carbon fiber electrode through the cardboard lid.
7. Sketch a diagram that shows how you can use your battery to help the coin react. Your diagram
should show how to connect the nickel and the carbon fiber post to the battery so that the COIN WILL BE
OXIDIZED. Your diagram should SHOW THE DIRECTION OF ELECTRON FLOW between the battery and each
electrode. Once you have made a good sketch, ask for your instructor’s approval. DO NOT MAKE
ANY ELECTRICAL CONNECTIONS UNTIL YOU HAVE APPROVAL!!!
8. Once your instructor approves your sketch, hook up the battery and watch for signs of reaction in the
beaker. In particular, look for evolution of gases, for color appearing in the electrolyte, and for build-up
or dissolution of solid material. Try to figure out the CHEMISTRY occurring at each electrode. In this
experiment, MORE THAN ONE half-reaction is occurring at each electrode—try to figure out ALL of them.
9. After some time, you should see some solid material building up on the cathode. Observe this solid
carefully and try to determine what it is (hint: the solid material is a METAL). Figure out how and why
this metal is forming on the carbon fiber electrode. Also figure out why solid nickel metal is NOT forming
on this electrode. Note: the process you are running is an example of “electrorefining”. In chemistry, the word
“refine” is a synonym for “purify”. Explain how the term ELECTROREFINING applies to this laboratory experiment.
10. After your reaction has been running for 20 to 30 minutes, you should terminate the reaction and
retrieve your coin (please consult with your instructor to determine whether the time is right for you to
terminate). To isolate the refined metal that formed on the carbon fiber rod, decant the green electrolyte
solution into an empty beaker and rinse the refined metal 3 times with distilled water. Then transfer the
refined metal to a pre-weighed evaporating dish and heat it to obtain an accurate mass. Also weigh your
leftover coin.
11. Calculate the yield of refined metal as a percentage of the mass lost from the coin. Explain why the
percent yield must be significantly less than 100%.
12. RINSE THE ALLIGATOR CLIPS THOROUGHLY WITH WATER to wash away any acid. Dump your
leftover acid solution into the WASH ACID jug on the buffet table.
13. Either throw away your coin or take it home as a souvenir.