Download Dear 3EFG, Refer to your notes for the formula and other data. But

Dear 3EFG,
Refer to your notes for the formula and other data. But read the supplements here. I am
sure that these will be helpful. Try answering the problems first. If you cant arrive at a
sound answer, confer and discuss with your classmates first. I’ll post the answer key once I
knew that you’ve discussed it with your classmates. Sometimes you learn by discussing
concepts with your classmates. I made some questions in the long test that will require
you to think critically and convert. With the many conversions that we had, I am
confident that you can derive them all. Please do read your notes and your oh so helpful
textbook. Good luck and God bless!
Sir G
I. Enthalpy
Energy Matters – Heat Changes of State
If we supply heat to a solid, such as a piece of copper, the energy supplied is given to
the molecules. These start to vibrate more rapidly and with larger vibrations – the
molecules gain Kinetic Energy(Ek). The heat supplied has been converted to molecular
kinetic energy.
If the solid is cooled, the reverse happens (the molecules vibrate more slowly and these
vibrations become smaller). The molecules have lost kinetic energy and this lost Ek is
converted back into heat energy that is lost to the surroundings.
If we heat the solid to a high enough temperature it will eventually melt to form a liquid. If
we monitor the temperature of the material during heating, we would see that it varies
with time during heating as shown in the graph below.
Notice that, in general, the temperature goes up the longer the heating continues.
However, there are two horizontal flat parts to the graph. These happen when there is a
change of state. The plateaus are also called phase changes.
The first change of state is melting (changing from a solid to a liquid). The temperature
stays the same while a substance melts. For water, this temperature is 0°C because the
melting point for water is 0°C.
The second change of state is boiling (changing from a liquid to a gas). The
temperature stays the same while a substance boils. For water, this temperature is 100°C
because the boiling point for water is 100°C.
Different substances have different melting points and boiling points, but the shapes of
their heating curves are very similar. For example, this is the heating curve for iron, a
metal that melts at 1538°C and boils at 2861°C.
Sample problem:
1. Calculate the amount of heat required to completely convert 50 g of ice at -10
ºC to steam at 120 ºC.
2. The molar heat of combustion of a compound is 1,350 kJ/mol. If 0.875 moles of
this compound was burned in a bomb calorimeter containing 1.70 L of water,
what would the increase in temperature be?
3. How much energy is required in total to change 1.9kg of ice at -10 ̊C to steam at
100 ̊C?
II. Nuclear Chemistry
Two types of nuclear reactions
1) Radioactive decay - process in which a nucleus spontaneously disintegrates, giving
off radiation.
2) nuclear bombardment reactions - nuclear reaction in which a nucleus is bombarded
or struck by another nucleus or nuclear particle. Here fission or fusion may occur.
An example of nuclear reactions
1) A sample of Uranium-238 decays spontaneously over a period of billions of years. After
about 30 billion years it is nearly gone. Strontium -90 formed by nuclear reactions that
occur in nuclear weapons testing is essentially gone after several hundred years.
2) Example of a nuclear bombardment reaction is the fusion that goes on in the sun
which is essentially four protons and electrons combining to make He.
Radioactivity comes out of the nucleus of atoms. The nucleus is radioactive because it is
unstable. Like electrons in an excited state dropping back down to ground state and
releasing a photon, nuclei need an outlet for their excited state. This outlet is radiation or
nuclear reactions. Nuclear reactions, radioactivity, are spontaneous decays, there is no
way to tell when it will occur, but eventually they will decay. These radioactive decays
occur in any atom with more than 83 protons. Also, in any atom with an exceptionally
small or large proton to neutron ratio will be radioactive.
Atomic Notation: 3 Types
A = Atomic Mass = protons + neutrons
Z = Atomic Number = protons
X = Atomic Symbol = the elements letter designation
Type 1
Type 2
Type 3
A
Z
A
X
12
6
X
12
X–A
C
C
C -12
This type came first and gives the most information
This type came second as the first type is redundant. You
do not need to tell me carbon has an atomic number of 6,
all carbons atomic numbers are 6.
This type came last and is the easiest to type, and still
relays all the info you need. This symbol is spoken, “carbon
twelve.”
3 Main Types of Radioactive Decay:
 alpha decay – a helium nucleus
 beta decay – a high speed electron
 gamma decay – electromagnetic radiation – occurs in an excited nucleus, like
excited electrons



Other Types of Nuclear Reactions:
Positron Emission – A positive beta particle
Electron Capture – Electron in orbit around nucleus is drawn into the nucleus
combining with a proton to produce a neutron. The mass of the nucleus remains
constant but the atomic number drops by 1.
Neutron Emission – a neutron is emitted from the nucleus cause the mass to drop by
1.
Fusion and Fission
Nuclear Binding Energy
Puzzling fact that the mass of an atom is always less than the sum of the masses of its
constituent particles.
For instance the mass of a helium-4 atom is 4.00260 amu
Mass of 2 electrons = 2 x 0.000549 amu
= 0.00110 amu
Mass of neutron is 1.675x10- 2 4 g so 2 x 1.00867 amu = 2.01734 amu
Mass of proton is 1.673x10- 2 4 g so 2 x 1.00728 amu = 2.01456 amu
Sum of mass
= 4.03300 amu
-0.03040 amu. This mass difference is explained by the
fact that when the nucleons come close together they bind so energy must be lowered.
This is related to the binding energy.
The binding energy is the energy required to break a nucleus into its individual protons
and neutrons.
Example
The 1 9 9 F isotopes has an atomic mass of 18.9984 amu
Nucleus has 9 protons and 10 neutrons, 19 nucleons
proton mass - 1.007825 amu
neutron mass - 1.008655 amu
9 x 1.007825 + 10 x 1.008655 amu = 19.15708 amu
This is larger than the measured mass of 1 9 9 F
This is a difference between the mass of the atom and sum of the masses of the nucleons
is called the mass defect.
Convert mass defect into energy ac
- 19.15708 = -0.1587
-0.1587amu x (3.00x108 m/s)2 = -1.43 x 101 6 amu m2 /s2
1 amu = 1.066 x 10- 2 7 kg
1 J = 1 kg m2 /s2
2
-1.43 x 101 6 amu m2 /s2 x 1.066 x 10- 2 7 kg / amu = -2.37 x 10- 1 1 J
This is the energy released when on Fluorine-19 nucleus is formed from 9 protons and 10
neutrons.
Half-life is the time required for half of the atoms of a radioactive isotope to undergo
decay. Some isotopes are very stable, undergo decay very slowly, and have extremely
long half-lives. Uranium-238 has a half-life of 4.46 billion years! Other isotopes are
extremely unstable, and have short half-lives. The isotope francium-233 has a half-life of
22 minutes. That means that if you possessed 10 grams of francium-233, after only 22
minutes you would have 5 grams of francium-233, while the remainder of the atoms
would have been converted by some decay processes to other elements.
An exponential decay process can be described by any of the following three
equivalent formulas:
where
1. N0 is the initial quantity of the substance that will decay (this quantity may be
measured in grams, moles, number of atoms, etc.),
2. N(t) is the quantity that still remains and has not yet decayed after a time t,
3. t1/2 is the half-life of the decaying quantity,
4. τ is a positive number called the mean lifetime of the decaying quantity,
5. λ is a positive number called the decay constant of the decaying quantity.
The three parameters
,
, and λ are all directly related in the following way:
Nuclear Chemistry problems:
1. The alpha decay of iridium-174
2. The beta decay of platinum-199
3. Positron emission from sulfur -31
4. Krypton-76 undergoes electron capture
5. Write the symbols for an alpha particle, beta particle, gamma ray, and positron.
6. If the half-life for the radioactive decay of zirconium-84 is 26 minutes and I start with a
175 gram sample, how much will be left over after 104 minutes?
7.
What is the mass defect for 3Li6, the lithium isotope with 3 neutrons and an atomic
weight of 6.01513? Compute for the binding energy
8.
Calculate the mass defect for 17Cl35 whose mass is 34.9689 AMU. Afterwards
compute for the binding energy per nucleon
III. Chemical Reactions
Types of Chemical Reactions
Consider for a moment the number of possible chemical reactions. Because there are
millions of chemical compounds, it is logical to expect that there are millions of possible
chemical reactions. It would be very difficult to memorize the equations for all the
different chemical reactions that occur so chemists have grouped them according to
the similarities in the way they react. It is not quite as simple as this though because some
chemical reactions can belong to more than one type. One method groups nearly all
the chemical reactions into five main types.
1. synthesis (or combination)
2. decomposition
3. single-replacement (or single displacement)
4. double replacement (or double displacement)
5. combustion
1.
Synthesis reactions
When two reactants combine to form one product. For example:
Word
equation
Potassium
+
Chlorine

Potassium
Chloride
Chemical
equation
K(s)
+
Cl2(g)

2 KCl(s)
2.
Decomposition reactions
When one reactant breaks down into two or more products or two of more of the same
molecules. It is hard to predict the products of decomposition reactions. For example:
1.
2.
Word equation
Calcium
carbonate

Calcium
Oxide
+
Carbon
dioxide
Chemical equation
CaCO3(s)

CaO(s)
+
CO2(g)
Word equation
Mercury oxide

Mercury
+
Oxygen
Chemical equation
2 HgO(s)

2 Hg(l)
+
O2(g)
3.
Single Replacement reactions
Occur between a metal element and a compound. The metal element replaces an
element in the compound. Common elements that swap are two metals or a metal and
hydrogen. For example:
1.
2.
Word
equation
Magnesium
+
Zinc nitrate

Magnesium
nitrate
+
Zinc
Chemical
equation
Mg(s)
+
Zn(NO3)2(aq)

Mg(NO3)2(aq)
+
Zn(s)
Word
equation
magnesium
+
Hydrochloric
acid

Magnesium
chloride
+
Hydrogen
Chemical
equation
Mg (s)
+
2 HCl (aq)

MgCl2 (aq)
+
H2(g)
In this reaction the element magnesium is replaced by the element hydrogen, H in
hydrochloric acid. Hydrogen gas, H2 becomes the product.
4.
Double Replacement reactions
Occur between two compounds. The two positive metal ions in each reactant swap
places with one another. For example:
1.
2.
Word
equation
Potassium
Carbonate
+
Barium
Chloride

Potassium
Chloride
+
Barium
carbonate
Chemical
equation
K2CO3(aq)
+
BaCl2(aq)

2 KCl (aq)
+
BaCO3(s)
ions
K+, CO32-
Ionic
Equation
CO32-(aq)
+
Word
equation
Sodium
sulfide
+
Magnesium
nitrate

Sodium nitrate
+
Magnesium
sulfide
Chemical
equation
Na2S(aq)
+
Mg(NO3)2(aq)

NaNO3 (aq)
+
MgS(s)
ions
Na+, S2-
Ionic
Equation
S2-(aq)
Ba2+, ClBa2+(aq)
K+, Cl
Mg2+, NO3+
Mg2+(aq)
BaCO3(s)
Na+, NO3
Ba2+, CO32-
Mg2+, S2-
MgS(s)
In these reactions the two positive metal ions swap places with one another to form two
new compounds. In most double displacement reactions the reactants are aqueous
solutions. The products formed are a solid and another aqueous solution. The solid
product formed when two aqueous solutions react is called a precipitate.
Reactions between acids and bases are also double displacement reactions. They are
also called neutralization reactions. For example the reaction between nitric acid and
the base sodium hydroxide.
Word
equation
Nitric acid
+
Sodium
hydroxide

Sodium nitrate
+
water
Chemical
equation
HNO3 (aq)
+
NaOH (aq)

NaNO3 (aq)
+
H2O(l)
5.
Combustion reactions
Reactions that involve the reactant reacting with oxygen are called combustion
reactions by scientists. We commonly called these reactions burning. For example paper
burning in oxygen in the air is a combustion reaction. Combustions reactions are all
exothermic because they release large amounts of heat energy. Energy because it is
produced is written on the product side of the chemical reaction.
For example if an element is burned in oxygen the element and oxygen combine to form
on product like in a synthesis reaction.
Word
equation
Carbon
+
Oxygen

Carbon
dioxide
+
Heat
energy
Chemical
equation
C(s)
+
O2(g)

CO2(g)
+
Heat
energy
If a compound burns in oxygen the products are either carbon dioxide and water or
carbon monoxide and water.
Word
equation
methane
+
Oxygen

Carbon
dioxide
+
water
+
Heat
energy
Chemical
equation
CH4(s)
+
2 O2(g)

CO2(g)
+
2 H2O(g)
+
Heat
energy
Stoichiometry involves quantitative relationships in a chemical reaction. Stoichiometric
factors—also called mole ratios—are based on the coefficients in the balanced equation
and are used to relate moles of one reactant or product to another. Molar masses and
stoichiometric factors, together with other factors, are used to determine information about
one reactant or product in a chemical reaction from known information about another. The
strategy for reaction stoichiometry calculations can be outlined diagrammatically, as
suggested below.
I. Mass-mass problems can be solved by following these steps:
1. Write a balanced equation for the chemical reaction involved.
2. Change the mass given into moles.
3.Compare the moles of the substance given to the moles of the substance for which the
mass is asked.
4. Calculate the number of moles required.
5. Change moles asked for into mass asked for.
Example: The balanced equation for the synthesis of ammonia is 3 H2(g) + N2(g) --> 2
NH3(g). Calculate the mass in grams of NH3 formed from the reaction of 64.0 g of N2
From the balanced equation, it is known that:
1 mol N2 ∝ 2 mol NH3
Use the periodic table to look of the atomic weights of the elements to calculate the
weights of the reactants and products:
1 mol of N2 = 2(14.0 g) = 28.0 g
1 mol of NH3 is 14.0 g + 3(1.0 g) = 17.0 g
These relations can be combined to give the conversion factors needed to calculate the
mass in grams of NH3 formed from 64.0 g of N2:
mass NH3 = 64.0 g N2 x 1 mol N2/28.0 g NH2 x 2 mol NH3/1mol NH3 x 17.0 g NH3/1 mol
NH3
mass NH3 = 77.7 g NH3
Exercises:
Balance the following reactions and indicate which of the six types of chemical reaction
are being represented:
1)
____ NaBr + ____ Ca(OH)2  ___ CaBr2 + ____ NaOH
Type of reaction: _____________________________
2)
____ NH3+ ____ H2SO4  ____ (NH4)2SO4
Type of reaction: _____________________________
3)
____ C5H9O + ____ O2  ____ CO2 + ____ H2O
Type of reaction: _____________________________
4)
____ Pb + ____ H3PO4  ____ H2 + ____ Pb3(PO4)2
Type of reaction: _____________________________
5)
____ Li3N + ____ NH4NO3  ___ LiNO3 + ___ (NH4)3N
Type of reaction: _____________________________
6)
____ HBr + ___ Al(OH)3  ___ H2O + ___ AlBr3
Type of reaction: _____________________________
Mass to Mass problem
1. Calculate the mass of the sulfate compound produced if 47.5 g of Potassium
Iodide reacted with Aluminum Phosphate.