Download Activity 5 Chemical Energy

Active Chemistry Cool Chemistry Show
Activity 5
Chemical Energy
7HAT$O9OU3EE
GOALS
What Do You Think?
In this activity you will:
Injury seems to be part of professional sports. Athletic trainers have large
kits with first-aid items to treat athletes during a game. One item they use
is a cold pack.
• Make hot packs and cold packs.
• Observe energy changes when
matter undergoes a change.
• Determine whether energy
changes are endothermic or
exothermic from a particular
point of reference.
• Do the instant ice packs found in first-aid kits require refrigeration?
• How do the cold packs work?
Record your ideas about these questions in your Active Chemistry log.
Be prepared to discuss your responses with your small group and the class.
Investigate
1. To make a cold
pack, place 10 g of
ammonium nitrate
in a quart-size,
resealable plastic
bag. Add 20 mL
of water to the
bag and seal.
a) Record your
observations.
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Activity 5 Chemical Energy
In an endothermic chemical
reaction, energy in the form of
heat is absorbed in the process. In
an exothermic chemical reaction,
energy in the form of heat is
given off in the process.
b) Was the cold pack an example
of an exothermic or endothermic
chemical reaction?
2. Make a hot pack by placing 20 g
of sodium carbonate (or calcium
chloride) in a quart-size, resealable
plastic bag. Add 20 mL of water to
the bag and seal.
a) Record your observations.
b) Was the reaction exothermic
(heat generating) or endothermic
(heat absorbing)?
3. Your teacher will perform the
following demonstration.
To a flask containing 16 g of
ammonium thiocyanate, 32 g
of barium hydroxide is added.
A rubber stopper is placed in the
mouth of the flask. The flask is
shaken vigorously.
The stoppered flask is placed on a
wood board that has been wet down
so that there are puddles of water.
Safety goggles
and a lab apron
must be worn
at all times in a
chemistry lab.
a) Record your observations.
Be careful when
working with
the sodium
hydroxide
pellets. Wear
rubber gloves
and eye
protection. If
you should
accidentally
drop a pellet,
do not try
to pick it up
with your bare
hands as it may
burn them. Use
gloved hands
to retrieve the
pellets.
b) Was the reaction exothermic
(heat generating) or endothermic
(heat absorbing)?
Wash your
hands and arms
thoroughly after
the activity.
a) Record your observations.
Cool chemistry!
b) Was the reaction exothermic
(heat generating) or endothermic
(heat absorbing)?
4. Using a chemical scoop, transfer a
few pellets of sodium hydroxide to a
test tube half-full of water. Carefully
feel the side of the test tube.
5. Dispose of the materials as directed
by your teacher. Clean up your
workstation.
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Active Chemistry Cool Chemistry Show
Chem Words
endothermic change
(reaction): a change in
which energy in the
form of heat is absorbed
from the surrounding
environment resulting
in an increase in the
internal energy of the
system.
exothermic change
(reaction): a change in
which energy in the
form of heat is released
from a system resulting
in a decrease in the
internal energy of the
system.
activation energy:
the minimum energy
required for a chemical
reaction.
kinetic energy: the
energy of motion.
KE = _1 mv2
2
ENDOTHERMIC AND EXOTHERMIC PROCESSES
Reactions That Absorb and Release Heat Energy
A process is described as endothermic when heat energy is absorbed,
increasing the internal energy of the system. The cold pack you made
with ammonium nitrate is an example of an endothermic reaction.
Another example is the decomposition of potassium chlorate. In this
reaction, energy must be added to the system in order to cause the
decomposition of the potassium chlorate to form the products of
oxygen gas and potassium chloride. If you touch a container that holds a
spontaneous endothermic process, it will feel cool to the touch.
An exothermic process results when heat energy is released,
decreasing the internal energy of the system. If you touch a container
that holds an exothermic process, it will feel warm or hot to the touch.
The hot pack you made with sodium carbonate is an example of an
exothermic reaction. Another example
is the combining of sodium hydroxide
solution with hydrochloric acid. This
reaction produces sodium chloride
and water and releases energy to the
environment. The terms endothermic and
exothermic can be used when describing
both physical and chemical changes.
Importance of Energy
Why is energy so important? In order for a chemical reaction to take
place, the particles (reactants) involved in the reaction must interact.
Not all collisions result in a chemical reaction. The particles involved
must have enough energy to enable them to react with each other.
The colliding particles must have enough kinetic energy to break the
existing bonds in order for new bonds to be formed. (Physics reminder:
1
Kinetic energy is the energy of motion. KE __2 mv2) The minimum
energy required for a chemical reaction is called the activation energy
for the reaction. Bond breaking is an endothermic process and requires
an addition of energy. Bond formation is an exothermic process and is
accompanied by a release of energy.
As products form, energy is released as new bonds are created. If this
released energy is greater than the energy needed to break the bonds
of the reactants (bond-forming bond-breaking), the reaction is
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Activity 5 Chemical Energy
exothermic, as
shown in the graph.
Exothermic Reaction
The prefix exo
(activation energy)
means “leaving” and
Ea
C6H12O6 + 6O2
thermo means heat
energy and so, heat
P.E.
(heat of reaction) H
energy is leaving.
6 CO2 + 6H2O
The products have
less energy than
the reactants. For
Reaction Coordinate
example, the cells
in your body use
glucose to get energy for cellular functions. Glucose plus oxygen (from
breathing) provides you with energy.
C6H12O6
6O2
→
6CO2
Reactants
6H2O
energy
Products
This energy is used by the body. When the released energy from bond
formation is less than the energy needed to break the bonds of the
reactants (bond-forming bond-breaking), the reaction is called
endothermic, as shown in the second graph. The prefix endo means
“taking in” and so the products have more energy than the reactants.
The reverse reaction, which is performed by plants during photosynthesis,
requires energy and is endothermic.
12H2O 6CO2 energy (sunlight) → C6H12O6 6O2 6H2O
Plants produce glucose for their own growth and release oxygen for
living things (including the plants) to breathe.
In the process of
photosynthesis,
plants use energy
from light to break
the bonds of
carbon dioxide and
water in order to
form the products,
glucose and oxygen.
Endothermic Reaction
C6H12O6 + 6O2 + 6H2O
Ea (activation
energy)
H
6CO2 + 12H2O
(heat of reaction)
P.E.
Reaction Coordinate
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Active Chemistry Cool Chemistry Show
Chem Words
Law of Conservation
of Energy: states that
the energy absorbed
from the surroundings
or released to the
soundings is equal to the
change in the energy of
the system.
gravitational potential
energy: is a stored
energy determined by
an object’s position in a
gravitational field.
Conservation of Energy
Energy transfer is an important feature in all chemical changes. Energy
is transferred whether the chemical reaction takes place in the
human body, like the metabolism of carbohydrates, or in a car, like the
combustion of gasoline. In both of these cases energy is released into
the surroundings. The photosynthesis that occurs in living plants and the
decomposition of water into hydrogen and oxygen both require energy
from the surroundings. Energy is the great organizing principle of all
science. The conservation of energy allows you to better understand the
world around you.
Energy exists as light energy, heat energy, sound energy, nuclear energy,
kinetic energy, and chemical energy, as well as other forms. According
to the Law of Conservation of Energy, the energy absorbed from
the surroundings or released to the surroundings is equal to the change
in the energy of the system. The total energy in any isolated (perfectly
insulated), closed system remains the same. In the simplest physical
systems, it is quite easy to describe the energy changes. When a bowling
ball falls, its original gravitational potential energy becomes kinetic
energy as the ball increases its speed. After the ball hits the ground,
this kinetic energy is converted to sound energy (you hear the crash)
and heat energy (you can measure a temperature rise of the ball and
ground) and the compression and vibration of the ground. Each of these
can be measured and it is always found that the total energy before
an event is equal to the total energy after the event. When a human
being is involved, you notice other energy interactions. A person is
able to eat food and digest the food. The energy released from this
slow-burn of the food (metabolism) is able to keep the body at about
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Activity 5 Chemical Energy
Chem Words
37ºC. As all non-living things
in a room cool down to room
temperature, humans are able
to stay warm in the 22ºC room
environment. People also use
the food energy that they ingest
for moving muscles, keeping the
heart pumping, and operating
all human functions. Living
organisms are superb energy
conversion systems.
heat: a form of energy
that results from the
motion of atoms and
molecules.
temperature: a measure
proportional to the
average kinetic energy
of all the particles of a
material.
Heat and Temperature
Both heat and temperature have been mentioned in this activity. It is
important to note that heat and temperature are not the same, although
they are related. Heat is one form of energy. When two materials of
different temperatures interact, they exchange heat energy until they
arrive at the same temperature. Temperature is a number associated
with how hot or cold something is. On a molecular level, temperature
is related to the average kinetic energy of the atoms in the material. All
particles in a material are in a constant state of motion. The temperature
of the material is a measurement of this molecular motion. If the kinetic
energy of the particles increases, the temperature increases.
A thermometer is an instrument that measures temperature. Heat is
the transfer of energy, which often results in a change in the kinetic
energy of particles. That is, it results in a change in the temperature of
the system.
Checking Up
1. Describe an
endothermic reaction.
2. Describe an exothermic
reaction.
3. Explain whether each
of the following
is endothermic or
exothermic:
a) bond breaking
b) bond forming
4. Distinguish between
heat and temperature.
What Do You Think Now?
At the beginning of the activity you were asked:
• Do the instant cold packs found in first-aid kits require refrigeration?
• How do the cold packs work?
How would you answer these questions now that you have completed this activity?
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Active Chemistry Cool Chemistry Show
What does it mean?
Chemistry explains the macroscopic phenomenon (what you observe)
with an explanation of what happens at the nanoscopic level (atoms and
molecules) using symbolic structures (formulas and measurements) as a way
to communicate. Complete the chart below in your Active Chemistry log:
MACRO
What do you observe when
heat energy is absorbed during
a chemical reaction? What do
you observe when heat energy
is released during a chemical
reaction?
NANO
What happens to atoms and
molecules as they become
heated?
SYMBOLIC
How can you graphically depict
an exothermic reaction and
the activation energy required
for the reaction to take place?
How do you know?
In the activity, what chemical reaction did you observe that was an
exothermic reaction? What evidence did you have that the process
was exothermic?
Why do you believe?
Consider the chemical reactions that you encounter in daily life. Which
are more common, endothermic reactions or exothermic reactions?
Hand and feet warmers are used in the winter. Why do you think that
hand and feet coolers are not used in the summer?
Why should you care?
Experiences that involve more than one sense (as in sight, sound, etc.)
add interest to a presentation. Consider and explain how the concept
of chemical energy can safely incorporate the sense of touch in your
chemistry show.
Reflecting on the Activity and the Challenge
Energy is involved in any process that requires the breaking or making of bonds.
The process may be a chemical or physical change. Sometimes energy changes are
not noticed or measured, but other times an energy change is significant enough
to be detected. In this activity you explored both endothermic and exothermic
processes. As you select activities to include in the Cool Chemistry Show, you must
be aware of any heat energy released or absorbed. The audience may be interested
to learn about the energy changes that accompany chemical processes. In addition,
your awareness will ensure that the presentations are safe for both the presenters
and the audience.
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Activity 5 Chemical Energy
1. Identify the following changes as endothermic or exothermic. (Ask yourself
whether the reaction requires the addition of heat energy to occur or does it
release energy in the form of heat.)
a) Melting ice
b) Lighting a match c) Dry ice subliming into carbon
dioxide gas
d) Frying an egg e) Burning gasoline
f) Explosion of hydrogen gas when it
reacts with oxygen gas
2. The water in a teapot is heated on a stovetop. The temperature of the water
increases. Is this an endothermic or exothermic process?
3. If a red-hot piece of iron is dropped into a bucket of water, what type of heat
change takes place in reference to the water? What type of heat change takes
place in reference to the iron? Use the conservation of energy to describe the event.
4. Explain in terms of energy flow how a cold pack works on a sprained ankle.
5. If a sample of ice is at –20ºC and you set a beaker of the ice on a warm hot
plate, explain why the ice does not appear to start melting immediately.
6. Given the reaction:
S(s) O2(g) → SO2(g) energy
Which diagram best represents the potential energy changes for this reaction?
2EACTION#OORDINATE
0OTENTIAL%NERGY
2EACTION#OORDINATE
D
0OTENTIAL%NERGY
2EACTION#OORDINATE
C
B
0OTENTIAL%NERGY
0OTENTIAL%NERGY
A
2EACTION#OORDINATE
7. Rumor has it that a leading cola producer is preparing a new can of cola that
does not need refrigeration! That’s right, you simply take the can right out of
the cupboard, open the can, and after waiting for five minutes you’re able to
enjoy an ice-cold can of your favorite soda!
a) Do you think this “instant cold” is possible? Give an explanation for
your answer.
b) How much would you be willing to pay for such a feature (in addition to the
cost of a regular can of soda)?
8. Preparing for the Chapter Challenge
Review the chemical reactions that you have so far planned to feature in your
Cool Chemistry Show. Have you included both endothermic and exothermic
reactions? In a paragraph describe the difference between the two types of
reactions. Be sure to mention why energy transfers are important.
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Active Chemistry Cool Chemistry Show
Inquiring Further
1. Commercial cold and hot packs
Research several cold and hot packs. What materials are used in the packs, and how
is the chemical reaction activated? You should be aware that there are a variety of
commercially available hot and cold packs. Some are reusable and some are not.
Those that are not reusable are typically chemical reactions. Those that are reusable
are typically physical changes. Design a process to make a cold pack, using your
research. Have your teacher approve your design before you actually try it out.
2. One colligative property of a solvent
A salt solution will depress the freezing point of water. This is commonly known as
the colligative property of a solution. When you add anti-freeze (ethylene glycol) to
water, you find that the freezing point of the solution is lowered, which prevents the
water in the car radiator from freezing at 0ºC. It also elevates the boiling point of
the solution, which will prevent the water solution from boiling at 100ºC. Design an
experiment to demonstrate the colligative property of a solvent. With the approval
of your teacher carry out your experiment.
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