Potential Energy Diagrams POGIL
Why?
Collision theory states that in order for a chemical reaction to occur, the reacting molecules must collide with
the proper energy and orientation. “Proper energy” refers to the average kinetic energy of the reacting
molecules. Remember that the average kinetic energy is reflected in the temperature. Additionally, each
compound has potential energy – energy stored in the chemical bonds. We can graphically represent this
energy in a potential energy diagram that shows the potential energy changes over the course of the reaction.
Success Criteria
*Interpret a potential energy diagram in regard to potential energy of the reactants, potential energy of the
products, the activation energy of the reaction, and the heat of the reaction
*Determine if a reaction is exothermic or endothermic based on its potential energy diagram
*Create a potential energy diagram that reflects both a catalyzed and uncatalyzed reaction.
*Determine if a reaction is exothermic or endothermic based on its H value.
Vocabulary
exothermic
endothermic
potential energy
activation energy
catalyst
heat of reaction (H)
Model 1.
The following chemical reaction occurs in a beaker: substance A
decomposes to yield substance B, substance C, and energy.
Key Questions
1. Which letter(s) represent(s) the reactant(s)?
2. Which letter(s) represent(s) the product(s)?
3. Should the “energy” term be shown as a reactant or product? How do you know?
4. Write the chemical equation for the reaction described above.
This type of reaction is called exothermic because the energy is being released (given off) and is written as a
product in the chemical reaction.
5. Why do we write “and energy”/”+ energy” if the energy is being given off?
6. If energy is being absorbed (taken in), where would we write the “energy” term in the equation? This type
of reaction is called endothermic.
7. The chemical reaction shown below is an endothermic reaction. Add the “energy” term to the appropriate
side of the balanced chemical equation.
HC2H3O2(aq) + Na2CO3 (aq) -----> NaC2H3O2 (aq)
+ H2O (l) + CO2 (aq)
Model 2.
2HCl(aq) + Mg(s)
MgCl2(aq) + H2(g)
Task: Proceed to the lab and using appropriate safety equipment, add 1 small piece of magnesium metal to
30.0 mL of 3M HCl in a small beaker. Record your observations of the reaction below. Be sure to make
observations regarding the temperature of the reaction (Either by touch or with a thermometer.)
Observations:
Key Questions
1. Which indicators of a chemical reaction are evident?
2. Is this reaction exothermic or endothermic? How do you know?
3. Add the word “energy” to the appropriate side of the balanced chemical equation.
2HCl(aq) + Mg(s)
MgCl2(aq) + H2(g)
The energy that is released in an exothermic reaction comes from the energy that was originally stored in the
reactants. Therefore, in an exothermic reaction, the products have less energy than the reactants do.
4. In an exothermic reaction, energy is released. Where is the released heat coming from?
5. After the reaction has occurred, the potential energy of the products is 300. kJ/mol. What is ONE possible
value for the potential energy of the reactants? How do you know?
Potential Energy (kJ/mole)
6. Graph the potential energy of the reactant that you predicted in question 5. The potential energy of the
product has been graphed for you. Do NOT write on the shaded part of the graph!!
700
500
300
100
7. If the reactants had a potential energy of 750. kJ/mol and the products had 1000. kJ/mol of potential
energy, would the reaction be endothermic or exothermic? How do you know?
Model 3.
Observe the teacher demonstration.
Write any notes/observations below:
Part 1 – teacher holds up a piece of magnesium in air
Part 2 – teacher holds up a piece of magnesium and attempts to light it using a match
Part 3 – teacher holds a piece of magnesium in a lit Bunsen burner
Key Questions
1. Does a chemical reaction occur in each of the three parts of the demonstration? How do you know?
Part 1
Part 2
Part 3
2. Remember that in order for a reaction to occur, the atoms have to collide with the proper speed and
orientation. In the demonstration, which factor – speed or orientation – is likely to be lacking in the part(s)
where a reaction does not occur? How do you know?
3. The majority of chemical reactions require the input of some energy to get the reaction started – even if
reaction is an exothermic one. In the demonstration where did the energy come from?
4. The amount of energy needed to get the reaction started is called the activation energy. If the reactants
have a potential energy of 500. kJ/mol and a total 700. kJ/mol are required for the reaction to actually occur,
how much more energy do you need for the reaction to occur?
5. Of the three potential energy diagrams shown below, which one correctly illustrates the concept of
activation energy as described in question 4? How do you know?
Reaction Progress
Reaction Progress
Reaction Progress
6. In the demonstration, magnesium reacts with oxygen to form magnesium oxide. The reaction produces 602
kJ/mol of energy. Let’s assume that magnesium and oxygen (our reactants) have a potential energy value of
810. kJ/mol. The activation energy is 255 kJ/mol. On the axes provided, create a potential energy
diagram for this reaction.
Potential Energy (kJ/mol)
Label the potential energy of the reactants, the potential energy of the products, and the
activation energy.
Reaction Progress
Exercises
For each question, select the correct answer.
_____21. Which potential energy diagram represents an exothermic reaction?
1.
3.
2.
4.
_____22. Which interval in the potential energy diagram represents the activation energy of the forward
reaction?
1. A
2. B
3. C
4. D
_____23. The graph represents a chemical reaction. The reaction is best described as
1. endothermic, because energy is absorbed
2. endothermic, because energy is released
3. exothermic, because energy is absorbed
4. exothermic, because energy is released
(kJ/mol)
_____24. The potential energy diagram of a chemical reaction is shown. What is the minimum amount of
energy that must be added to initiate the forward reaction?
1. 30 kJ
2. 50 kJ
3. 60 kJ
4. 80 kJ
_____25. Base your answer on the potential energy diagram of a chemical reaction. The reaction is best
described as an
1. exothermic reaction in which energy is released
2. exothermic reaction in which energy is absorbed
3. endothermic reaction in which energy is released
4. endothermic reaction in which energy is absorbed
_____26. The potential energy diagram of a chemical reaction is shown. Interval B represents the
1. activation energy
2. activated complex
3. potential energy of the products
4. potential energy of the reactants
_____27. Which potential energy diagram represents the reaction A + B → C + energy?
1.
3.
2.
4.
_____28. Given the potential energy diagram of a chemical reaction:
Which arrow represents the potential energy of the products?
1. A
2. B
3. C
4. D
_____29. Assume that the potential energy of the products in a chemical reaction is 60 kilojoules. This
reaction would be exothermic if the potential energy of the reactants were
1. 20 kJ
2. 30 kJ
3. 50 kJ
4. 80 kJ
Model 3.
Joe has to walk over Mount
Tellurium every day to get to
and from work.
Mount Te
Joe
Key Questions
1. What is Joe’s goal? How does he reach his goal?
2. How could we make it easier for Joe to get from one side Mount Tellurium to the other? List as many
possibilities as you can.
3. Which would be easier for Joe (require less of his energy) – walking over the mountain as shown in Figure A
OR walking through the mountain in Figure B? How do you know?
Figure A
Figure B
tunnel
Joe
Joe
4. If we think of Joe getting to the other side of the mountain as if it were a chemical reaction, we know that a
certain amount of energy – the activation energy—has to be put into the reaction by Joe to get himself to the
other side of the mountain. Which “reaction” would require less activation energy – Joe in Figure A or Joe in
Figure B? How do you know?
Catalysts speed up chemical reactions by enabling the reaction to happen by an alternate reaction pathway
that requires less activation energy than the “normal” (uncatalyzed) reaction pathway.
Model 4.
Figure A – Joe walking over the mountain
Figure B – Joe walking through the mountain
600. kJ/mol
350. kJ/mol
250.
250.
kJ/mol
kJ/mol
100.
Joe
tunnel
kJ/mol
100.
Joe
Reaction Progress
kJ/mol
Reaction Progress
Key Questions
1. On Figure A, label the following:
A. Joe’s potential energy before he walks over the mountain
B. Joe’s potential energy after he walks over the mountain
C. the activation energy needed to the other side of the mountain.
2. On Figure B, label the following:
A. Joe’s potential energy before he walks over the mountain
B. Joe’s potential energy after he walks over the mountain
C. the activation energy needed to the other side of the mountain.
3. Which potential energy diagram (Figure A or Figure B) has higher activation energy? How do you know?
4. Which potential energy diagram (Figure A or Figure B) represents a catalyzed reaction? How do you know?
5. How does Joe’s potential energy BEFORE he starts to the other side of the mountain in Figure A compare to
Joe’s potential energy BEFORE he starts to the other side of the mountain in Figure B?
6. How does Joe’s potential energy AFTER he reaches the other side of the mountain in Figure A compare to
Joe’s potential energy AFTER he reaches the other side of the mountain in Figure B?
7. Mathematically, what is the difference between the Joe’s potential energy BEFORE he starts to the other
side of the mountain and Joe’s potential energy AFTER he reaches the other side of the mountain?
Chemists call the difference between the potential energy of the products and the potential energy of the
reactants is called the heat of reaction. It is always calculated by subtracting the potential energy of the
products from the potential energy of the reactants. It is represented by H. A negative H value indicates
that a reaction is exothermic. A positive H value indicates that a reaction is endothermic.
8. Given the potential energy diagram shown below, calculate the value of H and determine if the reaction is
exothermic or endothermic.
Exercises
1. Explain how a catalyst may increase the rate of a chemical reaction.
2. Given the diagram to the right:
a. Does the diagram illustrate an exothermic or an endothermic
reaction? State one reason, in terms of energy, to support your
answer.
b. On the diagram provided, draw a dashed line to indicate a potential energy curve for the reaction if a
catalyst is added.
3. Describe how the potential energy diagram shown below will change if a catalyst is added.
4. Explain, in terms of the function of a catalyst, why the curves on the potential energy diagrams shown
below are different.
5. During a bread-making process, glucose is converted to ethanol and carbon dioxide, causing the bread
dough to rise. If this reaction is left to happen on its own, it takes longer than a human life span to occur!
However, when zymase, an enzyme produced by yeast, is added to bread dough, bread will rise in as short as
15 to 20 minutes.
What is the function of zymase?