# Potential Energy Diagram Problem Set Part 1

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Rates, Temperature and Potential Energy Diagrams Problem Set
Problems:
1. Which diagram is described by each of
these statements?
a) an exothermic reaction that is unlikely to
occur at room temperature.
b) an endothermic reaction.
c) the activation energy (Ea) is greater than
the energy released (∆H)
d) a spontaneous exothermic reaction.
2. Draw a potential energy (Ep) diagram for a reaction in which ∆H = -80 kJ/mol and
Ea = +28kJ/mol. Label the axes, activation energy, ∆H, site of the activated
complex, reactants and products.
3. Using the potential energy diagrams for an endothermic and
exothermic reaction shown, choose the letter that best fits
each statement.
Reaction I
Reaction 2
a) Ep of the reactants
b) Ep of the products
c) ∆H of the reaction
d) activation energy of the
forward reaction
e) site of activated complex
f) Ep of the activated
complex
g) activation energy of the
reverse reaction
4. In the following reaction, the enthalpy change of the forward
reaction is ∆H = -36 kJ/mol and the activation energy for the
forward reaction is 73 kJ/mol.
A +
B
C + 36 kJ
a) Draw a potential energy diagram for the the reaction.
b) What is the activation energy of the reverse reaction?
5. The activation energy of a forward and reverse reaction are as follows:
i) C2H4 (g) + H2 (g)
ii) C2H6 (g)
C2H6 (g)
C2H4 (g) + H2 (g)
Ea = 180 kJ/mol
Ea = 317 kJ/mol
a) Draw a potential energy diagram for this reversible reaction.
b) Calculate the enthalpy change (∆H) for each reaction.
6. Compare these reactions:
i) C2H5Cl (l)
C2H4 (g) + HCl (g)
ii) C2H5Br (l)
C2H4 (g) + HBr (g)
Ea = 254 kJ/mol
Ea = 219 kJ/mol
Which of these two reactants would decompose more rapidly under the same
reaction conditions and temperature? Explain your response.
7. A reaction consists of two steps as follows:
Step 1:
Step 2:
A + B
A + E
C + E
2D
Ea (kJ/mol)
+ 45
+ 80
∆H (kJ/mol)
- 72
+28
a) Write the overall reaction equation.
b) What is meant by the “rate-determining step”?
c) Which of these steps is the rate-determining step? Why?
d) What is the effect on the overall rate of increasing the concentration of A? What
is the effect on the overall rate of increasing the concentration of B? Explain.
8. Examine the two Boltzmann distributions showing the
distribution of kinetic energy possessed by the reactant molecules
in two different reactions and the activation energy (Ea) for each
reaction.
a) Which reaction will be fastest at room temperature? Explain
b) When the temperature eis increased to 60oC, what will happen
to the rate of (i) Reaction 1 and (ii) Reaction 2? Why?
c) In which case does the activation energy requirement change
when the temperature is increased?
9. For the reaction:
H2 + Cl2
2 HCl + 184 kJ
the activation energy for the process is 156 kJ/mol. What is the activation energy for
the decomposition of HCl to produce H2 and Cl2?
10. Using the concepts of kinetic energy and surface area, explain why sugar cubes
can be used in hot coffee but granulated sugar is preferable for making iced tea.
11.Given the kinetic energy distribution curves and
threshold energy (Ea) for reactions A and B:
a) Which reaction will be faster at room temperature?
b) Which reaction will show the greatest increase in
the rate of reaction if the temperature is increased?
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