1. The rate constant in an elementary reaction's rate equation can be modeled by
the Arrhenius equation: !
.
a. The coefficient A is the frequency factor. What are the units of A for a firstorder reaction? Justify your answer mathematically. (2 points)
b. Describe how the value of A is affected by molecular collisions. (1 point)
c. Is the effect of the reaction temperature on A high, low, or moderate? Explain.
(1 point)
d. Is the effect of the reaction temperature on the term !
moderate? Explain. (1 point)
high, low, or
2. The diagram shows possible collisions between two molecules for a reaction.
!
a. Identify and explain two factors that contribute to the effectiveness of a
collision resulting in a product. (2 points)
b. Would the probability factor in the rate equation be greater or lower if the
particles colliding were individual atoms instead of molecules? Why? (2 points)
3. Consider an elementary reaction: X → Y + Z.
a. The energy diagram for the reaction is shown below.
!
What do each of the labeled energy differences on the diagram represent? (2
points)
b. Refer to the diagram to explain how the types of species that are present
change as the reaction progresses. (2 points)
!
c. The diagram below shows the Boltzmann distribution comparing the fraction of
collisions with the collision energy at a temperature T. (2 points)
Draw a curve on the diagram that shows how the fraction of collisions changes if
the temperature increases. Then explain the changes and predict how they will
affect the rate of the reaction.
4. Consider the reaction F2 + 2NO2 → 2NO2F.
The following are proposed elementary steps for the reaction:
!
a. Evaluate the proposed elementary steps. Why would these steps be more
reasonable than the complete reaction in one step? (2 points)
b. Does the reaction have any intermediates? If not, explain why. If so, identify
the intermediates. (1 point)
c. Consider the following possible rate laws.
!
Which is consistent with the proposed elementary steps? Explain why the rate
law you chose matches the steps and why the one you did not choose does not
match the steps. (1 point)
5. People who are lactose intolerant are unable to digest the sugar in milk, called
lactose. One solution is to use an enzyme called lactase that acts as a catalyst in
the reaction. The diagram demonstrates how this works.
!
a. The diagram below represents the energy if the reaction progresses without
the catalyst. Draw on the diagram how the energy would change with the
catalyst. (1 point)
!
b. Refer to the energy diagram to explain how lactase helps a person digest milk
sugar. (1 point)
6. The figure shows an ethene molecule and two hydrogen gas molecules.
!
The reaction of the molecules to produce ethane is C2H4(g) + H2(g) → C2H6(g).
A mechanism for this reaction is shown below:
!
a. Explain the mechanism shown in the figure. (2 points)
b. Explain how this mechanism increases the rate of the reaction. (2 points)
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