Name _________________________________________________ Date ____________________ Period ___
Homework
Chapter 14: Chemical Kinetics
Exercises: Sections 14.6 - 14.7 : Reaction Mechanisms; Catalysts
1.
(a) What is meant by the term molecularity and what is an elementary step?
(b) Why are termolecular elementary steps very rare?
(c) What is an intermediate in a mechanism and how is it different from a catalyst? (You will need
to read 14.7 to answer the last part of this question.)
2. What is the molecularity of each of the following elementary processes? Write the rate law for each.
(a) CH2(g) + Cl2(g)
→ N2(g) + ClO(g) : molecularity =
______________________
Rate Law:
CH2
(b) H2C−CH2(g) →
CH2═CH−CH3(g) : molecularity =
______________________
Rate Law:
1
2. Continued:
(c) SO3(g) →
SO2(g) + O(g) : molecularity =
______________________
Rate Law:
3. Using Figure 14.20 on page 606 of the textbook as your basis,:
(a) draw the energy profile for the bromide ion-catalyzed decomposition of hydrogen peroxide.
(b) Label the curve with the activation energies for reactions [14.30] and [14.31] found on page 606 of
the textbook.
(c) Notice that when Br – (aq) is added initially, Br2 accumulates to some extent during the reaction.
What does this tell us about relative rates of reactions [14.30] and [14.31]
2
4. Consider the following reaction:
H2(g) + 2ICl(g) → 2HCl(g) + I2(g)
The rate law for this reaction is first order in both H2 and ICl: Rate = k[H2][ICl]. Which of the following
mechanisms are consistent with the observed rate law? Explain your answer.
(a)
2 ICl(g) + H2(g) → 2 HCl(g) + I2(g) (termolecular reaction)
(b) H2(g) + ICl(g) → HI(g) + HCl(g)
(slow)
HI(g) + ICl(g) → HCl(g) + I2(g)
(fast)
(c) H2(g) + ICl(g) → HI(g) + HCl(g)
(fast)
HI(g) + ICl(g) → HCl(g) + I2(g)
(d) H2(g) + ICl(g) → HICl(g) + H(g)
(slow)
(slow)
H(g) + ICl(g) → HCl(g) + I(g)
(fast)
HICl(g) → HCl(g) + I(g)
(fast)
I(g) + I(g)
→ I 2(g)
(fast)
3
5.
(a) Based on the reaction profile shown above, how many intermediates are formed in the reaction: A → D?
(b) How many transition states are there?
(c) Which step is fastest?
(d) Is the reaction A → D exothermic or endothermic? Explain.
4
6. The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to
proceed by a two-step mechanism:
STEP 1:
H2O2 (aq) + I – (aq) → H2O (l) + IO – (aq)
(slow)
STEP 2:
IO –
(fast)
(aq)
+ H2O2 (aq) → H2O (l) + O2 (g) + I – (aq)
(a) Write the rate law for each of the elementary reaction of the mechanism.
(b) Write the chemical equation for the overall process.
(c) Identify the intermediate, if any, in the mechanism.
(d) Assuming that the first step of the mechanism is rate determining, predict the rate law for the overall
process.
7. The reaction 2 NO (g) + Cl2 → 2 NOCl (g) obeys the rate law, rate = k[NO]2[Cl2]. The following
mechanism has been proposed for this reaction:
STEP 1:
NO (g) + Cl2 (g) →
NOCl2 (g)
STEP 2:
NOCl2 (g) + NO (g) → 2 NOCl (g)
(a) What would the rate law be if the first step were rate determining?
(b) Based on the observed rate law, what can we conclude about the relative rates of the two steps?
5
8.
(a) Most heterogeneous catalysts of importance are extremely finely divided solid materials. Why is
particle size important?
(b) What role does adsorption play in the action of a heterogeneous catalyst?
9. NO catalyzes the decomposition of N2O, possibly by the following mechanism:
STEP 1:
NO(g) + N2O(g) → N2(g) + NO2(g)
STEP 2:
2NO2(g) → 2NO(g) + O2(g)
(a) What is the chemical equation for the overall reaction? Show how the two steps can be added to
give the overall equation.
(b) Why is NO considered a catalyst and not an intermediate?
(c) If experiments show that during the decomposition of N2O, NO2 does not accumulate in measurable
quantities, does this rule out the proposed mechanism? If you think not, suggest what might be
going on.
6