Kinetics. 1. Consider the following reaction: 3 A 2 B How is the average rate of appearance of B related to the average rate of disappearance of A?

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1 Kinetics 1. Consider the following reaction: 3 A 2 B How is the average rate of appearance of B related to the average rate of disappearance of A? A. [A]/ t = [B]/ t B. [A]/ t = (2/3)( [B]/ t) C. [A]/ t = (3/2)( [B]/ t) D. [A]/ t = (1/2)( [B]/ t) E. [A]/ t = (1/3)( [B]/ t) 2. The combustion of ethylene proceeds by the reaction: C 2 H 4 (g) + 3O 2 (g) 2CO 2 (g) + 2H 2 O(g) When the rate of disappearance of O 2 is.23 M s 1, what is the rate of appearance of CO 2 in M s 1? A..15 B..77 C..69 D..35 E Given the following kinetic data for the reaction, A + B + C products Experiment [A] (M) [B] (M) [C] (M) [A]/ t (M/s) The rate law for this reaction is: A. Rate = k[a][b][c] B. Rate = k[a] 2 [B] C. Rate = k[a] 2 [C] D. Rate = k[a][b][c] 2 E. None of these is the correct rate law.

2 4. The decomposition of ethanol (C 2 H 5 OH) on an alumina (Al 2 O 3 ) surface was studied at 6K. C 2 H 5 OH (g) C 2 H 4 (g) + H 2 O (g) Analysis of the data revealed the adjacent straightline plot with a slope of 4. x 1 5 M 1 s 1 1 [C 2 H 5 OH] If the initial concentration of C 2 H 5 OH was, 1.25 x 1-2 M, calculate the half-life for this reaction. A. 2. x 1 6 s B. 1.73x 1 4 s C x 1 3 s D s E. 16 s Time 5. Two substances A and B react with each other in such a way that A is 5% consumed in 33 minutes and 75% consumed in 66 minutes. Changing the concentration of B has no effect on the results. This reaction is: A. zero order in both A and B. B. 1st order in both A and B. C. 2nd order in both A and B. D. 1st order in A and zero order in B. E. 2nd order in A and zero order in B. 6. It requires 9. seconds for a first-order reaction to go 25.% of the way to completion. What is the half-life of the reaction? A. 18 seconds B. 217 seconds C. 5 seconds D seconds E. None of the above is within 5% of the correct answer.

3 7. A second order reaction is observed to be 1.31 times faster at 42. o C than it is at 3. o C. This means that the activation energy for the reaction is A. 236 J/mol B kj/mol C. 178 J/mol D kj/mol E. None of the above is within 5% of the correct answer. 8. Which of the following statements are correct? 1. A reaction is endothermic if the activation energy is greater for the reverse reaction than for the forward reaction. 2. A reaction is exothermic if the activation energy is greater for the reverse reaction than for the forward reaction. 3. The activated complex is what the reactants are called just before the chemical reaction occurs. 4. Formation of the activated complex is what requires the activation energy. A. l only B. 2 only C. l and 3 only D. 2 and 3 only E. 2 and 4 only 9. Below are five statements about the temperature dependence of a reaction rate constant, k. Which one statement holds true for most chemical reactions? A. k increases with temperature mainly because there are fewer collisions per second. B. k increases with temperature, mainly because a higher proportion of the collisions are energetic enough to produce reaction. C. k decreases with T, mainly because of the decrease in effectiveness of the attractive long-range intermolecular forces. D. k does not change with temperature. E. k increases or decreases with temperature, depending on whether the reaction is endothermic or exothermic.

4 1. If the reaction aa + bb cc is an elementary reaction, the most likely values of a and b are: A. 1 and 1 B. 1 and 2 C. 2 and 1 D. 2 and 2 E. insufficient information 11. Nitrogen monoxide is reduced by hydrogen to give water and nitrogen by the following reaction. 2 H 2 (g) + 2 NO(g) N 2 (g) + 2 H 2 O(g) For the following proposed mechanism Step 1 (fast) 2 NO(g) N 2 O 2 (g) Step 2 (slow) N 2 O 2 (g) + H 2 (g) N 2 O(g) + H 2 O(g) Step 3 (fast) N 2 O(g) + H 2 (g) N 2 (g) + H 2 O(g) A. N 2 O 2 is a transition state B. N 2 O 2 is an activated complex C. N 2 O 2 is an intermediate D. N 2 O 2 is a a catalyst E. N 2 O 2 is both a catalyst and a transition state. 12. Two mechanisms have been proposed for the gas phase reaction 2 NO(g) + O 2 (g) 2 NO 2 (g) Scheme I: 2 NO(g) N 2 O 2 (g) fast N 2 O 2 (g) + O 2 (g) 2 NO 2 (g) slow Scheme II: NO(g) + O 2 (g) NO 3 (g) fast NO 3 (g) + NO(g) 2 NO 2 (g) slow Which mechanism is consistent with the observed rate law rate = k[no] 2 [O 2 ] A. Scheme I only. B. Scheme II only. C. Neither Scheme I nor II. D. Both Scheme I and II. E. There is insufficient information to make this determination.

5 13. For the reaction 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) the following mechanism is proposed: N 2 O 5 (g) NO 2 (g) + NO 3 (g) fast NO 2 (g) + NO 3 (g) NO 2 (g) + NO(g) + O 2 (g) slow NO 3 (g) + NO(g) 2 NO 2 (g) fast The observed rate law that is consistent with this mechanism is A. first order in N 2 O 5. B. first order in NO 2 and first order in NO 3. C. first order in NO 2 and NO 3 and second order in N 2 O 5. D. second order in N 2 O 5. E. None of these is consistent with the above mechanism. 14. The following reaction is one step in the industrial manufacture of nitric acid: 4 NH 3 (g) + 5 O 2 (g) 6 H 2 O(g) + 4 NO(g) + heat The reason for using the Pt catalyst is A. to change the standard heat of reaction so as to make the process feasible. B. to increase the standard entropy change of the reaction, which will favor the forward reaction. C. to absorb the excess heat generated by the reaction. D. to increase the reaction rate. E. to eliminate the back reaction. 15 Enzymes speed up chemical reactions by (a) Lowering the energy of the reaction products. (b) Increasing the Arrhenius factor, A. (c) Lowering the activation energy, E a A. (a) only B. (a) and (b) C. (a) and (c) D. (b) and (c) E. (a), (b), and (c)

6 16. The decomposition of acetaldehyde, CH 3 CHO, can be catalyzed by I 2 (g). uncatalyzed reaction: E a = 19 kj/mol CH 3 CHO(g) CH 4 (g) + CO(g) catalyzed reaction: E a = 136 kj/mol CH 3 CHO(g) + I 2 (g) CH 3 I(g) + HI(g) + CO(g) CH 3 I(g) + HI(g) CH 4 (g) + I 2 (g) Which of the following reaction profiles is most appropriate for the catalyzed and uncatalyzed reactions? (The dotted lines are only a guide for your eye.) Heats of formation of the molecules are also given. Molecule CH 3 CHO(g) CH 4 (g) CO(g) H f 166 kj/mol 74.9 kj/mol 11.5 kj/mol Uncatalyzed catalyzed

7 17. The data of [A]/[A] 1 vs. time, shown below, was obtained for the first order reaction of the decomposition of A to yield B. Suppose the reaction was then carried out with the same [A] as above, but in the presence of a heterogeneous catalyst, such that the reaction mechanism still obeyed first order kinetics. If the rate of the catalyzed decomposition was found to increase by a factor of 2 with respect to the uncatalyzed reaction, what would the resulting plot of [A]/[A] vs. time look like for the catalyzed system with respect to the initial system? A C x 1 [A]/[A] B 1 D E The plots would be identical