Ch 20 Practice Problems - KEY

Chem 121 Fritsch

Ch 20 Practice Problems - KEY The following problems are intended to provide you with additional practice in preparing for the exam. Questions come from the textbook, previous quizzes, previous exams, and other sources. A solutions manual is supplied in a separate document. 1. When the concentrations of reactant molecules are increased, the rate of reaction increases. The best explanation for this phenomenon is that as the reactant concentration increases, A. the average kinetic energy of molecules increases. B. the frequency of molecular collisions increases. C. the rate constant increases. D. the activation energy increases. E. the order of reaction increases. Answer: B 2. With respect to the reaction profile diagram, which choice correctly identifies all the numbered positions?

2 3 4 A. transition state intermediate product B. transition state intermediate product C. transition state activation energy catalyst D. intermediate transition state product E. intermediate intermediate transition state

Answer: B

Chem 121 Fritsch

3. In the General Chemistry Laboratory you and your lab partners seek to determine overall rate law for the reaction of iodine monochloride with hydrogen which has the following overall reaction:

2ICl + H2 I2 + 2HCl Determine the rate law and calculate the value of the rate constant, k.

Expt [ICl]0 (M) [H2]0 (M) Initial Rate

(M•s-1)

1 0.1004 0.1001 1.48 x 10-3

2 0.2002 0.0998 6.04 x 10-3

3 0.0999 0.0493 1.51 x 10-3

4 0.3009 0.1997 1.37 x 10-2

Chem 121 Fritsch

4. Use the data at the right that were collected in a study of the isomerization of cis-butene to trans-butene. Analysis of the data gave a straight line relationship when the ln [cis-butene] was plotted versus time.

Calculate the concentration at 95 s.

time (s) [cis-butene] (M)

0 0.0894 5 0.0765

10 0.0621 15 0.0527 20 0.0446 25 0.0381 30 0.0317 40 0.0231

Chem 121 Fritsch

5. The degradation of iodide by hypochlorous acid relies on a two-step mechanism where the first elementary step is an equilibrium followed by a rate determining step as shown below.

1

fast, Kc = 2.98 x 10-4

2

slow

a. Write the rate laws for each of the elementary step.

b. Using a fast, pre-equilibrium analysis, show that the rate law for the production of chloride is overall first order.

Chem 121 Fritsch

6. Beginning with the rate law for first-order kinetics, derive the equation for half-life for first-order kinetics. Be explicit with each step.

Start here End here

Chem 121 Fritsch

7. Answer the questions below regarding reaction profiles A and B.

Which profile has more transition states? A

Which profile will have the slower first reaction step? A

In which profile will the return of products to reactants occur at the fastest rate?

A

Which profile has the slowest step converting an intermediate to final products?

B

Which profile is appropriate for an endothermic reaction? A

Chem 121 Fritsch

8. The data at the right were collected in a study of the photochemical degradation of ozone, O3, by light. Analysis of the data gave a straight line relationship when the [O3]-1 was plotted versus time.

a. Calculate the concentration at 523 s.

[A]t = 7.101 x 10-4 M

time (s) [O3] (M)

0 2.98 x 10-3

50 2.40 x 10-3

100 1.90 x 10-3

150 1.60 x 10-3

200 1.35 x 10-3

250 1.18 x 10-3

300 1.07 x 10-3

400 8.65 x 10-4

Chem 121 Fritsch

9. The reaction below was studied in aqueous solution and the data in

the table were collected. The concentrations were held constant

between experiments, [C2H5I]0 = 0.00203 M and [OH-]0 = 1.413M. (The

equation below represents the one elementary step in the reaction.)

C3H7I(aq) + OH-(aq) C3H7OH(aq) + I-(aq)

What is the activation energy, Ea, for the reaction in kJ/mol?

Expt # T (°C) k (s-1)

1 12.2 0.0616

2 23.7 0.200

3 42.5 1.20

4 68.8 10.8

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10. Find the activation energy, Ea, and collision frequency, A, from the data provided below:

Answer: Ea = 69.3 kJ/mol

A = 1.24 x 1013 s-1

11. For the reaction below, at a particular time, [BrO3]/t = 1.5 102 M/s. What is [Br]/t at the

same instant?

BrO3 + 5Br+ 6H+ 3Br2 + 3H2O

Answer: 7.5 102 M/s 12. For the overall chemical reaction shown below, which one of the following statements can be rightly assumed?

2H2S(g) + O2(g) 2S(s) + 2H2O(l) A. The reaction is third-order overall. B. The reaction is second-order overall. C. The rate law is, rate = k[H2S]2 [O2]. D. The rate law is, rate = k[H2S] [O2]. E. The rate law cannot be determined from the information given. Answer: E

Chem 121 Fritsch

13. When the concentrations of reactant molecules are increased, the rate of reaction increases. The best explanation for this phenomenon is that as the reactant concentration increases, A. the average kinetic energy of molecules increases. B. the frequency of molecular collisions increases. C. the rate constant increases. D. the activation energy increases. E. the order of reaction increases. Answer: B

14. The thermal decomposition of acetaldehyde, CH3CHO CH4 + CO, is a second-order reaction. The

following data were obtained at 518C inside a 1.164 L flask. Based on the data given, what is the half-life for the disappearance of acetaldehyde?

time (s) Pressure CH3CHO (mm Hg)

0 364

42 330

105 290

720 132

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15. Use the following data to determine the rate law for the reaction

2NO + H2 N2O + H2O

Expt # [NO]0 (M) [H2]0 (M) Initial rate

(M/s) (10-2)

1 0.205 0.0649 1.461

2 0.210 0.2621 1.459

3 0.419 0.0651 5.839

16. A first-order reaction has a rate constant of 7.5 103 s-1. Find the time required for the reaction to convert 60% of A to B.