CHEM 102 Winter 10 Exam 2(a)

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1 CHEM 102 Winter 10 Exam 2(a) On the answer sheet (scantron) write your Name, Student ID Number, and Recitation Section Number. Choose the best (most correct) answer for each question AND ENTER IT ON YOUR ANSWER SHEET. b ± b 2 4 a c Potentially useful information: x = 2 a ln(c) = ln(c 0 ) kt 1. Please choose the letter a as your answer for this question. 1/C 1/C 0 = kt For gas-phase equilibria, all equilibrium constants are K p values. 2. At elevated temperatures, dinitrogen pentoxide decomposes to nitrogen dioxide and oxygen: 2N 2 O 5 (g) 4NO 2 (g) + O 2 (g) When the rate of formation of NO 2 is M/s, what is the rate of decomposition of N 2 O 5? a M/s b M/s c M/s d M/s e M/s 3. Which of the following factors will usually affect the rate of a reaction? a. the presence of a catalyst b. the properties (particularly, molecular structure and bonding) of reactants and products c. the concentrations of the reactants d. the temperature at which the reaction occurs e. all of the above 4. Which statement about a rate constant (k) is correct? a. Its units are independent of the order of the reaction. b. Its value does not change when a catalyst is added. c. Its value is independent of the concentration of the reactant(s). d. Its value is independent of temperature. e. It is calculated as the average change in the concentration of a reactant or product over some unit of time. Page 1 of 6

2 5. Which energy difference in the energy profile below corresponds to the activation energy for the forward reaction? a. x b. y c. x + y d. x - y e. y - x 6. The rate law for a given reaction is rate = k[reactant] 2, with k = M -1 min -1. If the initial concentration is M, what is the initial rate, with the correct units? a M min -1 b min -1 c M min -1 d M min -1 e min The rate constant for a particular second-order reaction is 0.47 M -1 s -1. If the initial concentration of reactant is 0.25 mol/l, it takes for the concentration to decrease to 0.13 mol/l. a. 7.9 s b. 1.4 s c. 3.7 s d. 1.7 s e s 8. For a reaction that is first-order with respect to reactant A, and third-order overall a. Rate = k [A] [B] b. Rate = k [A] c. Rate = k [A] [B] 2 d. Rate = k [A] [B] 3 e. Rate = 3 k [A] Page 2 of 6

3 9. Consider the reaction NO 2 + O 3 NO 3 + O 2 Use the data shown to determine the rate law for the reaction. Experiment [NO 2 ], M [O 3 ], M Rate, M s a. rate = k[no 2 ] [O 3 ] b. rate = k[no 2 ] 2 [O 3 ] c. rate = k[no 2 ] [O 3 ] 2 d. rate = k[no 2 ] e. rate = k[no 3 ] [O 3 ]/ [NO 2 ] [O 2 ] 10. The half-life for the first-order conversion of A to B is 2.22 hr. What is the rate constant? a hr -1 b hr -1 c hr -1 d hr -1 e hr The reaction below is first order in [H 2 O 2 ]: 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) A solution originally at M H 2 O 2 is found to be M after 54 min. The half-life for this reaction is min. a. 6.8 b. 18 c. 14 d. 28 e The mechanism for formation of the product X is: Step 1 A + B C + D (slow) Step 2 B + D X (fast) Which of the following statements is true? a. C is an intermediate. b. Step 2 likely has higher activation energy than Step 1. c. Step 1 likely is the rate-determining step. d. B is the activated complex. e. D is a catalyst in this reaction. Page 3 of 6

4 13. Which statement is true about a catalyst? a. It increases the activation energy involved in a reaction. b. It must be the same phase as the reactants. c. It is formed during an early step in the reaction and consumed in a later step. d. It is only effective at high temperatures. e. It changes the reaction pathway. 14. As the temperature of a reaction is increased, the rate of the reaction increases because the a. reactant molecules collide less frequently. b. reactant molecules collide with greater energy per collision. c. activation energy is lowered. d. orientation of the molecules changes. e. reaction is exothermic. 15. If a catalyst is added to a chemical reaction, the equilibrium yield of a product will be, and the time taken to come to equilibrium will be than before. a. higher; less b. lower; the same c. higher; the same d. the same; less e. lower; less 16. The equilibrium expression for the reaction below is. a. 2O 3 (g) 3O 2 (g) b. c. d. e. Page 4 of 6

5 17. If the equilibium constants for the two reactions and 2 HCl(g) H 2 (g) + Cl 2 (g) ICl(g) ½ I 2 (g) + ½ Cl 2 (g) are denoted K 1 and K 2, respectively, then the equilibrium constant for the reaction a. (K 1 /K 2 ) 2. b. K 1 /K 2 2. c. K 1 K 2. d. K 1 +K 2. e. K 1 K 2 /2. 2 HCl(g) + I 2 (g) 2 ICl(g) + H 2 (g) equals 18. Consider the reaction H 2 (g) + Br 2 (g) 2 HBr(g). If the partial pressures in an equilibrium mixture of H 2, Br 2, and HBr are atm, atm, and 5.07 atm, respectively, what is the value of K p for this reaction at this temperature? a b c d e K p = at 721 K for the reaction 2HI(g) H 2 (g) + I 2 (g) If the initial partial pressure of HI is 0.84 atm, what is the partial pressure of H 2 at equilibrium? a atm b atm c atm d atm e. 11 atm 20. The value of K for the reaction A(aq) B(aq) is At equilibrium a. the amount of A is slightly less than the amount of B. b. the amount of A is much larger than the amount of B. c. the amount of A is much less than the amount of B. d. the amount of A is very close to the amount of B. e. more information is needed to make any statement about the relative amounts of A and B. Page 5 of 6

6 21. To decide whether a reaction mixture is at equilibium, a student determines the value of Q, the reaction quotient, and finds that it is less than K. Therefore, the mixture is a. at equilibrium, since there is as much reaction as required. b. not at equilibrium, and will react to the right, to increase the amounts of products. c. not at equilibrium, and will react to the left, to increase the amounts of reactants. d. not at equilibrium, and will react to the right, to increase the amounts of reactants. e. not at equilibrium, and will react to the left, to increase the amounts of products. 22. For the reaction 2A(aq) 3B(aq) K = If the concentrations of A and B are equal, what is the value of that concentration? a M b M c M d M e M 23. An endothermic reaction that results in an increase in moles of gas will be most product-favored under conditions of pressure and temperature. a. high; high b. high; moderate c. high; low d. low; high e. low; low 24. Consider the equilibrium system C(s) + CO 2 (g) 2CO(g). If more C(s) is added, the equilibrium will ; if CO is removed the equilibrium will. a. shift to the left; shift to the left b. shift to the right; shift to the right c. shift to the right; shift to the left d. be unchanged; shift to the left e. be unchanged; shift to the right 25. Consider the following reaction at equilibrium: 2NH 3 (g) N 2 (g) + 3H 2 (g) ΔH = kj Le Châtelier's principle predicts that adding N 2 (g) to the system at equilibrium will result in a. a decrease in the concentration of NH 3 (g). b. a decrease in the concentration of H 2 (g). c. an increase in the value of the equilibrium constant. d. a lower partial pressure of N 2 (g). e. removal of all of the H 2 (g). Page 6 of 6

7 chem102w10exam2a Answer Section MULTIPLE CHOICE 1. ANS: A PTS: 0 2. ANS: D PTS: 1 3. ANS: E PTS: 1 4. ANS: C PTS: 1 5. ANS: A PTS: 1 6. ANS: D PTS: 1 7. ANS: A PTS: 1 8. ANS: C PTS: 1 9. ANS: A PTS: ANS: A PTS: ANS: B PTS: ANS: C PTS: ANS: E PTS: ANS: B PTS: ANS: D PTS: ANS: E PTS: ANS: B PTS: ANS: A PTS: ANS: C PTS: ANS: C PTS: ANS: B PTS: ANS: D PTS: ANS: D PTS: ANS: E PTS: ANS: B PTS: 1 Page 7 of 6