CHEM 121b Exam 3 Spring 1999

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1 Name SSN CHEM 121b Exam 3 Spring 1999 This exam consists of 10 multiple choice questions (each worth 3 points), and 5 written problems (points noted below for each). There are a total of 100 possible points. Please print your name and the Test Color on your Scantron sheet. Carefully mark the appropriate answer to each question on the Scantron sheet, and show any work in the space provided. Please hand in both the Exam and the Scantron sheet. Multiple Choice 1. Which of the following statements about equilibrium is not true? a. A system that is disturbed from an equilibrium condition responds in a manner to restore equilibrium. b. Catalysts are an effective means of changing the position of an equilibrium. c. Equilibrium in molecular systems is dynamic, with two opposing processes balancing one another. d. An exothermic reaction shifts toward reactants when heat is added to the reaction. 2. If, at a given temperature, the equilibrium constant for the reaction: H 2 (g) + Cl 2 (g) = 2 HCl(g) is K p, then the equilibrium constant for the reaction: can be represented as: a. 1\K p 2 b. K p 2 c. K p 1/2 d. K p 1/2 HCl(g) = 1/2 H 2 (g) + 1/2 Cl 2 (g) 3. For the chemical system, CO(g) + Cl 2 (g) = COCl 2 (g); K = 4.6 x 10 9 L/mol. How do the equilibrium concentrations of the reactants compare to the equilibrium concentration of the product? a. They are much smaller. b. They are much bigger. c. They are about the same. d. You can t tell from the information given. 4. For the same chemical system as in question 3 (above), if the concentration of the product were to double, what would happen to the equilibrium constant? a. It would double its value. b. It would become half its current value. c. It would quadruple its value. d. It would not change its value.

2 Chem 121b Exam 3 Page 2 5. For the same chemical system as in question 3 (above), what is the symbolic expression for K p in terms of K? a. K(RT) b. K/(RT) c. K(RT) 2 d. K/(RT) 2 6. Which of the following is true for a system whose equilibrium constant is relatively small? a. It will take a short time to reach equilibrium. b. It will take a long time to reach equilibrium. c. The equilibrium lies to the left. d. The equilibrium lies to the right. 7. Equilibrium is reached in a chemical reaction when: a. the rates of the forward and reverse reactions become equal. b. the concentrations of reactants and products become equal. c. the temperature shows a sharp rise. d. all chemical reactions stop. 8. In class, Shana showed the following aqueous equilibrium reaction as a chemical demonstration: Co(H 2 O) Cl = CoCl H 2 O (pink) (blue) Which statement below describes the change that the system will undergo if hydrochloric acid is added? a. It should become more blue. b. It should become more pink c. The equilibrium will shift to the right. d. Both a and c. 9. Which statement below describes the change that the system in question 8 (above) will undergo if water is added? a. More chloride anions will be produced. b. More water will be produced. c. The equilibrium will shift to the right. d. There will be less of the hydrated cobalt ion at the new equilibrium position. 10. Which statement below describes the change that the system in question 8 (above) will undergo if silver nitrate is added? (Hint: Recall that AgCl(s) is insoluble.) a. It should become more blue. b. It should become more pink. c. The silver ion will react with the CoCl 4 2. d. Nothing will change.

3 Chem 121b Exam 3 Page 3 Written Problems Write out the solution to each written problem. Please show all of your work. 11. Write the expression for K c and K p for the following reactions. In each case indicate whether the reaction is homogeneous or heterogeneous. (15 pts.) a. 3 NO(g) = N 2 O(g) + NO 2 (g) b. CH 4 (g) + 2 H 2 S(g) = CS 2 (g) + 4 H 2 (g) c. Ni(CO) 4 (g) = Ni(s) + 4 CO(g) d. Fe 2 O 3 (s) + 3 H 2 (g) = 2 Fe(s) + 3 H 2 O(g) e. 2 N 2 O 5 (g) = 4 NO 2 (g) + O 2 (g)

4 Chem 121b Exam 3 Page Consider the following reaction: CS 2 (g) + 4 H 2 (g) = CH 4 (g) + 2 H 2 S(g) The equilbrium constant K c is 0.28 M 2 at 900 C. What is K p at this temperature? (12 pts.) 13. At 400 K, K c = 7.0 for the equilibrium: Br 2 (g) + Cl 2 (g) = 2 BrCl(g). If 0.50 mol of Br 2 and 0.50 mol of Cl 2 are introduced into a 1.0-L container at 400 K, what will be the equilibrium concentrations of Br 2, Cl 2, and BrCl? (10 pts.)

5 Chem 121b Exam 3 Page At 250 C, the reaction: PCl 5 (g) = PCl 3 (g) + Cl 2 (g) has an equilibrium constant K c = 1.80 M. If M PCl 5 is added to a 5.00-L vessel, what are the concentrations of PCl 5, PCl 3, and Cl 2 at equilibrium? (15 pts.)

6 Chem 121b Exam 3 Page Consider the following equilibrium, for which H < 0: 2 SO 2 (g) + O 2 (g) = 2 SO 3 (g). How will each of the following affect the amount of SO 2 in the equilibrium mixture of the three gases? (Circle the appropriate answer for each.) (18 pts). a) O 2 (g) is added to the system. b) The reaction mixture is heated. c) The volume of the reaction vessel is doubled. d) A catalyst is added to the mixture. e) The total pressure of the system is increased by adding a noble gas. f) SO 3 (g) is removed from the system. Multiple Choice (30) Problem 11 (15) Problem 12 (12) Problem 13 (10) Problem 14 (15) Problem 15 (18) Exam 3 Total (100)