Chem 128, Exam III April 25, 2002

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1 I. (41 points) A. (4 points) Write the equilibrium equation and K sp expression for Co 2 S 3. Pay attention to chemical state designations, charges and stoichiometric coefficients! equilibrium equation: K sp expression: B. (14 points) K sp for PbCrO 4 (M = g/mol) at 25 Cis2x mg of PbCrO 4 is added to 1.0 L of H 2 O at this temperature. 1. How many mg of the of the PbCrO 4 dissolves? 2. How many mg of the of the PbCrO 4 remains undissolved? 3. If mg of PbCrO 4 are added to 1.00 L of M Na 2 CrO 4 at 25 C, how many mg of the of the PbCrO 4 dissolves? 4. A mixture containing mg of PbCrO 4 in 1.0 L of water is heated to 95 C. A saturated solution is obtained, i.e., all the PbCrO 4 just dissolves. What is K sp for PbCrO 4 at 95 C? 1

2 C. (14 points) Consider the equilibrium curve for AgCl shown on the right. Questions 1 5 refer to a solution at point A on the curve. Answer the questions below, using Y (for true ), F (for false ), or MI (for more information required ) in the blanks provided. 1. The solution is saturated and at equilibrium. 2. Addition of NaCl increases the concentration of Cl in solution. 3. Addition of NaCl increases the concentration of Ag + in solution. 4. Addition of Ag + results in the precipitation of AgCl. 5. Addition of solid NaNO 3 to the solution without change in volume does not change [Ag + ]or[cl ]. 6. At equilibrium, [Ag + ] is always equal to [Cl ]. 7. The molar solubility s, of AgCl can be expressed in terms of K sp as K sp =2s 2. D. (9 points) The addition of OH to a saturated solution of PbCl 2 increases its solubility due to the formation of the complex ion [Pb(OH) 3 ]. 1. Write a balanced net ionic equation for this process. 2. The K sp for PbCl 2 =1.7x10 5 ;K f for [Pb(OH) 3 ] = 3.8 x Calculate K for this process. 2

3 II. (42 points) A. (12 points) Write your answers on the blanks provided. Use LT, GT, and EQ for less than, greater than and equal to respectively, when appropriate. 1. At what temperature (in K) is G = H? (1). 2. What is the value of Q when G = G? (2). 3. S o for steam is (3) (LT, GT, EQ) S o for water. 4. The value for G at equilibrium is (4). 5. For a reaction, H < 0, S < 0, and G = 0 at 452 K. The reaction is spontaneous at a temperature (5) (LT, GT, EQ) 452 K. 6. S o for Ag(s) at 25 C is (5) (<, >, =)0. B. (6 points) Answer the questions below, using LT (for is less than ), GT (for is greater than ), EQ (for is equal to ), or MI (for more information required ) in the blanks provided. The reaction given below takes place in a cylinder which feels warm to the touch after the reaction is complete. A 2 (g) + B 2 (g) 2AB(s) 1. At all temperatures, S At all temperatures, H At all temperatures, G 0. C. (6 points) Given the following molar entropies of formation (J/mol-K) H 2 O(l) = 69.9 NH 3 (g) = NO (g) = O 2 (g) = Calculate S for the following reaction: 4NH 3 (g)+5o 2 (g) 4NO(g)+6H 2 O(l) 3

4 D. (18 points) For the reaction 2 CuCl (s) + 2 OH (aq) Cu 2 O (s) + 2 Cl (aq) H = 54.3 kj, and S J/K. 1. Calculate G at 200 C. 2. At 165 C, G for the reaction is kj. What is K for the reaction at 165 C? 3. Calculate Q and ln Q for the reaction if [OH ] = M and [Cl ] = M Q= lnq= 4. At 165 C, and the conditions given in (3), what is G for the reaction. Is it spontaneous? 4

5 III. (42 points) A. (4 points) Write a balanced chemical equation for the cell represented by Sn Sn 2+ MnO 4 Mn2+ (Pt) The MnO 4 Mn2+ half cell is in acidic medium. B. (16 points) Use the following data to answer the questions below: Al 3+ (aq) + 3 e Al (s) E red = 1.68 V Fe 2+ (aq) + 2 e Fe (s) E red = V Tl + (aq) + e Tl (s) E red = V Cu 2+ (aq) + 2 e Cu (s) E red = V Au 3+ (aq) + 3 e Au (s) E red = V Write your answers on the blanks provided with the best word or phrase from the following list: greater than less than equal to true false unknown electrolytic voltaic oxidation reduction Nernst Faraday You may use a term more than once. 1. It is (1) that aluminum metal can reduce Au Cu Cu 2+ Au 3+ Au is a(n) (2) cell. 3. The E ox for gold metal is (3) 1 V. 4. E ox forcuis (4) E red for Tl+. 5. The ability of Tl to reduce Cu 2+ is (5) the ability of Fe to reduce Cu The (6) constant has the value 96,480 J/mol-V. Using the same data write the number required in the blank to make the statement true. 7. E for the cell Fe Fe 2+ Au 3+ Au is (7). 8. The coefficient for Tl in the balanced equation for the cell Tl Tl + Al 3+ Al is (8). 5

6 C. (15 points) Consider the following reaction at 25 C: 2NO 3 (aq) + 3 Cu (s) + 8 H+ (aq) 2 NO (g) + 3 Cu 2+ (aq) + 4 H 2 O 1. What is n? E = V 2. What is G? 3. What is K? 4. What is Q for the following conditions: [NO 3 ] = 2.34 M, ph = 0, Cu =2g,[Cu2+ ] = M, NO = 1.00 atm 5. What is E under the conditions of (4)? 6

7 D. (7 points) An electrolytic cell is made up of a g strip of copper in M Cu(NO 3 ) 2 and a g strip of Cr in M Cr(NO 3 ) 3. The overall reaction is: 3 Cu (s) + 2 Cr 3+ (aq) 3Cu 2+ (aq) + 2 Cr(s) E = V An external battery provides 5 amperes for 4 hours and 10 minutes with 100% efficiency. 1. How many coulombs are supplied by the battery? 2. What is the mass of the chromium strip after the battery is disconnected? IV. (25 points) The last page of this exam packet is a schematic diagram for the qualitative analysis of Groups I, II, and III ions. Tear that page out and write your answers to the questions on the blanks below. The numbers below correspond to the answers to the circled numbers on the diagram

8 BONUS (15 points) All or nothing. The bonus should be done only after you have completed the main part of this exam and checked your work for errors. The time alloted for this exam does not include time for the bonus. Trial and error solutions will not be accepted. K sp for silver chloride at 25 C is x At35 C, K sp is x What are H and S for the dissociation of AgCl? 8

Chem 128, Final Exam May 11, 2003

Chem 128, Final Exam May 11, 2003 I. (70 points) This part of the final corresponds to Exam I. It covers the material in Chapters 10, 11, and 12. A. (10 points) Answer the questions below, using LT (for is less than ), GT (for is greater