chem 128 exam two march 02, 2006 dr. rod wolfcall schoonover name: This examination packet contains 07 pages, composed of 5 pages of questions, this cover sheet and a sheet of possibly useful information on the back. Be sure that you have all the pages. Partial credit may be given for partially correct answers; however, there is no obligation on the instructor's part to give partial credit. Once you begin, you are not allowed to leave the testing room until you are finished. If you do not finish by 01:15 pm, please remain seated until the end of the period. FORM krusty Write this on your scantron!!! part section points possible I objective 36 II short answer 26 III problems 38 total 100 I understand that if I am caught copying off another person's test, using crib sheets, a cell phone or other outside material (including memory cards for calculators), or any other form of cheating, I will receive an F on this exam and may be expelled from the course. I pledge that the work herein is my own. signed
PART I: Multiple Choice. Circle the answer below AND put it on the SCANTRON. [3 points each] 01. The reaction POCl 3 (g) POCl(g) + Cl 2 (g) is at equilibrium. Which of the following statements describes the behavior of the system after POCl is added to the container? a) The forward reaction will proceed to establish equilibrium. b) The reverse reaction will proceed to establish equilibrium. c) The partial pressures of POCl 3 and POCl will remain constant while the partial pressure of Cl 2 increases. d) The partial pressure of Cl 2 remains steady while the partial pressures of POCl 3 and POCl increase. e) The partial pressure of Cl 2 will increase while the partial pressure of POCl decreases. 02. Nitrogen dioxide dissociates to nitric oxide and oxygen: 2 NO 2 (g) 2 NO(g) + O 2 (g); H = 114 kj Under which conditions would you expect to have the highest yield of oxygen at equilibrium? a) high temperature, small volume b) high temperature, large volume c) low temperature, small volume d) low temperature, large volume e) a rose-tinted flask with polka dots 03. The following exothermic reaction is at equilibrium in a sealed container: N 2 (g) + 3 H 2 (g) 2NH 3 (g). Which of the following actions will increase the value of the equilibrium constant, K c? a) adding a catalyst b) removing N 2 c) increasing the pressure d) increasing the temperature e) none of the above 04. Which of the following results in a decrease in the entropy of the system? a) O 2 (g, 300K) O 2 (g, 400K) b) H 2 O(s, 0 C) H 2 O(, 0 C) c) N 2 (g, 25 C) N 2 (aq, 25 C) d) NH 3 (, 30 C) NH 3 (g, -30 C) e) 2 H 2 O 2 (g, 100 C) 2 H 2 O(g, 100 C) + O 2 (g, 100 C) 05. You are given pure samples of ethane, C 2 H 6 (g), and toluene, C 7 H 8 ( ). What prediction would you make concerning their standard molar entropies at 298K? a) S ethane is greater than S toluene b) S ethane is less than S toluene c) S ethane is approximately equal to S toluene d) S ethane is approximately equal to 1/3 of S toluene e) both S ethane and S toluene are negative values 06. For a process for which S < 0, which of the following statements is true? a) The process will definitely be spontaneous if it is exothermic. b) The process will definitely be spontaneous if H < T S. c) The process is nonspontaneous. d) The process will definitely be spontaneous, regardless of H. e) The process will definitely be spontaneous if S surr > 0. 2
07. The system X(aq) + Y 2+ (aq) X 2+ (aq) + Y(aq) is at equilibrium. Which of the following statements is necessarily true? a) I only b) II only c) III only d) I and II e) II and III I. G = 0 II. S surr = 0 III. E = 0 08. Which of the following represents the correctly balanced reaction for the voltaic cell Al(s) Al 3+ (aq) Ni 2+ (aq) Ni(s)? a) 2 Ni 2+ (aq) + 3 Al(s) 2 Ni(s) + 3 Al 3+ (aq) b) 3 Ni 2+ (aq) + 2 Al(s) 3 Ni(s) + 2 Al 3+ (aq) c) Ni(s) + Al 3+ (aq) Ni 2+ (aq) + Al(s) d) 3 Ni(s) + 2Al 3+ (aq) 3 Ni 2+ (aq) + 2 Al(s) e) 3 Ni 2+ (aq) + 2 Al 3+ (aq) 3 Ni(s) + 2 Al(s) 09. Which of the following statements is true for the reaction: 3CuO(s) + 2NH 3 (aq) N 2 (g) + 3H 2 O(l) + 3Cu(s)? a) CuO is the oxidizing agent and is reduced. b) CuO is the oxidizing agent and is oxidized. c) NH 3 is the oxidizing agent and is reduced. d) NH 3 is the oxidizing agent and is oxidized. e) This is not a redox reaction. 10. When metal X(s) is placed in a solution of metal ions Y 2+ (aq), a reaction occurs, producing X 2+ (aq) in solution. When metal Y(s) is placed in acid solution, gas bubbles form on its surface. When metal X(s) is placed in a solution of metal ions Z 2+ (aq), no reaction occurs. Which of the following reactions would not occur spontaneously? a) Z(s) + 2H + (aq) H 2 (g) + Z 2+ (aq) b) Z(s) + X 2+ (aq) Z 2+ (aq) + X(s) c) Y(s) + Z 2+ (aq) Y 2+ (aq) + Z(s) d) X(s) + 2H + (aq) H 2 (g) + X 2+ (aq) e) Y(s) + 2H + (aq) H 2 (g) + Y 2+ (aq) ------------------------------------balancing REDOX reactions-----------------------------------------------weeeeee!!!- 11. Consider the following redox equation: I (aq) + NO 3 (aq) NO(g) + I 2 (s) [acidic] When the equation is balanced with smallest whole number coefficients, what is the coefficient for the iodide ion? a) 2 b) 3 c) 6 d) 8 e) none of the above 12. Consider the following redox reaction: Br 2 ( ) BrO 3 (aq) + Br (aq) [basic] When the equation is balanced with smallest whole number coefficients, what is the coefficient for the bromide ion? a) 4 b) 5 c) 10 d) 11 e) 22 3
Part II: Short Answers. [1 point each] [08] 01. Fill in the blank. the law of thermodynamics says that if A B and B C then A C a synonymous term for a voltaic cell is a(n) cell the name of the constant whose value is equal to the charge on one mole of electrons [04] 02. Consider the following endothermic equilibrium: A(g) + 2B(g) X(s) + 2Y(g). All gases are orange except for B, which is colorless. Predict the effect of each of the following actions on the appearance of the reaction vessel. a) decrease the volume more orange less orange no change b) increase the temperature more orange less orange no change c) add X more orange less orange no change d) remove Y more orange less orange no change [04] 03. For the following reactions/processes, predict the sign of the following: a) freezing of water at 1 C G is: positive negative zero not known b) dissolving NaCl in water at 25 C H is: positive negative zero not known c) ice freezing at -1 C S surr is: positive negative zero not known d) a lion chasing an antelope G is: this is a really stupid question [02] 04. Will there be a reaction if a piece of aluminum is dropped into PbCl 2 (aq)? yes no Will there be a reaction if a piece of cobalt is dropped into Mg(NO 3 ) 2 (aq)? yes no [08] 05. Some yesno s. I keep trying to get out and they keep pulling me back in!! a) Entropy equals disorder. yes no b) An unpeeled banana is an example of an open system. yes no c) For a spontaneous process, S must be greater than 0. yes no d) A salt bridge provides a path for electrons to move between the anode and cathode compartments. yes no e) For a spontaneous process, G represents the maximum work obtainable from the system. yes no f) At a given temperature, G, E and K are all constants. yes no g) A molecule with a lot of ways to disperse energy will tend to have a large entropy. yes no h) The " " in G means all gases are 1 atm, all solutions are 1 M and the temperature is 25 C. yes no 4
Part III: PROBLEMS. Show all your work. [05] 01. At 50 C, K C = 0.0500 for the following reaction. Calculate the number of moles of I 2 at equilibrium if 3.00 moles of I 2 and 4.00 moles of Cl 2 are initially put into a 1-L vessel. I 2 (g) + Cl 2 (g) 2 ICl(g) [10] 02. Consider the following setup involving half-cells A, B and C. Switch S, when set to position a, connects A and B while position b connects B and C. Glass tubes, filled with KNO 3 (aq), are used as salt bridges and there are two voltmeters which, when connected, will measure the voltage. For S set to position a: a) What is the half-reaction that occurs at the cathode? mol b) What will be the measured voltage (on the left voltmeter)? V c) Will the manganese electrode lose or gain mass? lose gain d) Which electrode will be negative? nickel manganese For S set to position b: d) Which direction will the K + ions move through the salt bridge? left right f) What will be the measured voltage (on the right voltmeter)? V [05] 03. Calculate the voltage produced by the voltaic cell: Cr(s) Cr 3+ (0.100 M) Sn 4+ (2.0 M), Sn 2+ (0.10 M) Pt(s) V 5
[05] 04. From the table of values at right, determine the boiling point of bromine (in C): H f [kj/mol] G f [kj/mol] S [J/mol. K] Br 2 (l) 152.23 Br 2 (g) 30.91 3.13 245.38 Br(g) 111.9 82.40 174.90 Br (g) 218.9 102.9 80.71 C [07] 05. The equilibrium constant is 2.8 E5 at 15 C for the reaction: A(aq) + 2 B(s) 3 Y(aq) + Z(aq) a) Calculate G. b) Calculate G for the condition where all solutions on the left are 0.250 M and those on the right are 0.450 M. kj/mol [05] 06. Consider the reaction A(g) 2B(g). At 25 C, experiments show that [A] = 2.5 mol/l and [B] = 4.5 mol/l at equilibrium. When the temperature is raised to 50 C, the equilibrium shifts so that [B] = 5.5 mol/l. Calculate H for the reaction. kj/mol [01] 07. Draw a picture showing how much you love entropy. mol/l 6
R = 0.08206 L.atm/mol.K R = 8.314 J/mol.K R = 8.314 E-3 kj /mol K F = 96485 C/mol T K = T C + 273.2 1 atm = 760.0 mmhg 1 cm 3 = 1 ml!g =!H " T!S!G =!G o + RT lnq!g o = "RT ln K!G = "nfe!!k P = K c ( RT )!n gas E = E o " RT nf lnq E = E o " 0.0592V n # logq S = k lnw!! ln K & 2 $ % K 1 ' ( = "!H o # 1 " 1 & R $ % T 2 T 1 ' ( 7