Chemistry 105 B Practice Exam 3 Dr. Jessica Parr First Letter of last Name PLEASE PRINT YOUR NAME IN BLOCK LETTERS Name: Last 4 Digits of USC ID: Lab TA s Name: Question Points Score Grader 1 18 2 14 3 10 4 20 5 10 6 10 7 8 8 10 Total 100 Please Sign Below: I certify that I have observed all the rules of Academic Integrity while taking this examination. Signature: Instructions: 1. You must show work to receive credit. 2. If necessary, please continue your solutions on the back of the preceding page (facing you). 3. YOU MUST use black or blue ink. (No pencil, no whiteout, no erasable ink.) 4. There are 8 problems on 11 pages. Please count them before you begin. A periodic table and some useful equations can be found on the last page. 5. Good luck!! =) 1
1. (18 pt) Answer the following multiple choice questions. i. For which process is ΔS sys negative? a. evaporation of 1 mol of liquid CCl 4 b. mixing 5 ml of ethanol with 25 ml of water c. freezing of liquid benzene to solid benzene d. raising the temperature of 100 g of He from 245 K to 295 K e. grinding a large crystal of KCl to powder ii. A chemical reaction will be spontaneous at all temperatures if it is accompanied by: a. increasing enthalpy and increasing entropy b. decreasing enthalpy and increasing entropy c. increasing enthalpy and decreasing entropy d. decreasing enthalpy and decreasing entropy iii. The second law of thermodynamics states that: a. the entropy of a perfect crystal is zero at 0 K b. the entropy of the universe is constant c. the energy of the universe is increasing d. the entropy of the universe is increasing e. the energy of the universe if constant iv. Which of the following processes decreases the atomic number by 1? a. gamma-ray production b. electron capture c. beta-particle production d. positron production 2
e. at least two of the above processes decrease the atomic number by 1 v. The number of half-lives needed for a radioactive element to decay to about 6% of its original activity is (choose the nearest number): a. 2 b. 3 c. 4 d. 5 e. 6 vi. If a tree dies and the trunk remains undisturbed for 13,750 years, what percentage of the original 14 C is still present? (half-life of 14 C = 5730) a. 5.20% b. 19.0 % c. 2.20 % d. 45.0 % 3
2. (14 pt) Consider the reaction: 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g), at 25 o C for which the following data are relevant: N 2 O 5 NO 2 O 2 o ΔH f 11.29 kj/mol 33.15 kj/mol 0 kj/mol S o 355.4 J/K*mol 239.9 J/K*mol 204.8 J/K*mol a. Calculate ΔH o for the reaction. b. Calculate ΔS o for the reaction. c. Calculate ΔG o for the reaction. d. Calculate ΔG for the reaction when the partial pressures of all the gases are 0.75 atm at 25 o C. e. The reaction is allowed to proceed until all substances involved have reached equilibrium concentrations. Under these conditions what is ΔG for the reaction? 4
3. (10 pt) Consider the reaction: H 2 (g) + Br 2 (g) 2 HBr (g) where ΔH o = -103.8 kj/mol. In a particular experiment, equal moles of H 2 (g) at 1.00 atm and Br 2 (g) at 1.00 atm were mixed in a 1.00 L flask. at 25.0 o C and allowed to reach equilibrium. Then the molecules of H 2 at equilibrium were counted using a very sensitive technique, and 1.10 x 10 13 molecules were found. For this reaction, calculate the values of K, ΔG o, and ΔS o. 5
4. (20 pt) Answer the following multiple choice questions based on the galvanic cell shown here: Pt Pt - 2+ + MnO 4, Mn, H - MnO 4 (0.10 M) Mn 2+ (0.20 M) H + (0.10 M) porous disk Cr 3+ 2- +, Cr 2 O 7, H Cr 3+ (0.40 M) - Cr 2 O 7 (0.30 M) H + (0.10 M) The standard reduction potentials are as follows: MnO 4 - (aq) + 8 H + (aq) + 5 e - Mn 2+ (aq) + 4 H + (aq) Cr 2 O 7 2- (aq) + 14 H + (aq) + 6 e - 2 Cr 3+ (aq) + 7 H 2 O (l) ε o = 1.51 V ε o = 1.33 V i. What is the oxidation state of Cr in Cr 2 O 7 2-? ii. Under standard conditions, when the current is allowed to flow, which species is oxidized? iii. Under standard conditions, when the current is allowed to flow, which species is reduced? iv. What is the value of ε o cell? 6
v. What is the value of the equilibrium constant at 25 o C for the net spontaneous cell reaction? vi. In the balanced cell reaction what is the stoichiometric coefficient for H +? vii. How many electrons are transferred in the balanced reaction? viii. What is the value of Q, the reaction quotient, for this cell reaction? ix. In which direction do electrons flow in the external circuit? a. left to right b. right to left c. no current flows; the cell is at equilibrium d. cannot be determined x. What is the cell potential at 25 o C as read on a digital voltmeter? 7
5. (10 pt) Fill in the following blanks: 191 a. 75Re 0 1e + 235 1 139 1 b. 92U + 0n 53I + 20n + 235 1 139 1 c. 92U + 0n 56Ba + 30 n + 241 4 1 d. 95 Am+ 2He 2 0n + 40 e. K + 0 19 1e 6. (10 pt) It takes 15 kwh (kilowatt-hours) of electrical energy to produce 1.0 kg of aluminum metal from aluminum oxide by the Hall-Heroult process. Compare this to the amount of energy necessary to melt 1.0 kg of aluminum metal. Why is it economically feasible to recycle aluminum cans? Must show work to support your answer! (ΔH fus = 10.7 kj/mol, 1 watt = J/s) 8
7. (8 pt) The standard free energies of formation of several aqueous species are given in the following table. What is the standard reduction potential of methanoic acid in aqueous solution? HCOOH (aq) + 4 H + (aq) + 4 e - CH 3 OH (aq) + H 2 O (l) ΔG o (kj/mol) H + (aq) 0 H 2 O (l) -237 CH 3 OH (aq) -163 HCOOH (aq) -351 e- 0 9
8. (10 pt) Use the atomic and particle masses to determine the binding energy in MeV per nucleon for the 4 He nucleus and the 8 Be nucleus. Atom or particle Mass in amu Electron 5.4858 x 10-4 Proton 1.00728 Neutron 1.00866 4 He 4.00260 8 Be 8.00531 Which is more stable two 4 He nuclei or a 8 Be nucleus, why? 10
I 1 H 1.008 3 Li 6.941 11 Na 22.99 19 K 39.10 37 Rb 85.47 55 Cs 132.9 87 Fr (223) II III IV V VI VII 4 Be 9.012 12 Mg 24.31 20 Ca 40.08 38 Sr 87.62 56 Ba 137.3 88 Ra 226.0 21 Sc 44.969 39 Y 88.91 57 La 138.9 89 Ac 227.0 22 Ti 47.88 40 Zr 91.22 72 Hf 178.5 104 Rf 261) 23 V 50.94 41 Nb 92.91 73 Ta 180.9 105 Db (262) 24 Cr 51.996 42 Mo 95.94 74 W 183.85 106 Sg (263) 25 Mn 54.9380 43 Tc (99) 75 Re 186.2 107 Bh (262) 26 Fe 55.847 44 Ru 101.1 76 Os 190.2 108 Hs (265) 27 Co 58.9332 45 Rh 102.9 77 Ir 192.2 109 Mt (268) 28 Ni 58.69 46 Pd 106.4 78 Pt 195.09 110 Ds (271) 29 Cu 63.546 47 Ag 107.9 79 Au 197.0 111 Rg (272) 30 Zn 65.377 48 Cd 112.4 80 Hg 200.6 112 Uub 5 B 10.81 13 Al 26.98 31 Ga 69.72 49 In 114.8 81 Tl 204.4 113 Uut 6 C 12.01 14 Si 28.09 32 Ge 72.59 50 Sn 118.7 82 Pb 207.2 114 Uuq 7 N 14.01 15 P 30.97 33 As 74.921 6 51 Sb 121.8 83 Bi 209.0 115 Uup 8 O 16.0 16 S 32.07 34 Se 78.96 52 Te 127.6 84 Po (209) VIII 9 F 19.00 17 Cl 35.45 35 Br 79.90 53 I 126.9 85 At (210) 2 He 4.003 10 Ne 20.18 18 Ar 39.95 36 Kr 83.80 54 Xe 131.3 86 Rn (222) Lanthanides Actinides 58 Ce 140.1 90 Th 232.0 59 Pr 140.9 91 Pa 231.0 60 Nd 144.2 92 U 238.0 61 Pm (145) 93 Np 237.0 62 Sm 150.4 94 Pu (244) 63 Eu 151.96 95 Am (243) 64 Gd 157.3 96 Cm (247) 65 Tb 158.9 97 Bk (247) 66 Dy 162 98 Cf (251) 67 Ho 164.9 99 Es (252) 68 Er 167.3 100 Fm (257) 69 Tm 168.9 101 Md (258) 70 Yb 173. 0 102 No (259) 71 Lu 175 103 Lr (26) Useful Equations and Constants: F = 96,485 C/mol e - ΔG o = -RT ln(k) R = 8.314 J/K*mol = 0.08206 L*atm/K*mol ΔG o = ΔH o - TΔS o ΔS surr = -ΔH/T ΔS univ = ΔS sys + ΔS surr ΔG = ΔG o + RT ln(q) N A = 6.022 x 10 23 ε = ε o (RT/nF)lnQ ε = ε o (0.0591/n)logQ ΔG = -nfε ΔE = mc 2 c = 3.00 x 10 8 m/s 1 amu = 1.6605 x 10-27 kg 1A = 1C/s 1 MeV = 1.60 x 10-13 J ln[ A ] = kt + ln[ A Rate = k[a] ] 0 t 1 / 2 = 0.693 k 11