Chem 112, Fall 05 (Weis/Garman) Exam 4A, December 14, 2005 (Print Clearly) +2 points

Transcription

1 +2 pints Befre yu begin, make sure that yur exam has all 7 pages. There are 14 required prblems (7 pints each) and tw extra credit prblems (5 pints each). Stay fcused, stay calm. Wrk steadily thrugh yur exam. YOU MUST: Put yur name and student ID n the bubble sheet crrectly. Put all yur answers n the bubble sheet; nthing n this exam will be used fr grading. Sign the statement n the last page f the exam. Turn in bth the exam and bubble sheet when yu are dne. Gd Luck! Physical Cnstants: R = kj/(k ml) F = kj/(ml ev) N = 6.02 x F = 96,485 C/(ml e) Useful quatins: Binmial Distributin: (p + q) n ΔS = q rev /T q rev = ΔH sys ΔS sys = Σ S(prducts) Σ S(reactants) ΔS univ = ΔS surr + ΔS sys ΔG sys = ΔH sys - TΔS sys ΔG rxn = Σ ΔG f (prducts) Σ ΔG f (reactants) ΔG = ΔG + (RT)lnQ ΔG = -(RT)lnK w max = -ΔG sys page 1

2 Useful quatins (cnt d): = cathde - ande (1/2 reactins are expressed as reductins) = (RT/nF)lnQ = (reductin ½ rxn) + (xidatin ½ rxn) At T = K, = (( V)/n)lnQ w max = nf ΔG = -nf = (RT/nF)lnK Current (amperes, A) = [electric charge (culmbs, C)]/[time (secnds, s)] lectric charge (culmbs, C) = [n (mles f electrns transferred)] x [F (culmbs/mle e - )] TABLS f DATA All the Necessary Data are Presented with the Prblem page 2

3 1. Which f the fllwing linear alkane mlecules is likely t have the largest standard entrpy? A. CH 4 B. CH 3 CH 3 C. CH 3 CH 2 CH 3 D. CH 3 CH 2 CH 2 CH 3. CH 3 CH 2 CH 2 CH 2 CH 3 2. All f the fllwing statements cncerning entrpy are true XCPT A. entrpy values fr substances are greater than r equal t zer. B. entrpy is a thermdynamic state functin. C. entrpy is zer fr elements under standard cnditins. D. a psitive change in entrpy dentes a change tward greater disrder. 3. Which reactin is likely t have a negative change in entrpy? A. 2 NH 3 (g) N 2 (g) + 3 H 2 (g) B. CaCO 3 (s) CaO(s) + CO 2 (g) C. N 2 O 4 (g) 2 NO 2 (g) D. 2 CO(g) 2 C(s) + O 2 (g) 4. The disslutin f ammnium nitrate (NH 4 NO 3 ) ccurs spntaneusly in water at 25 C. As it disslves, the temperature f the water decreases. What are the signs f ΔH, ΔS, and ΔG fr this prcess? A. ΔH > 0, ΔS < 0, ΔG > 0 B. ΔH > 0, ΔS > 0, ΔG < 0 C. ΔH > 0, ΔS > 0, ΔG > 0 D. ΔH < 0, ΔS < 0, ΔG < 0 page 3

4 5. What is the crrect equilibrium cnstant expressin fr the balanced reactin shwn belw? C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(l) A. B. C. D. 6. At what temperatures will a reactin be spntaneus if ΔH = +158 kj and ΔS = +411 J/K? A. All temperatures belw 384 K B. All temperatures abve 384 K C. Temperatures between 158 K and 411 K D. The reactin will be spntaneus at any temperature.. The reactin will never be spntaneus. 7. Calculate ΔG fr the fllwing reactin at 425 C, at which temperature K eq = 56. H 2 (g) + I 2 (g) 2 HI(g) (R = J/K ml) A kj B kj C kj D kj kj page 4

5 8. In the reductin f HClO 4 t Cl - (HClO 4 Cl - ), the xidatin state f Cl changes by A. -7 B. +7 C. -8 D Write a balanced half-reactin fr the reductin f ClO 3 - (aq) t Cl 2 (g) in an acidic slutin. A. 2 ClO - 3 (aq) + 6 H + (aq) + 10 e - Cl 2 (g) + 6 OH - (aq) B. 2 ClO - 3 (aq) + 12 H + (aq) + 5 e - Cl 2 (g) + 6 H 2 O(l) C. 2 ClO - 3 (aq) + 10 e - Cl 2 (g) + 6 H 2 O(l) + 3 O 2 (g) D. 2 ClO - 3 (aq) + 12 H + (aq) + 10 e - Cl 2 (g) + 6 H 2 O(l). nne f these are balanced 10. Use the standard reductin ptentials belw t determine which element r in is the best reducing agent. Pd 2+ (aq) + 2 e - Pd(s) = V Sn 2+ (aq) + 2 e - Sn(s) = V Cr 2+ (aq) + 2 e - Cr(s) = V A. Pd 2+ B. Cr(s) C. Sn 2+ D. Cr 2+. Pd(s) 11. Which f the fllwing statements abut the reactin is crrect? (Cr e - Cr, = V) (Cd e - Cd, = V) 3Cd 2+ (aq) + 2Cr(s) 3Cd(s) + 2Cr 3+ (aq) A. < 0 B. ΔG < 0 C. 6 mles f electrns are transferred in the reactin D. A and C. B and C page 5

6 12. In an electrchemical... A. the xidatin prcess ccurs at the ande. B. the reductin prcess ccurs at the cathde. C. the salt bridge enables electrical cnnectin thrugh the by making a physical cnnectin between the ande and cathde cmpartments. D. All f the abve.. Just A and B. 13. Given the fllwing tw half-reactins, determine which f the verall reactins is spntaneus and calculate its standard ptential. Al 3+ (aq) + 3 e - Al(s) = V Cu 2+ (aq) + 2e - Cu(s) = V A. 2 Al 3+ (aq) + 3 Cu(s) 2 Al(s) + 3 Cu 2+ (aq) B. 2 Al 3+ (aq) + 3 Cu(s) 2 Al(s) + 3 Cu 2+ (aq) C. 2 Al(s) + 3 Cu 2+ (aq) 2 Al 3+ (aq) + 3 Cu(s) D. 2 Al(s) + 3 Cu 2+ (aq) 2 Al 3+ (aq) + 3 Cu(s) = V = V = V = V 14. Calculate fr the fllwing electrchemical reactin at 298 K where Sn 2+ (aq) + 2AgI(s) Sn 4+ (aq) + 2I - (aq) + Ag(s) [Sn 2+ ] = 0.50 M [Sn 4+ ] = 0.50 M [I - ] = 0.15 M AgI(s) + e - Ag(s) + I - (aq) = V Sn 4+ (aq) + 2 e - Sn 2+ (aq) = V A V B V C V D V V page 6

7 XTRA CRDIT (5 pints each) 15. Citric Acid (Cit) binds reversibly t Citric Acid binding prtein (CBP) with an equilibrium cnstant f 2.96 x The enthalpy f binding is kj ml -1. What is the entrpy f binding at 25 C? CBP + Cit CBP:Cit A J/K B J/K C J/K D J/K 16. T generate 1 kilgram (1000 grams) f Al frm Al 2 O 3 in an electrlytic in 1 hur, what is the electrical current (in amperes) that must be supplied t the? A x 10 7 B x 10 6 C D. 990 Please sign the fllwing statement at the cmpletin f the exam. I, (print name) pledge nt t discuss this test with anyne wh has nt it the tests until are graded and returned. page 7