CH302 Spring 2009 Worksheet 7 on Electrochemistry Answer Key Use the following table of standard reduction potentials: http://www.jesuitnola.org/upload/clark/refs/red_pot.htm 1. Consider the redox reaction: Cd(s) + Co3+(aq) Cd2+(aq) + Co2+(aq) a. Balance it Cd(s) + 2 Co3+(aq) Cd2+(aq) + 2 Co2+(aq) b. Calculate E°cell E°cell = E°cathode - E°anode = 1.842 - -0.403 = 2.445 2. Consider the redox reaction: Fe2+(aq) Fe(s) + Fe3+(aq) a. Balance it 3 Fe2+(aq) Fe(s) + 2 Fe3+(aq) b. Calculate E°cell E°cell = E°cathode - E°anode = -0.440 - 0.771 = -1.211 3. Consider the redox reaction: VO2+(aq) + MnO4-(aq) MnO2(s) + VO2+(aq) a. Balance it in acid H2O(l) + 3 VO2+(aq) + MnO4-(aq) MnO2(s) + 3 VO2+(aq) + 2 H+(aq) b. Balance it in base 2 OH-(aq) + 3 VO2+(aq) + MnO4-(aq) MnO2(s) + 3 VO2+(aq) + H2O(l) c. Calculate E°cell E°cell = E°cathode - E°anode = 0.59 - 1.00 = -0.41 4. Consider the redox reaction: BiO+(aq) + 2Cr3+(aq) Bi(s) + Cr2O72-(aq) a. Balance it in acid 5 H2O(l) + 2 BiO+(aq) + 2Cr3+(aq) 2 Bi(s) + Cr2O72-(aq) + 10 H+(aq) b. Balance it in base 10 OH- + 2 BiO+(aq) + 2Cr3+(aq) 2 Bi(s) + Cr2O72-(aq) + 5 H2O(l) c. Calculate E°cell E°cell = E°cathode - E°anode = 0.32 -1.33 = -1.01 5. What effect will raising the pH have on E°cell for a. the reacion in question 3 above? it will increase E°cell b. the reacion in question 4 above? it will increase E°cell 6. Consider the answer to 1b above, If [Co3+] and[Co2+] were both 1 M, what concentration of [Cd2+] could be used to make a 2.5 V battery based on this reaction? Ecell = E°cell - (0.05916/ne)logQ 2.5 = 2.445 - (0.05916/2)log([Cd2+]·1/1) 0.005 = -(0.05916/2)log([Cd2+]) -0.169 = log([Cd2+]) [Cd2+] = 0.678 M 7. Consider the answer to 2b above. Using the Nernst equation, what ratio of [Fe3+] and [Fe2+] would be present if this cell were allowed to reach equilibrium at room temperature?Ecell = E°cell - (0.05916/ne)logQ = 0 at equilibrium 0 = -1.211 - (0.05916/2)logQ 1.211 = - (0.05916/2)logQ -40.94 = logQ Q = [Fe3+]/[Fe2+] = 1.15 x 10-41 8. If E°cell for a particular reaction is negative, a. what are two things one could do to make the Ecell positive? You could lower the concentration of aqueous species or gases on the right side of the equation or raise the concentration of aqueous species or gases on the left side of the equation. b. what is one thing you could do to make E°cell positive? Reverse the cell. 9. Rank the following species from worst to best oxidizing agent: PbSO4, Na+, Fe3+, Cl2 and AgCl. worst Na+ PbSO4 AgCl Fe3+ Cl2 best 10. Rank the following species from best to worst reducing agent: Mn2+, Ni, H2, Al, Hg22+. worst Al Ni H2 Hg22+ Mn2+ best 11. What will be produced at the cathode a. for the reaction in question 1? Co2+(aq) b. for the reaction in question 2? Fe(s) c. for the reaction in question 3? MnO2(s) d. for the reaction in question 4? Bi(s) 12. What will be consumed at the anode a. for the reaction in question 1? Cd(s) b. for the reaction in question 2? Fe2+(aq) c. for the reaction in question 3? VO2+(aq) d. for the reaction in question 4? 2Cr3+(aq) 12. Calculate ΔG a. for the reaction in question 1 ΔG = -nFE = -2·96,485·2.455 = -473,741 J·mol-1 = -473.7 kJ·mol-1 b. for the reaction in question 2 ΔG = -nFE = -2·96,485·-1.211 = 233,687 J·mol-1 = 233.7 kJ·mol-1 c. for the reaction in question 3 ΔG = -nFE = -3·96,485·-0.41 = 118,677 J·mol-1 = 118.7 kJ· mol-1 d. for the reaction in question 4 ΔG = -nFE = -6·96,485·-1.01 = 584,699 J·mol-1 = 584.7 kJ·mol-1 13. Calculate Ka. for the reaction in question 1 K = e^(nFE/RT) = e^(2·96,485·2.455/8.314·298) = 1.10 x 1083 b. for the reaction in question 2 K = e^(nFE/RT) = e^(2·96,485·-1.211/8.314·298) = 1.09 x 10-41 c. for the reaction in question 3 K = e^(nFE/RT) = e^(3·96,485·-0.41/8.314·298) = 1.58 x 10-21 d. for the reaction in question 4 K = e^(nFE/RT) = e^(6·96,485·-1.01/8.314·298) = 3.22 x 10-103 14. If a current of 1 ampere can plate 3 moles of a certain product in 10 hours, a. how much of the same product could it plate in 20 minutes? (3 moles / 10 hours) x 1/3 hours = 0.1 moles b. what current would be needed to plate 6 moles in just 2 hours? 1 ampere x (10 hours / 3 moles) x (6 moles / 2 hours) = 10 amperes 15. If a 5.5845 g sample of Fe is plated from an unknown solution in 482 minutes at a constant current of 1 ampere, what was the oxidation state of the Fe ion is solution? 5.5845 g Fe x (1 mol / 55.845 g) = 0.1 moles Fe 482 minutes x (60 seconds / minute) = 28,920 (I· t)/(ne·F) = moles (1·28,920)/(ne·96485) = 0.1 ne = 3, Therefore, the solution contained Fe3+. 16. Complete the table below from memory E ΔG K Reduction Oxidation Cathode Anode Battery + - K>1 Cathode Anode + - Electrolytic - + 0<K<1 Cathode Anode - + 17. When electroplating metal (producing a solid metal from an ion by an electrolytic reaction), the amount of metal produced is a. directly proportional to current (I) and time (t) b. inversely proportional to number of electrons (n) 18. Using the table mentioned at the top of this document, what species could a. reduce O2(g) to H2O(l) but not NO3-(aq) to NO(g) Br-(aq) b. reduce Cu2+(aq) to Cu+(aq) but not Pb2+(aq) to Pb(s) Sn2+(aq) or H2(g) c. oxidize Ag(s) to Ag+(aq) but not H2O2(aq) to O2(g) Fe3+(aq) 19. Determine the number of electrons gained/lost by the carbon atom in each reaction below a. CH4 + 2 O2 + CO2 + 2 H2O 8 electrons lost b. CH4 + O2 + CH2O + H2O 4 electrons lost c. 2 CH4 + O2 + 2 CH3OH 2 electrons lost 20. Consider the answer to question 19 above. How does a breaking a C-H bond and forming a C-Obond effect the oxidation state of C? It raises the oxidation number of the carbon by
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