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Unformatted text preview: 5. Check that atoms/charges are balanced. 1. MnO4‐Mn2+ Fe2+Fe3+ ‐+ 8H+ + 5e‐ Mn2+ + 4H O 2+Fe3+ + 1e‐ 2. MnO4
5Fe2+5Fe3+ + 5e‐ 3. MnO4‐+ 8H+ + 5e‐ Mn2+ + 4H2O ‐+ 8H+ + 5e‐ Mn2+ + 4H O 4. MnO4
2 5Fe2+5Fe3+ + 5e‐ ________________ MnO4‐ + 5Fe2+ + 8H+Mn2+ + 5Fe3+ + 4H2O *coefficient for Fe2+ is 5. 17. 1. Cr2O72‐Cr3+ H2SS 2‐+ 14H+ + 6e‐ 2Cr3+ + 7H O + + 2e‐ 2. Cr2O7
H2S S + 2H 2
2‐+ 14H+ + 6e‐ 2Cr3+ + 7H O + + 6e‐ 3. Cr2O7
3H2S 3S + 6H
4. Cr2O72‐+ 14H+ + 6e‐ 2Cr3+ + 7H2O + + 6e‐________________ 3H2S 3S + 6H Cr2O72‐+ 3H2S + 8H+ 2Cr3+ + 3S + 7H2O *coefficient for S is 3. 18. Best reducing agentmost easily oxidizedlowest reduction potentialcopper. But remember, Cu+ gets oxidized, not Cu2+(gets reduced), so Cu+ is the best reducing agent. 19. (A) NO‐although the oxidation of Fe to Fe2+ is favorable by +0.44 V, the reduction of Zn2+ to Zn is unfavorable by ‐0.76 V. The overall potential is negative, so this reaction will NOT occur. 2+ to Fe3+ is unfavorable by ‐0.77 V. (B) NO‐ The oxidation of Fe
(C) NO‐ The reduction of Zn2+ to Zn is unfavorable by ‐0.76 V. 2+ to Fe is unfavorable by ‐0.44 V, the (D) YES Although the reduction of Fe
the oxidation of Zn to Zn2+ is favorable by +0.76 V, making the overall potential positive. 20. Best oxidizing agentmost easily reducedhighest reduction potentialZ. It is Z+ that gets reduced (Z gets oxidized), so Z+ is the best oxidizing agent. 21. We need the most positive E°, with one species being reduced and the other being oxidized. The most positive reduction potential corresponds to Cu+ being reduced to Cu (s) (+0.52 V). The most positive oxidation potential (most negative reduction potential) corresponds to Cr2+ being oxidized to Cr3+ (NOT Cr3+‐this gets reduced) (+0.41 V). Therefore, the answer is B. Dr. Fus Electrochemistry Problem Set Solutions Chemistry 123 22. A voltaic cell uses a spontaneous redox reaction to produce an electric current. Therefore, the E° for the overall reaction must be positive if the redox reaction is to be spontaneous. One of the two species given must be reduced, and one must be oxidized. The most negative reduction potential for the half‐reactions given corresponds to the species that will undergo oxidation. The less negative reduction po...
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- Spring '06