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Standard reduction potentials are given: I2 + 2e- -> 2I-, E° = +0.54 V MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O, E° = +1.52 V Fe3+ + e- -> Fe2+, E° = +0.77 V Sn4+ + 2e- -> Sn2+, E° = +0.10 V Which one of the following reactions is spontaneous?

1. Sn4+ + 2Fe2+ -> Sn2+ + 2Fe3+
2. 2Fe2+ + I2 -> 2Fe3+ + 2I-
3. Sn4+ + 2I- -> Sn2+ + I2
4. Sn2+ + I2 -> Sn4+ + 2I-

Correct answer: Sn2+ + I2 -> Sn4+ + 2I-

Solution

For spontaneity the oxidant (reduced species) must have a higher E° than the reductant (oxidized species). I2 (E° = +0.54 V) can oxidize Sn2+ (Sn4+/Sn2+ E° = +0.10 V) because 0.54 > 0.10, giving E°cell = +0.44 V > 0. The other options reverse a favorable direction and give negative E°cell.

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