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Two standard reduction potentials are given: Fe3+(aq) + e- -> Fe2+(aq), E° = 0.771 V and I2(g) + 2e- -> 2I-(aq), E° = 0.536 V. What is the standard cell potential E°cell for the reaction 2Fe3+(aq) + 2I-(aq) -> 2Fe2+(aq) + I2(g)?

1. (2 * 0.771 - 0.536) = 1.006 V
2. (0.771 - 0.5 * 0.536) = 0.503 V
3. (0.771 - 0.536) = 0.235 V
4. (0.536 - 0.771) = -0.235 V

Correct answer: (0.771 - 0.536) = 0.235 V

Solution

Fe3+ is reduced to Fe2+ (cathode, higher E°) while I- is oxidised to I2 (anode). Since E° is intensive, you must NOT multiply it by the stoichiometric coefficient even though 2 electrons are transferred. Hence E°cell = 0.771 - 0.536 = 0.235 V.

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