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Question:

A solution contains Fe2+, Fe3+ and I− ions. This solution was treated with iodine at 35oC. E0 for Fe3+/Fe2+ is +0.77V and E0 for I2/2I− = 0.536V. The favourable redox reaction is:

I2 will be reduced to I−

there will be no redox reaction

I− will be oxidised to I2

Fe2+ will be oxidised to Fe3+

Solution:

The favorable reaction is the oxidation of iodide ion to iodine.
I− will be oxidised to I2
2e− + Fe+3 → Fe2+; E=0.77V
2I− → I2 + 2e− E=0.536V

2Fe+3 + 2I− → 2Fe+2 + I2; E=Eox+Ered=0.77−0.536=0.164V