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Given the standard electrode potentials Fe2+/Fe = −0.44 V and Fe3+/Fe2+ = +0.77 V, if a block of iron is placed in contact with Fe3+ ions, what is expected to happen?

1. The amount of Fe3+ rises
2. The amount of Fe3+ falls
3. The concentration of Fe2+ stays the same
4. The amount of Fe2+ falls

Correct answer: The amount of Fe3+ falls

Solution

Iron metal reduces Fe3+: Fe + 2Fe3+ -> 3Fe2+ (E_cell = 0.77 - (-0.44) > 0, spontaneous). Therefore the amount of Fe3+ falls while Fe2+ rises.

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