E⁰ (Fe³⁺/Fe²⁺) is positive . This means Fe can be reduced to +2 from +3 but then enthalpy of hydration is also negative ( when oxidizing Fe+2 to Fe+3 because Fe+3 has more charge density so it will make system stable nd release energy). Then why is E⁰(Fe3+/Fe2+) positive?

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Solution

Reduction potential is depending on two factors.
1. Stability of oxidation state
2. Hydration enthalpy
In case of Fe2+ and Fe3+, the stability of oxidation state strongly suggests that Fe3+ should be stable due to half filled d5 configuration. Similarly hydration enthalpy also suggests that Fe3+ should be stable due to high charge density.
As both the factors are supporting any one can easily say that Fe2+ will easily oxidize to Fe3+ rather than reduction.
But reduction potential is decided with reference to SHE. This means that if you combine Fe3+ solution with Hydrogen gas, Fe3+ should be reduced to Fe2+ and hydrogen gas should be oxidized. This is exactly happening in experiments. So reduction potential of Fe3+/Fe2+ is positive

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