Which Iron Ion Is Truly More Stable, Fe2+ or Fe3+?

[chatelier]

In terms of electronic structure, Fe3+ is more stable than Fe2+ because it has a half filled d-orbital which gives extra stability.
i.e. Fe3+ : [Ar]3d5
Fe2+ : [Ar]3d6
But the standard electrode potential of the following reaction is 0.77V,
Fe3+(aq) + e- => Fe2+(aq)
which means formation of Fe2+is favored and Fe2+ should be more stable in this case.

so, actually, Fe2+ or Fe3+ is more stable?

 

[Bystander]

What state is the iron in when you're looking at electronic configurations? What state is the iron in when you're looking at the reduction potential?

 

[chatelier]

Plz comment on my way of thinking:
Fe2+ is more stable than Fe3+ in aqueous solution because of the more -ve hydration energy of Fe2+
so in what situation will Fe3+ be more stable than Fe2+??

 

[Bystander]

Plz comment on my way of thinking:
Fe2+ is more stable than Fe3+ in aqueous solution because of the more -ve hydration energy of Fe2+

Okay.

so in what situation will Fe3+ be more stable than Fe2+??

You aren't restricted to aqueous solution. You aren't forbidden the use of chelating agents. You're not forbidden the formation of complexes.

 

[GCT]

in addition to what bystander noted, I think what you mean by "stability" is in regard to the ionization energies (1st, 2nd, 3rd). You'll need to consider the trend in ionization energies in relation to electronic configurations, the answer can be found in an inorganic text.