Wxplain why `E^(@)` for `Mn^(3+)//Mn^(2+)` couple is more positive than that for `Fe^(3+)//Fe^(2+)`(At. Nos. Mn=25,Fe=26)?

Answer

`._25Mn^(2+)=[Ar]3r^(5),._25Mn^(3+)=[Ar]3d^(4)`
`._26Fe^(2+)=[Ar]3d^(6),._26Fe^(3+)=[Ar]3d^(5)`
Thus, `Mbn^(2+)` has more stable configuration than `Mn^(3+)` while `Fe^(3+)` has more stable configuration than `Fe^(2+)` consequently,large third ionization enthalpy is required to change `Mn^(2+) to Mn^(3+)`.As `E^(@)` is the sum of enthalpy of atomization,ionizatio enthalpy and hydration enthalpy,therefore,`W^(@)` for `Mn^(3+)//Mn^(2+)` couple is more positive than `Fe^(3+)//Fe^(2+).