Which of the following couples has the most positive `E^(Theta)` value ?

Options

A. `V^(3+)//V^(2)`
B. `Cr^(3=)//Cr^(2+)`
C. `Fe^(3+)//Fe^(2+)`
D. `Mn^(3+)//Mn^(2+)`

Answer

D

Explanation

The standerd electrode potential for the oxidation of a metal is a composite property that depends on `Delta G^(@)` for the sublimation of the metal, the the ionization enthalpies of the metal atom, and `Delta G^(@)` for the hydrogen of the hydration of the metal ion. `E(Theta)` value for the `Mn^(3+)//Mn^(2+)` cople `(+1.57)` is much more positive than that for others: `V^(3+)//V^(2+)` `(-0.26V)`, `Cx^(3+)//Cr^(2+)` `(-0.41)` and `Fe^(2+)//Fe^(2+)` `(+0.77V)` because of much higher third ionization enthalpy of Mn as the required change is `d^(5)` to `d^(4)` :
`Mn^(2+) (3d^(5)) rarr Mn^(3+) (3d^(4))`
Thus
`Mn^(3+) + bar e rarr Mn^(2+)`
is quite favourable and hence `Mn^(3+)` is an oxidizing agent. This clearly explains why the `+3` state of Mn is of little importance.