For `M^(2+)//M` and `M^(3+)//M^(2+)` systems, the `E^(@)` values for some metals are as follows :
`{:(Cr^(2+)//Cr,,=-0.9V,,Cr^(3+)//Cr^(2+),,=-0.4V),(Mn^(2+)//Mn,,=-1.2V,,Mn^(3+)//Mn^(2+),,=+1.5V),(Fe^(2+)//Fe,,=-0.4V,,Fe^(3+)//Fe^(2+),,=+0.8V):}`
Use this data to comment upon :
(a) the stability of `Fe^(3+)` in acid solution as compared to that of `Cr^(3+)` or `Mu^(3+)` and
(b) the ease with which iron can be oxidised as compared to the similar process for either chromium or manganese metals.

Explanation

(a) `Cr^(3+)//Cr^(2+)` has a negative reduction potential. Hence, `Cr^(3+)` cannot be reduced to `Cr^(2+)`, i.e., `Cr^(3+)` is most stable. `Mn^(3+)//Mn^(2+)` has large positive `E^(@)` value. Hence, `Mn^(3+)` can be easily reduced to `Mn^(2+)`, i.e., `Mn^(3+)` is least stable. `E^(@)` value for `Fe^(3+)//Fe^(2+)` is positive but small. Hence, `Fe^(3+)` is more stable than `Mn^(3+)` but less stable than `Cr^(3+)`. Thus, the stability follows the order :
`Cr^(3)gt Fr^(3+)gt Mn^(3+)`
(b) Oxidation potentials for the given pairs will be `+0.9V,+1.2V` and `+0.4" V"`. Thus, the order of getting oxidised will be :
`Mn gt Cr gt Fe`