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

Explanation

(a) As `E_(Cr^(3+)//Cr)^(@)` is negative (-0.4 V) , this means that `Cr^(3+)` ions in solution cannot be reduced to `Cr^(2+)` ions or `Cr^(3+)` ions are very stable. As further comparison of `E^(@)` values shows that `Mn^(3+)` ions can be reduced to `Mn^(2+)` ion more readily than `Fe^(3+)` ions. Thus, in the light of this, the order of realtive stabilities of different ions is :
`Mn^(3+) lt Fe^(3+) lt Cr^(3+)`.
(b) From the `E^(@)` values, the order of oxidation of the metal to the divalent cations is :
`Mn gt Cr gt Fe`.