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

Explanation

(i) The `E^(ϴ)` value for `Fe^(3+)//Fe^(2+)` is higher than that for `Cr^(3+)//Cr^(2+)` and lower than that for `Mn^(3+)//Mn^(2+)`. So, the reduction of `Fe^(3+)` to `Fe^(2+)` is easier than the reduction of `Mn^(3+)` to `Mn^(2+)`, but not as easy as the reduction of `Cr^(3+)` to `Cr^(2+)`. Hence, `Fe^(3+)` is more stable than `Mn^(3+)`, but less stable than `Cr^(3+)`. These metal ions can be arranged in the increasing order of their stability as: `Mn^(3+) lt Fe^(3+) lt Cr^(3+)`
(ii) The reduction potentials for the given pairs increase in the following order.
`Mn^(2+)//Mn lt Cr^(2+)//Cr lt Fe^(2+)//Fe`
So, the oxidation of Fe to `Fe^(2+)` is not as easy as the oxidation of Cr to `Cr^(2+)` and the oxidation of Mn to `Mn^(2+)`. Thus, these metals can be arranged in the increasing order of their ability to get oxidised as: `Fe lt Cr lt Mn`