For `(M^(2+))/(M)` and `(M^(3+))/(M^(2+))` systems the `E^(ɵ)` 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) -0.4, (Fe^(3+))/(Fe^(2+)) +0.8V`
Use this data comment upon:
(i). The stability of `Fe^(3+)` in acid solutio as compared to that of `Cr^(3+)` or `Mn^(3+)` and ltbtgt (ii). The case 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`