8.17 For M2+/M and M3+/M2+ systems the  EƟ values for some metals are as follows:

Cr2+/Cr               -0.9V                Cr3/Cr2+             -0.4 V

Mn2+/Mn            -1.2V                 Mn3+/Mn2+      +1.5 V

Fe2+/Fe              -0.4V                 Fe3+/Fe2+      +0.8 V

Use this data to comment upon:

(i) the stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+ and

(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.

(i) The Evalue for Fe3+/Fe2+ is higher than that for Cr3+/Cr2+ and lower than that for

Mn3+/Mn2+. So, the reduction of Fe3+ to Fe2+ is easier than the reduction of Mn3+ to

Mn2+, but not as easy as the reduction of Cr3+ to Cr2+. Hence, Fe3+ is more stable than

Mn3+, but less stable than Cr3+. These metal ions can be arranged in the increasing

order of their stability as: Mn3+ < Fe3+< Cr3+

(ii) The reduction potentials for the given pairs increase in the following order.

Mn2+ / Mn < Cr2+ / Cr < Fe2+ /Fe

So, the oxidation of Fe to Fe2+ is not as easy as the oxidation of Cr to Cr2+ and the oxidation

of Mn to Mn2+. Thus, these metals can be arranged in the increasing order of their ability

to get oxidised as: Fe < Cr < Mn