Question

For M2+/M and M3+/M2+ systems, the $E^\ominus$ 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.

Answer 1

(i) The $E^\ominus$ value 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+

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(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