Question

What is meant by 'disproportionation'? Give two examples of disproportionation reaction in aqueous solution.

Answer 1

It is found that sometimes a relatively less stable oxidation state undergoes an oxidation - reduction reaction in which it is simultaneously oxidised and reduced. This is called disproportionation.
For example,
(i) $3Cr^{3-}_4+8H^+ \rightarrow 2Cr^{2-}_4+Cr^{3+}+4H_2O$
Cr( V ) Cr ( VI ) Cr ( III )
Cr(V) is oxidized to Cr(VI) and reduced to Cr(III)
(ii) $3MnO^{2-}_4+4H+\rightarrow2MnO^-_4+MnO_2+2H_2O$
Mn( VI ) Mn ( VII ) Mn( IV )
Mn (VI) is oxidized to Mn (VII) and reduced to Mn (IV).