`MnO_(4)^(2-)` undergoes disproportionation reaction in acidic medium but `MnO_(4)^(-)` does not. Given reason.

Explanation

In `MnO_(4)^(-)` Mn is in the highest oxidation state of +7 and hence it can undergo disproportionation
In contrast the O.N of Mn in `MnO_(4)^(-)` is +6 therefore it can increase its O.N to +7 or decrease its cannot to some lower value thus `MnO_(4)^(2-)` undergoes disporportionation according to the following reactio
`3MnO_(4)^(2-)+4H^(+)rarr2MnO_(4)^(-)+MnO_(2)^(-)+2H_(2)O`
here the O.N of Mn increase from +6 in `MnO_(4)^(2-)` to +7 in `MnO_(4)^(-)` and decreases to +4 in `MnO_(2)` thus `MnO_(4)^(2-)` undergoes disproportionation in acidic medium