Question

(mno4)2- undergoes disproportionation reaction in acidic medium but (mno4)- does not why?

Answer 1

Disproportionation is a type of redox reaction in which a species is simultaneously reduced and oxidised forming two different products.
In MnO42-, the oxidation state of manganese is +6. It can disproportionate to form MnO2 and MnO4-.
3MnO42- + 4H+ → MnO2 + 2MnO4- + 2H2O

​However, the oxidation state of Mn in MnO4- is +7 which is the maximum possible oxidation state of Mn (atomic number 25, 1s2 2s2 2p6 3s23p6 3d5 4s2) and hence it cannot undergo disproportionation reaction. 
 

Answer 2

MnO42- undergoes disproportionation reaction in acidic medium but MnO-4 does not.

Explanation:

For MnO42-, the oxidation state is +6.

The maximum oxidation state Mn can exhibit is +7.

It has the tendency to lose one more electron and undergo oxidation. The Mn nucleus is electron deficient with +6 ON, MnO42- undergoes reduction.

For MnO4-, the oxidation state is +7. It can no longer donate any electron and hence can no longer undergo oxidation.

MnO42- undergoes disproportionation reaction in acidic medium but MnO-4 does not.