144.

The Mn³⁺ ions is unstable in solution and undergoes disproportionation to give Mn²⁺, MnO₂ and H⁺ ion. Write a balanced ionic equation for the reaction.

The skeleton equation is
Mn³⁺ (aq) → Mn²⁺ (aq) + MnO₂(s) + H⁺(aq)
Let us balance the above equation by ion electron method.

1. Write the oxidation and reduction half-reactions by observing the changes in oxidation number and writing these separately
Oxidation half-reaction:
            +3                    +4
            
  Reduction half reaction:
             +3                         +2
             

2. Balancing the oxidation half reaction
(i) Add 1 electron towards R.H.S. to balance the charge on Mn.
                  
(ii) Balance the charges by adding four H⁺ towards R.H.S.
               
                               [Balanced oxidation half reaction]
(iii) Balance O atoms by adding two H2O molecules downwards L.H.S.
          
                              [Balanced oxidation half reaction]
3. Balancing the reduction half reaction:
         Add 1 electron towards L.H.S. to balance the charge on Mn. 
                   
                                   (Balanced reduction half reaction]
4. Adding balanced oxidation half reaction and balanced reduction half-reaction. 
                     
This is balanced redox equation for disproportionation reaction.