145.

Balance the following redox reactions by ion-electron method:

The skeleton equation is:
    
(i) Separation of the equation in two half reactions. 
    Write the O.N. of the atoms involved in the equation. 
             +7-2      +4-2            +2            +1 + 6-2
            
(ii) Identify the atoms which undergo change in O.N.
             +7             +4          -2            - 6
       
(iii) Find out the species involved in the oxidation and reduction half reactions.

Oxidation half reaction:
                              
Reduction half reaction:
                       
Balancing the oxidation half reaction:
    The oxidation half reaction is:
                     
(i) As the increase in O.N. is 2, therefore add two electrons on the product side to balance change in O.N.

(ii) Balance O atoms by adding two H₂O molecules on the reactant side and balance H atoms by adding three H⁺ on the product side. 
                ...(1)
Balancing the reduction half reaction:
The reduction half reaction is:
            
(i) As the decrease in O.N. is 5, therefore, add 5e^- on the reactant side. 
                  
(ii) Balance O atoms by adding four H₂O molecules on the product side and balance H atoms by adding eight H⁺ on the reactant side. 
               
Adding the two half reactions.
In order to equate the electrons, multiply equation (1) by 5 and equation (2) by 2. Add the two equations.