Balancing Equations by the Method of Half-Reactions

In the half-reaction method, we usually begin with a skeleton ionic equation showing only the substances undergoing oxidation and reduction. In such cases, we assign oxidation numbers only when we are unsure whether the reaction involves oxidation-reduction. We will find that (for acidic solutions), OH (for basic solutions), and H2O are often involved as reactants or products in redox reactions. Unless H, OH , or H2O is being oxidized or reduced, these species do not appear in the skeleton equation. Their presence, however, can be deduced as we balance the equation. 

For balancing a redox reaction that occurs in acidic aqueous solution, the procedure is as follows  

Divide the equation into one oxidation half-reaction and one reduction halfreaction. 

This specific sequence (a) —(d) is important, and it is summarized in the diagram in the margin. At this point, you can check whether the number of electrons in each half-reaction corresponds to the changes in oxidation state. 

Multiply half-reactions by integers as needed to make the number of electrons lost in the oxidation half-reaction equal the number of electrons gained in the reduction half-reaction. 

or H2O is being oxidized or reduced, these species do not appear in the skeleton equation. Their presence, however, can be deduced as we balance the equation. 

This specific sequence (a)—(d) is important, and it is summarized in the diagram in 

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