Q. How to balance a redox equation?

Ans.

Redox equations may be written and balanced by the ionic half-reaction method. Let's use the half-reaction method to balance the following equation:

Ag⁺ + Zn -> Ag + Zn²⁺

1. Separate ionic half-reactions are written for the reduction half-reaction and for the oxidation half-reaction.

Ag⁺ + e^- -> Ag (reduction)

Zn -> Zn²⁺ + 2e^- (oxidation)

2. Multiply each half-reaction by a number such that the number of electrons gained in one half-reaction is equal to that lost in the other.

In the example, this is done by multiplying the Ag half-reaction by 2:

2Ag⁺ + 2e^- -> 2Ag

There are now two moles of electron gained in the reduction half-reaction, and two moles of electrons gained in the oxidation half-reaction. Therefore, the electron gain and loss are now balanced.

3. The half-reactions are now added:

2Ag⁺ + 2e^- -> 2Ag

+ Zn -> Zn²⁺ + 2e^-

2Ag⁺ + Zn -> 2Ag + Zn²⁺

Note, that when adding the half reactions, there are 2e- on the left side of the equation and 2e- on the right side of the equation. Therefore, the electrons cancel each other out and do not appear in the net reaction equation.

Double check to see if you balanced the reaction correctly. Notice that there is a total charge of +2 on both sides. Notice that the number of atoms on each side of the equation is also balanced.