Principles of balancing redox equations

Redox equations involve electron transfer from a reactant known as the reductant (reducing agent) to a reactant known as the oxidant (oxidising agent).

The reductant is the electron donor.

The oxidation number of an atom in the reductant is more positive in the products.

The oxidant is the electron acceptor.

The oxidation number of an atom in the oxidant becomes more negative in the products.

In the redox reaction:
Cu²⁺(aq) + Zn(s)

Zn²⁺(aq) + Cu(s)

Zn is the reductant. Cu²⁺ is the oxidant.

Redox reactions can be separated into two half equations, one showing the electron loss, the other showing the electron gain.

Cu²⁺(aq) + 2e^–

Cu(s)
Zn(s)

Zn²⁺(aq) + 2es^–

The overall charge on the reactants in each half equation equals the overall charge on the products. Thus the equations are balanced both for atoms and for charge.

The overall equation is the sum of the two half equations.
In the overall process electron loss must equal electron gain.

For the reaction above, the two half equations are simply added because the number of electrons lost equals the number of electrons gained.

If this is not the case, as in the example below the equations must be multiplied by suitable factors so that electrons cancel when the two half equations are added.

Ag⁺ reacts with iron to give silver and Fe²⁺. The half equations are:

Fe(s)

Fe²⁺(aq) + 2e^–
[Ag⁺(aq) + e^–

Ag(s)] × 2

Before combining, the silver half equation must be multiplied by 2.

2Ag⁺(aq) + Fe(s)

Fe²⁺(aq) + 2Ag(s)