Assign oxidation numbers, identify oxidation and reduction, and balance redox equations using half-equations.

What this dot point is asking

You must assign oxidation numbers, identify the oxidant and reductant, and balance redox equations using half-equations.

Oxidation and reduction

Rules for assigning oxidation numbers

1. An element in its pure form has oxidation number $0$ (e.g. $\text{Cu}$, $\text{O}_2$).
2. A monatomic ion's oxidation number equals its charge (e.g. $\text{Na}^+$ is $+1$).
3. Oxygen is usually $-2$ (except in peroxides, where it is $-1$).
4. Hydrogen is usually $+1$ (except in metal hydrides, where it is $-1$).
5. The sum of oxidation numbers equals the overall charge: $0$ for a neutral compound, or the ion charge for a polyatomic ion.

Identifying oxidation and reduction

Compare oxidation numbers before and after. If an element's oxidation number increases, it was oxidised; if it decreases, it was reduced.

Balancing with half-equations

Split the reaction into an oxidation half and a reduction half, balance each, then combine:

1. Balance the atoms other than O and H.
2. Balance O by adding $\text{H}_2\text{O}$.
3. Balance H by adding $\text{H}^+$ (in acidic solution).
4. Balance charge by adding electrons ($e^-$).
5. Multiply the half-equations so the electrons are equal, then add and cancel.