Assign oxidation numbers, identify oxidants and reductants, and balance redox half-equations and overall equations.

What this dot point is asking

You must assign oxidation numbers, identify what is oxidised and reduced, name the oxidant and reductant, and balance redox equations.

Oxidation and reduction

Redox reactions involve transfer of electrons.

• Oxidation is the loss of electrons; the oxidation number increases.
• Reduction is the gain of electrons; the oxidation number decreases.

The oxidant (oxidising agent) is the species that is reduced, because it takes electrons from something else. The reductant (reducing agent) is the species that is oxidised, because it supplies the electrons.

Assigning oxidation numbers

Use these rules in order:

• Free elements have an oxidation number of 0 (for example, $\text{Cu}$, $\text{O}_2$).
• A monatomic ion has an oxidation number equal to its charge (for example, $\text{Na}^+$ is +1).
• Oxygen is usually -2 (except in peroxides, where it is -1).
• Hydrogen is usually +1 (except in metal hydrides, where it is -1).
• The sum of oxidation numbers equals the overall charge of the species.

Writing half-equations

Split the reaction into an oxidation half and a reduction half. For each half-equation balance the atoms, then balance charge by adding electrons. In acidic aqueous conditions, balance oxygen with $\text{H}_2\text{O}$ and hydrogen with $\text{H}^+$.

State the oxidant and reductant by name or formula in your answer, since identifying them is often a separate mark from balancing the equation.