A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.

Consider the reaction below:

Cl₂ + 2Fe²⁺ → 2Cl⁻ + 2Fe³⁺

The two elements involved, iron and chlorine, each change oxidation state; iron from 2+ to 3+, chlorine from 0 to 1−. There are then effectively two half-reactions occurring. These changes can be represented in formulas by inserting appropriate electrons into each half-reaction:

Fe²⁺ → Fe³⁺ + e⁻

Cl₂ + 2e⁻ → 2Cl⁻

In the same way given two half-reactions it is possible, with knowledge of appropriate electrode potentials, to arrive at the full (original) reaction.

Guide for writing and balancing half-reaction equations

1. Identify the key element that undergoes an oxidation state change.
2. Balance the number of atoms of the key element on both sides.
3. Add the appropriate number of electrons to compensate for the change of oxidation state.
4. Add H+ (in acid medium), or OH− (in basic medium), to balance the charge on both sides of the half-reactions; and H₂O, if necessary, to balance the equations.

See also

• Electrode potential