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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−