Oxidation and Reduction (Redox) Reactions

This lesson covers oxidation, reduction and redox reactions as required by the AQA GCSE Combined Science Trilogy specification (8464). You must understand oxidation and reduction in terms of both oxygen transfer and electron transfer, and be able to write half equations for higher tier.

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Oxidation and Reduction — Oxygen Transfer

The simplest definition of oxidation and reduction involves the transfer of oxygen:

Term	Definition (Oxygen Transfer)	Example
Oxidation	Gain of oxygen	2Mg + O₂ → 2MgO — magnesium is oxidised
Reduction	Loss of oxygen	CuO + C → Cu + CO₂ — copper oxide is reduced

In any reaction where one substance is oxidised, another is reduced. These are called redox reactions (reduction–oxidation).

Example — Thermit Reaction

$2\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\text{Fe}(l)$2Al(s)+Fe2​O3​(s)→Al2​O3​(s)+2Fe(l)

• Aluminium is oxidised — it gains oxygen.
• Iron(III) oxide is reduced — it loses oxygen.
• Aluminium is the reducing agent (it causes the iron oxide to be reduced).
• Iron(III) oxide is the oxidising agent (it causes the aluminium to be oxidised).

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Oxidation and Reduction — Electron Transfer