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

A more precise definition involves the transfer of electrons. Use the mnemonic OIL RIG:

Mnemonic	Meaning
OIL	Oxidation Is Loss (of electrons)
RIG	Reduction Is Gain (of electrons)

flowchart LR
    A["Oxidation<br/>Loss of electrons<br/>OIL"] --- B["Redox<br/>Reaction"]
    B --- C["Reduction<br/>Gain of electrons<br/>RIG"]

    style A fill:#d32f2f,color:#fff
    style B fill:#7b1fa2,color:#fff
    style C fill:#1565c0,color:#fff

Electron Transfer in Metal Reactions

When a metal reacts, it loses electrons (is oxidised) to form positive ions:

$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$Mg→Mg2++2e−

The non-metal (or the ions of a less reactive metal) gains those electrons (is reduced):

$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$Cu2++2e−→Cu

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Half Equations (Higher Tier)

For AQA higher tier, you must be able to write half equations to show what happens to each substance separately.

Displacement Example: Mg + CuSO₄

Overall equation:

$\text{Mg}(s) + \text{CuSO}_4(aq) \rightarrow \text{MgSO}_4(aq) + \text{Cu}(s)$Mg(s)+CuSO4​(aq)→MgSO4​(aq)+Cu(s)

Oxidation half equation (Mg loses electrons):

$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-$Mg→Mg2++2e−

Reduction half equation (Cu²⁺ gains electrons):

$\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$Cu2++2e−→Cu

Exam Tip: The number of electrons lost in the oxidation half equation must equal the number of electrons gained in the reduction half equation. If they do not match, multiply one or both half equations.

Reaction of a Metal With Acid

$\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$Zn(s)+2HCl(aq)→ZnCl2​(aq)+H2​(g)

Oxidation: $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$Zn→Zn2++2e−

Reduction: $2\text{H}^+ + 2e^- \rightarrow \text{H}_2$2H++2e−→H2​

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Oxidising and Reducing Agents

Term	Definition	What Happens to It
Reducing agent	A substance that reduces another substance	It is oxidised (loses electrons / gains oxygen)
Oxidising agent	A substance that oxidises another substance	It is reduced (gains electrons / loses oxygen)

Key Insight

The reducing agent is the substance that donates electrons. The oxidising agent is the substance that accepts electrons.

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Common Redox Reactions to Know

Reaction	Oxidised	Reduced
2Mg + O₂ → 2MgO	Magnesium (gains oxygen)	Oxygen
CuO + H₂ → Cu + H₂O	Hydrogen (gains oxygen)	Copper oxide (loses oxygen)
Fe + CuSO₄ → FeSO₄ + Cu	Iron (loses electrons)	Cu²⁺ (gains electrons)
Zn + 2HCl → ZnCl₂ + H₂	Zinc (loses electrons)	H⁺ (gains electrons)
2Al + Fe₂O₃ → Al₂O₃ + 2Fe	Aluminium (gains oxygen)	Iron oxide (loses oxygen)

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Common Mistakes

Mistake	Correction
Saying "oxidation is gaining oxygen and gaining electrons"	Oxidation is gaining oxygen OR losing electrons (OIL)
Confusing oxidising agent and reducing agent	The reducing agent is oxidised; the oxidising agent is reduced
Writing unbalanced half equations	Ensure the charges and atoms balance on both sides
Forgetting that in a redox reaction, both oxidation AND reduction always occur	One cannot happen without the other

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Worked Example

Question: In the reaction 2Al + 3CuO → Al₂O₃ + 3Cu, identify the oxidising agent and reducing agent.

Answer:

1. Aluminium gains oxygen (Al → Al₂O₃), so aluminium is oxidised. Since it is oxidised, aluminium is the reducing agent.
2. Copper oxide loses oxygen (CuO → Cu), so copper oxide is reduced. Since it is reduced, copper oxide is the oxidising agent.

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Summary

• Oxidation = gain of oxygen OR loss of electrons (OIL).
• Reduction = loss of oxygen OR gain of electrons (RIG).
• Oxidation and reduction always occur together — these are redox reactions.
• A reducing agent reduces another substance and is itself oxidised.
• An oxidising agent oxidises another substance and is itself reduced.
• For higher tier, write half equations showing electron loss (oxidation) and electron gain (reduction) separately.
• The number of electrons in the two half equations must balance.

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Extended Worked Examples

Worked Example 1 — Writing a Half-Equation From a Displacement Reaction

Question: Iron is added to copper(II) sulfate solution. Write the overall ionic equation and the two half-equations.

Answer (step by step):

1. Full equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
2. Remove spectator ions (SO₄²⁻): Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s).
3. Oxidation half-equation (iron loses electrons): Fe → Fe²⁺ + 2e⁻.
4. Reduction half-equation (copper ions gain electrons): Cu²⁺ + 2e⁻ → Cu.
5. Electrons balance (2 lost and 2 gained). Correct.

Worked Example 2 — Identifying Oxidation and Reduction in an Oxygen-Transfer Equation

Question: In the reaction Fe₂O₃ + 2Al → 2Fe + Al₂O₃, identify which substance is oxidised, which is reduced, and which is the oxidising agent.