Redox – IGCSE Chemistry Definition

A redox reaction is one where oxidation and reduction occur simultaneously. The 0620 syllabus (Supplement) requires you to identify redox reactions, name the substance oxidised and reduced, and link the concept to electrolysis and displacement reactions.

The 0620 definition

A redox reaction is a chemical reaction in which oxidation and reduction take place at the same time. One substance is oxidised (loses electrons) while another is reduced (gains electrons).

OILRIG and redox

Oxidation Is Loss of electrons Reduction Is Gain of electrons

In every redox reaction, the electrons lost by one substance are gained by another. Oxidation and reduction cannot occur independently — they always happen together.

Examples of redox reactions

Displacement reactions

Mg + CuSO₄ → MgSO₄ + Cu

• Mg is oxidised: Mg → Mg²⁺ + 2e⁻ (loses electrons)
• Cu²⁺ is reduced: Cu²⁺ + 2e⁻ → Cu (gains electrons)
• The electrons lost by magnesium are gained by copper ions

Thermite reaction

2Al + Fe₂O₃ → Al₂O₃ + 2Fe

• Al is oxidised (gains oxygen / loses electrons)
• Fe₂O₃ is reduced (loses oxygen / gains electrons)

Electrolysis

• Anode: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation)
• Cathode: Cu²⁺ + 2e⁻ → Cu (reduction)

Both occur simultaneously — electrolysis is a redox process.

Reactions with oxygen

2Mg + O₂ → 2MgO

• Mg is oxidised (gains oxygen)
• O₂ is reduced (gains electrons from Mg)

How to identify a redox reaction

1. Look for electron transfer (one substance loses electrons, another gains)
2. Look for oxygen transfer (one gains, another loses)
3. Look for changes in oxidation number
4. If both oxidation and reduction are present, it is redox

Worked exam question

In the reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. (a) State which substance is oxidised. (1) (b) State which substance is reduced. (1) (c) Explain why this is a redox reaction. (1)

Mark scheme

(a) CO / carbon monoxide (it gains oxygen to become CO₂) [1]

(b) Fe₂O₃ / iron(III) oxide (it loses oxygen to become Fe) [1]

(c) Because both oxidation and reduction occur at the same time / one substance gains oxygen while another loses it [1]

Common exam mistakes

• Identifying only oxidation or only reduction and calling it “redox”. Both must be present — name both.
• Confusing which substance is oxidised and which is reduced. The substance that gains oxygen (or loses electrons) is oxidised. The substance that loses oxygen (or gains electrons) is reduced.
• Forgetting that all displacement reactions are redox reactions. When a more reactive metal displaces a less reactive one, electron transfer is occurring.