Oxidation and Reduction (Redox) — The Reactivity Series | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 6% of your exam marks.

stable

Low

Stable6%

Displacement reactions and determining order of reactivity from experimental data.

In terms of oxygen

Oxidation is the gain of oxygen by a substance
Reduction is the loss of oxygen from a substance
The two always happen together in the same reaction — a redox reaction
In Zn + CuO → ZnO + Cu:
- Zinc gains oxygen → zinc is oxidised → zinc is the (it caused the reduction of CuO)
- Copper(II) oxide loses oxygen → it is reduced → CuO is the

In terms of electrons

The more general definitions use electron transfer:
- is loss of electrons
- is gain of electrons
Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain)
For the displacement reaction Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s):
- Mg → Mg²⁺ + 2 e⁻ (oxidation — Mg loses electrons)
- Cu²⁺ + 2 e⁻ → Cu (reduction — Cu²⁺ gains electrons)
Mg is the reducing agent (gives away electrons); Cu²⁺ is the oxidising agent (accepts them)

Exam tip

Explaining why a displacement reaction is redox (two marks)

What comes up: a two-mark question asks you to explain why a metal displacement reaction (for example, aluminium reducing iron oxide) is a redox reaction.

Write (two marks): (1) one substance gains oxygen (or loses electrons) and is oxidised — state which one. (2) the other substance loses oxygen (or gains electrons) and is reduced — state which one. For example: aluminium gains oxygen and is oxidised; iron(III) oxide loses oxygen and is reduced.

Watch out: the mark scheme rejects "iron loses oxygen" as the description of what is reduced — it is iron(III) oxide (the compound) that loses oxygen, not iron itself. Name the compound, not the metal element.

Spectator ions and ionic equations

In the full equation Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s), the sulfate ion SO₄²⁻ appears on both sides unchanged
An ion that is unchanged in a reaction is a spectator ion — it does not take part in the chemistry
Leaving out the spectator ions gives the , which highlights the actual redox change: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

A practice prompt

In the reaction Cu(s) + 2 AgNO₃(aq) → Cu(NO₃)₂(aq) + 2 Ag(s), Cu loses two electrons to form Cu²⁺ (oxidation) while each Ag⁺ gains one electron to form Ag (reduction)
Copper is therefore the reducing agent here, and silver(I) is the oxidising agent