Electron Transfer Reactions (HT Only)

Section 1 of 7Oxidation and reduction (HT only)

Oxidation

• Oxidation A substance is oxidised when it loses electrons.

Reduction

• Reduction A substance is reduced when it gains electrons.

Redox

• Redox Both happen in the same reaction, so it is called a redox reaction.
• Memory aidOIL RIG, oxidation is loss, reduction is gain.

Section 2 of 7Splitting a redox reaction

1

Write the full equation: Fe + CuSO₄ → FeSO₄ + Cu.

2

Write it as an ionic equation, showing the ions: Fe + Cu²⁺ + SO₄²⁻ → Fe²⁺ + SO₄²⁻ + Cu.

3

Cross out spectator ions that appear unchanged on both sides, here sulfate: Fe + Cu²⁺ → Fe²⁺ + Cu.

4

Write a half equation for each change: Fe → Fe²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.

5

Label them: the iron loses electrons so it is oxidised; the copper ion gains electrons so it is reduced.

Section 3 of 7Electrolysis as redox

Electrolysis

At the cathode

• At the cathode Positive ions gain electrons, so reduction happens here. Example: Pb²⁺ + 2e⁻ → Pb.

At the anode

• At the anode Negative ions lose electrons, so oxidation happens here. Example: 2Br⁻ → Br₂ + 2e⁻.

Electron flow

• Electron flow Electrons are pushed by the power supply from the anode round the circuit to the cathode.

Section 4 of 7Identifying what is oxidised and reduced

Change	Electrons	Process
Zn → Zn2+	Lost: 2e−	Oxidation
Cu2+ → Cu	Gained: 2e−	Reduction
Fe3+ → Fe2+	Gained: 1e−	Reduction
V3+ → V4+	Lost: 1e−	Oxidation

Section 5 of 7Displacement as electron transfer

1More reactive metal

• More reactive metal Its atoms lose electrons easily to become ions.

2Electrons transferred

• Electrons transferred The electrons pass to the less reactive metal ion.

3Less reactive metal forms

• Less reactive metal forms The ions are reduced to atoms and appear as a solid.

Section 6 of 7Key terms

Term	Meaning
Oxidation	The loss of electrons by a substance.
Reduction	The gain of electrons by a substance.
Redox reaction	A reaction in which oxidation and reduction occur together.
Spectator ion	An ion present in the reaction mixture that does not change and takes no part in the reaction.
Half equation	An equation showing only the loss or gain of electrons by one species.

Section 7 of 7Exam technique

1

Reversing OIL RIG, and saying that oxidation is gain of electrons.

2

Leaving spectator ions in the ionic equation instead of cancelling them.

3

Forgetting that half equations must balance in both atoms and charge.

4

Writing electrons on the wrong sideelectrons appear on the right in oxidation and on the left in reduction.

5

Saying the metal ion is oxidised when it forms a metal atom; it is reduced.