Redox and Electron Transfer

Section 1 of 7Redox as electron transfer (HT only)

Oxidation

• Oxidation Oxidation is the loss of electrons by an atom, ion or compound.
  • For example, silver is oxidised to silver ions: Ag → Ag⁺ + e^-.

Reduction

• Reduction Reduction is the gain of electrons.
  • For example, oxygen is reduced to oxide ions by magnesium: O₂ + 4e^- → 2O^2-.

Always together

• Always together Electrons lost by one species are gained by another, so oxidation and reduction occur at the same time.
  • The reaction is called a redox reaction.
• Use OIL RIG to remember: Oxidation Is Loss, Reduction Is Gain.

Section 2 of 7Worked example: iron and copper sulfate (HT only)

1Full equation

• Full equation Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).

2Ionic equation

• Ionic equation Write the dissolved compounds as ions: Fe + Cu²⁺ + SO₄^2- → Fe²⁺ + SO₄^2- + Cu.

3Remove spectator ions

• Remove spectator ions The sulfate ions are the same on both sides and take no part in the change, so cancel them to leave Fe + Cu²⁺ → Fe²⁺ + Cu.

4Identify the changes

• Identify the changes Iron atoms lose two electrons to become Fe²⁺, so iron is oxidised.
  • Copper ions gain two electrons to become copper atoms, so copper is reduced.

Section 3 of 7Spotting oxidation and reduction from charges (HT only)

1

Write down each species that changes charge during the reaction.

2

Compare the charge before and after the arrow for each.

3

If the charge becomes more positive, electrons have been lost, so that species is oxidised.

4

If the charge becomes less positive, or more negative, electrons have been gained, so that species is reduced.

5

Example: V³⁺ → V⁴⁺ loses an electron so is oxidised, while Fe³⁺ → Fe²⁺ gains one so is reduced.

Section 4 of 7Comparing the ideas (HT only)

Idea	Oxidation	Reduction
Electrons	Lost	Gained
Oxidation number	Increases	Decreases
Example	Fe → Fe2+ + 2e-	Cu2+ + 2e- → Cu
In electrolysis	Takes place at the anode	Takes place at the cathode

Section 5 of 7Useful connections

Displacement reactions

• Displacement reactions A more reactive metal displaces a less reactive one from solution because it loses electrons more readily.
  • The more reactive metal is oxidised and the metal ion is reduced.

Same idea as oxygen transfer

• Same idea as oxygen transfer In Mg + O₂, magnesium is oxidised both because it gains oxygen and because it loses electrons.
  • The electron definition works for every redox reaction, including those with no oxygen.

Section 6 of 7Key terms

Term	Meaning
Oxidation	A reaction in which an element, ion or compound loses electrons.
Reduction	A reaction in which an element, ion or compound gains electrons.
Redox reaction	A reaction in which oxidation and reduction occur together.
Spectator ion	An ion present in the mixture that is unchanged and does not take part in the reaction.
Ionic equation	An equation showing only the particles that change, written as ions where they are in solution.

Section 7 of 7Exam technique

1

Defining oxidation only as gain of oxygen; at this level it must be given as loss of electrons.

2

Saying that a species 'gets oxidised' when its charge falls; a falling charge means reduction.

3

Leaving spectator ions in an ionic equation and so not showing the real change.

4

Forgetting that the electrons lost in oxidation equal the electrons gained in reduction, so the charges must balance.