Metal Displacement ReactionsOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is a metal displacement reaction?
A displacement reaction happens when a more reactive metal reacts with a compound (in aqueous solution) of a less reactive metal, taking the less reactive metal's place. The more reactive metal 'displaces' the less reactive one:
more reactive metal + salt of less reactive metal → salt of more reactive metal + less reactive metal
These reactions are used to establish the order of the reactivity series, since a metal will only displace another metal that is below it in reactivity.
Magnesium metal added to copper sulfate solution: Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s). The magnesium displaces the copper because it is more reactive.
Section 2
How do we know which metal is more reactive from a displacement reaction?
If a metal displaces another metal from its salt solution, the metal that has been added must be more reactive than the metal in the solution. By testing a series of metals against a series of salt solutions and observing which reactions occur, chemists can build up the full reactivity series.
Signs a displacement reaction has happened include: a colour change in the solution, a solid coating forming on the added metal, and a temperature rise (the reaction is exothermic).
When describing observations, mention the specific colour change (e.g. blue solution fading) as well as the solid deposit forming — examiners often want both.
Section 3
How are displacement reactions explained using oxidation and reduction?
Displacement reactions are redox reactions — oxidation and reduction happen simultaneously:
- The more reactive metal is oxidised — it loses electrons to form positive ions
- The less reactive metal ion is reduced — it gains electrons to form the metal
For magnesium displacing copper:
- Mg → Mg²⁺ + 2e⁻ (oxidation)
- Cu²⁺ + 2e⁻ → Cu (reduction)
Section 4
How do you write ionic equations for displacement reactions?
An ionic equation for a displacement reaction shows only the ions and atoms that actually change — 'spectator ions' that do not change are left out.
Full equation: Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)
Ionic equation: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
Here, the sulfate ion (SO₄²⁻) is a spectator ion — it appears unchanged on both sides of the full equation, so it is omitted from the ionic equation.
Always check that charges balance on both sides of an ionic equation — Mg²⁺ + Cu²⁺ style errors from unbalanced charge are a common mistake.
Must Know
- A more reactive metal displaces a less reactive metal from a solution of its salt
- Displacement reactions establish positions within the reactivity series
- The more reactive metal is oxidised (loses electrons); the less reactive metal ion is reduced (gains electrons)
- Displacement reactions are redox reactions
- Ionic equations omit spectator ions, e.g. Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
- Observable signs include colour change of solution and a solid metal deposit forming
That's the notes covered.
Carry on to the next subtopic.