Displacement Reactions

This lesson covers displacement reactions as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what displacement reactions are, predict when they will occur using the reactivity series, write word and symbol equations, and know about the thermit reaction.

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What Is a Displacement Reaction?

A displacement reaction occurs when a more reactive element takes the place of a less reactive element in a compound. The less reactive element is "displaced" — it is pushed out.

General Pattern

more reactive metal + compound of less reactive metal → compound of more reactive metal + less reactive metal

This only works if the free metal is higher in the reactivity series than the metal in the compound.

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Metal + Metal Salt Solution

The most common type of displacement reaction at GCSE involves placing a piece of metal into a solution containing a salt of a less reactive metal.

Example 1: Iron in Copper Sulfate Solution

iron + copper sulfate → iron sulfate + copper

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Observations:

• The iron nail becomes coated with a layer of brown/orange solid (copper metal).
• The blue copper sulfate solution turns pale green (iron sulfate forms).
• Iron is more reactive than copper, so iron displaces copper.

Example 2: Zinc in Copper Sulfate Solution

zinc + copper sulfate → zinc sulfate + copper

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Observations:

• The zinc becomes coated with brown copper metal.
• The blue solution becomes colourless (zinc sulfate is colourless).
• The reaction is exothermic — the test tube feels warm.

Example 3: Copper in Magnesium Sulfate Solution

copper + magnesium sulfate → no reaction

Copper is less reactive than magnesium, so it cannot displace magnesium from its compound. No colour change occurs.

Exam Tip: To predict whether a displacement reaction will occur, compare the two metals in the reactivity series. The free metal must be more reactive than the metal in the compound for displacement to happen.

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Predicting Displacement Reactions

Use the reactivity series to predict outcomes:

Free Metal	Solution	Reaction?	Reason
Mg	ZnSO₄	Yes	Mg is more reactive than Zn
Fe	CuSO₄	Yes	Fe is more reactive than Cu
Zn	MgCl₂	No	Zn is less reactive than Mg
Cu	FeSO₄	No	Cu is less reactive than Fe
Zn	FeSO₄	Yes	Zn is more reactive than Fe
Ag	CuSO₄	No	Ag is less reactive than Cu

graph TD
    A["Is the free metal<br/>MORE reactive than<br/>the metal in the compound?"]
    A -->|Yes| B["DISPLACEMENT occurs<br/>The more reactive metal<br/>takes the place of<br/>the less reactive metal"]
    A -->|No| C["NO REACTION<br/>The less reactive metal<br/>cannot displace<br/>the more reactive metal"]

    style A fill:#2c3e50,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#c0392b,color:#fff

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The Thermit Reaction

The thermit reaction is a dramatic displacement reaction between aluminium powder and iron(III) oxide. It is used to weld railway tracks together.

Equation

aluminium + iron(III) oxide → aluminium oxide + iron

2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l)

Why Does This Work?

• Aluminium is more reactive than iron in the reactivity series.
• Aluminium displaces iron from iron(III) oxide.
• The reaction is extremely exothermic — it releases so much heat that the iron produced is molten (liquid).
• The molten iron runs between the rail ends and solidifies, welding them together.

Observations

• A bright white/yellow glow or intense flash
• Sparks are produced
• Molten iron flows out of the reaction mixture
• The reaction is very violent and produces extreme heat (~2500 °C)

Exam Tip: The thermit reaction is a favourite exam question. Remember: aluminium displaces iron because aluminium is more reactive. The reaction is highly exothermic and produces molten iron.

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Ionic Equations for Displacement (Higher Tier)

For higher tier, you may be asked to write ionic equations for displacement reactions. In these equations, the spectator ions (ions that do not change) are removed.

Example: Iron + Copper Sulfate

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

Full ionic equation: Fe(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Fe²⁺(aq) + SO₄²⁻(aq) + Cu(s)

The sulfate ion (SO₄²⁻) appears on both sides and does not change — it is a spectator ion. Remove it:

Net ionic equation: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)

This shows that:

• Iron loses 2 electrons → Fe → Fe²⁺ + 2e⁻ (oxidation)
• Copper gains 2 electrons → Cu²⁺ + 2e⁻ → Cu (reduction)

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Displacement with Non-Metals — Halogens

Displacement also occurs with halogens (Group 7). A more reactive halogen will displace a less reactive halogen from a solution of its salt.

Halogen Added	Solution	Observation	Reaction?
Chlorine (Cl₂)	Potassium bromide (KBr)	Solution turns orange	Yes — Cl₂ displaces Br₂
Chlorine (Cl₂)	Potassium iodide (KI)	Solution turns brown	Yes — Cl₂ displaces I₂
Bromine (Br₂)	Potassium iodide (KI)	Solution turns brown	Yes — Br₂ displaces I₂
Iodine (I₂)	Potassium bromide (KBr)	No change	No — I₂ is less reactive than Br₂

Exam Tip: In halogen displacement, the order of reactivity is F₂ > Cl₂ > Br₂ > I₂. A halogen can only displace a halogen below it in Group 7.

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Summary