Displacement Reactions

This lesson covers displacement reactions involving metals and metal compounds, as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 4. You need to understand why displacement reactions occur, predict whether they will happen using the reactivity series, and write balanced equations.

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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. In the context of metals:

A more reactive metal displaces a less reactive metal from a solution of its salt (or from its oxide).

The general equation is:

reactive metal + less reactive metal compound → reactive metal compound + less reactive metal

This happens because the more reactive metal has a stronger tendency to form positive ions (lose electrons) than the less reactive metal.

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Displacement Reactions in Solution

When a piece of metal is placed into a solution containing ions of a less reactive metal, a displacement reaction occurs.

Example 1: Iron and Copper Sulfate

Word equation: iron + copper sulfate → iron sulfate + copper

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

Observations:

• The blue copper sulfate solution gradually turns pale green (as iron sulfate forms)
• A brown/orange coating of copper metal appears on the surface of the iron
• The iron nail becomes coated with copper and appears to change colour
• The solution warms up (exothermic reaction)

Example 2: Magnesium and Copper Sulfate

Word equation: magnesium + copper sulfate → magnesium sulfate + copper

Symbol equation: Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

Observations:

• The blue solution turns colourless (magnesium sulfate is colourless in solution)
• A brown/orange deposit of copper forms on the magnesium
• The reaction is exothermic — the test tube becomes noticeably hot
• The reaction is more vigorous than with iron (magnesium is more reactive)

Example 3: Zinc and Copper Sulfate

Word equation: zinc + copper sulfate → zinc sulfate + copper

Symbol equation: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Observations:

• The blue solution becomes colourless (zinc sulfate is colourless)
• A brown/orange deposit of copper appears on the zinc
• The reaction is exothermic

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Predicting Displacement Reactions

Use the reactivity series to predict whether a displacement reaction will occur:

Rule: If the standalone metal is higher (more reactive) in the reactivity series than the metal in the compound, a displacement reaction will occur. If it is lower (less reactive), no reaction takes place.

Combination	Prediction	Reason
Magnesium + zinc sulfate	Yes — reaction occurs	Mg is more reactive than Zn
Zinc + iron sulfate	Yes — reaction occurs	Zn is more reactive than Fe
Iron + copper sulfate	Yes — reaction occurs	Fe is more reactive than Cu
Copper + iron sulfate	No — no reaction	Cu is less reactive than Fe
Copper + magnesium sulfate	No — no reaction	Cu is less reactive than Mg
Silver + copper sulfate	No — no reaction	Ag is less reactive than Cu
Magnesium + sodium chloride	No — no reaction	Mg is less reactive than Na

Exam Tip: When predicting displacement reactions, always state which metal is more reactive and which is less reactive. Then state whether displacement will or will not occur. If no reaction occurs, write "no reaction" — do not leave it blank.

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The Thermite Reaction

One of the most dramatic displacement reactions is the thermite reaction:

Word equation: aluminium + iron(III) oxide → aluminium oxide + iron

Symbol equation: 2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l)

Observations:

• Extremely exothermic — produces temperatures above 2 000 °C
• The iron produced is molten (liquid iron) due to the extreme heat
• Bright white sparks and intense light
• The reaction is very vigorous once started

Why it works: Aluminium is more reactive than iron, so aluminium displaces iron from iron(III) oxide.

Uses: The thermite reaction is used to weld railway tracks together. The molten iron produced fills the gap between the rails and solidifies to form a strong joint.

Exam Tip: The thermite reaction is a favourite exam example. Remember: aluminium is above iron in the reactivity series, so it displaces iron. The reaction is extremely exothermic, producing molten iron.

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Displacement with Metal Oxides

Displacement reactions also occur when a reactive metal is heated with the oxide of a less reactive metal.

Example: Carbon and copper oxide

Word equation: carbon + copper oxide → copper + carbon dioxide

Symbol equation: 2C(s) + 2CuO(s) → 2Cu(s) + 2CO₂(g)

(More precisely: C(s) + 2CuO(s) → 2Cu(s) + CO₂(g))

Observations:

• The black copper oxide turns brown/orange (copper metal forms)
• The mixture glows when heated
• Limewater turns milky if the gas is passed through it (confirms CO₂)

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Temperature Changes in Displacement Reactions

Displacement reactions are exothermic — they release energy to the surroundings. This can be detected by:

• Measuring the temperature increase of the solution
• Observing that the test tube or beaker becomes hot to the touch

The size of the temperature change depends on the difference in reactivity between the two metals:

Reaction	Temperature Change	Explanation
Mg + CuSO₄	Large increase	Large difference in reactivity
Zn + CuSO₄	Moderate increase	Moderate difference in reactivity
Fe + CuSO₄	Small increase	Small difference in reactivity

Exam Tip: If a question asks about energy changes in displacement reactions, state that they are exothermic and that the temperature of the solution increases. The greater the difference in reactivity, the greater the temperature change.

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Ionic Equations for Displacement (Higher)

For the higher tier, you may need to write ionic equations for displacement reactions.

Iron displacing copper from copper sulfate:

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

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

The sulfate ions (SO₄²⁻) are spectator ions — they remain unchanged.

In this reaction:

• Iron is oxidised (it loses electrons): Fe → Fe²⁺ + 2e⁻
• Copper ions are reduced (they gain electrons): Cu²⁺ + 2e⁻ → Cu

This is a redox reaction.

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Practical: Investigating Displacement

A common practical involves adding different metals to solutions of metal salts and recording observations:

1. Place samples of magnesium, zinc, iron and copper into labelled test tubes.
2. Add copper sulfate solution to each and observe.
3. Repeat with zinc sulfate, iron sulfate and magnesium sulfate.
4. Record colour changes and any temperature changes.
5. Use the results to construct a reactivity order.