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3.2.2Atomic Structure and the Periodic Table

Metal Displacement Reactions

5 min read 5 parts Lesson 1 of 10 Unit quiz

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Section 1 of 5What Displacement Means

3.2.2 Metal Displacement Reactions

A displacement reaction is a redox reaction in which one metal takes the place of another in a compound.

Displacement
  • A more reactive metal displaces a less reactive metal from its compound.
Two methods
  • Displacement can happen by heating a metal with a metal oxide, or by adding a metal to a solution of a metal compound.
Redox
  • Oxidation is loss of electrons and reduction is gain of electrons, and both occur together in a displacement reaction.
Prediction
  • A reaction happens only when the metal added is higher in the reactivity series than the metal in the compound.

Section 2 of 5Worked Displacement Equations

Two examples show the same idea applied to a solid oxide and to a solution.

1
Heating zinc with copper(II) oxide gives zinc oxide and copper, because zinc is more reactive than copper: Zn + CuO → ZnO + Cu.
2
In this reaction zinc gains oxygen and is oxidised, while copper(II) oxide loses oxygen and is reduced.
3
Iron added to copper(II) sulfate solution displaces copper: Fe + CuSO₄ → FeSO₄ + Cu.
4
Removing the spectator ions gives the ionic equation Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s), which shows iron atoms losing electrons to copper ions.

Section 3 of 5Redox in Terms of Electrons

Redox in Terms of Electrons
Iron atom
  • Each Fe atom loses two electrons to form Fe²⁺ ions, so iron is oxidised.
Copper ion
  • Each Cu²⁺ ion gains two electrons to form a copper atom, so copper ions are reduced.
Spectator ions
  • Sulfate ions do not change during the reaction and are removed from the ionic equation.
Reactivity
  • Iron has a greater tendency to form ions than copper, which is why it displaces copper.

Section 4 of 5Worked Example

Question

  • Identify which of these pairs will react: (a) iron and magnesium(II) sulfate solution;
    • (b) zinc and copper(II) sulfate solution;
    • (c) iron(II) chloride solution and magnesium.

  1. Ask whether the metal added is more reactive than the metal in the compound.
  2. (a) Iron is less reactive than magnesium, so iron cannot displace magnesium: no reaction.
  3. (b) Zinc is more reactive than copper, so a reaction occurs.
    • (c) Magnesium is more reactive than iron, so a reaction occurs.

Answer

  • Pairs (b) and (c) react;
    • pair (a) does not.

Section 5 of 5Exam Checklist & Tips

3.2.2 – Key Points for the Exam

  • Displacement reaction A reaction in which a more reactive element takes the place of a less reactive element in a compound.
  • Spectator ion An ion that is present in a reaction but does not change, so it is left out of the ionic equation.
  • Ionic equation An equation that shows only the particles which change during a reaction.
  • Redox reaction A reaction in which oxidation and reduction occur together.

Common Mistakes

  • Predicting that a less reactive metal will displace a more reactive metal.
  • Including spectator ions in the ionic equation.
  • Describing oxidation only as gain of oxygen, rather than loss of electrons in electron terms.
  • Mixing up which species is oxidised and which is reduced in the copper(II) oxide reaction.
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