C5.2 Displacement Reactions

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how a more reactive metal pushes a less reactive metal out of its compounds, and what this tells us about oxidation and reduction. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Predict whether a displacement reaction will happen using the reactivity series, and explain it using the tendency of metals to form positive ions
• Explain how displacement-type evidence places carbon and hydrogen in the reactivity series
• Describe the observations in displacement reactions and write word and symbol equations
• Identify oxidation and reduction in terms of oxygen and (Higher tier) electrons
• Write ionic equations and half equations for displacement reactions, leaving out spectator ions (Higher tier)

Key words

• Displacement reaction: A reaction in which a more reactive element takes the place of a less reactive element in a compound.
• Oxidation: Gain of oxygen. In terms of electrons (Higher): loss of electrons.
• Reduction: Loss of oxygen. In terms of electrons (Higher): gain of electrons.
• Redox reaction: A reaction in which oxidation and reduction happen at the same time.
• Ionic equation: An equation that shows only the ions and atoms that change in a reaction (Higher).
• Spectator ion: An ion that is present but does not change during the reaction (Higher).
• Half equation: An equation that shows what happens to one species, with electrons written as e− (Higher).

Predicting displacement

A more reactive metal will displace a less reactive metal from a solution of one of its salts. If the metal added is less reactive than the metal in the compound, there is no reaction.

iron + copper sulfate → iron(II) sulfate + copper

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Why does this happen? In a displacement reaction the metal atoms compete to exist as positive ions. The more reactive metal has the greater tendency to lose electrons and form positive ions, so it ends up as ions in solution, and the less reactive metal is left as atoms.

Signs of a displacement reaction: a coating of the less reactive metal forms; the colour of the solution may change (blue copper sulfate fades); the mixture gets warmer, because displacement reactions are exothermic. The bigger the gap in the reactivity series, the bigger the temperature rise.

A displacement grid

A class added small pieces of three metals to solutions of their nitrates. ✓ = reaction seen; ✗ = no reaction.

• magnesium; magnesium nitrate –; iron(II) nitrate ✓ dark grey iron forms; silver nitrate ✓ grey silver crystals form
• iron; magnesium nitrate ✗; iron(II) nitrate –; silver nitrate ✓ grey silver crystals form
• silver; magnesium nitrate ✗; iron(II) nitrate ✗; silver nitrate –

The metal that displaces the most others is the most reactive: magnesium (2 reactions) > iron (1) > silver (0).

Key fact: Displacement also works with solid oxides. In the thermite reaction, aluminium displaces iron: 2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l). Aluminium gains oxygen (oxidised); iron oxide loses oxygen (reduced).

Fitting carbon and hydrogen into the series

Carbon and hydrogen are non-metals, but they can also be put “in competition” with metals, so they get places in the reactivity series.

• Hydrogen: a metal that reacts with a dilute acid is displacing hydrogen from the acid (the H⁺ ions become H₂ gas). Iron and even lead do this (lead very slowly); copper and silver cannot. So hydrogen goes between lead and copper.
• Carbon: when heated, carbon takes oxygen away from the oxides of zinc, iron, lead and copper, but not from aluminium oxide or magnesium oxide. So carbon goes between aluminium and zinc.

Exam tip: Use these positions to predict: a metal above hydrogen reacts with dilute acids; a metal oxide below carbon can be reduced by heating with carbon.

Oxidation and reduction in terms of electrons (Higher tier)

In a displacement reaction, atoms of the more reactive metal lose electrons and become ions. Ions of the less reactive metal gain those electrons and become atoms.

Exam tip: OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.

Ionic and half equations (Higher tier)

In magnesium + copper sulfate, the sulfate ions are in solution before and after. They do not change, so they are spectator ions and we leave them out.

Worked example: writing the equations

Full equation: Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)
Ionic equation: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
Half equation (oxidation): Mg → Mg²⁺ + 2e−
Half equation (reduction): Cu²⁺ + 2e− → Cu
Check: atoms balance and the total charge is the same on both sides (+2 = +2).

Answer: Mg is oxidised; Cu²⁺ is reduced

When silver nitrate is used, two silver ions are needed for every magnesium atom, because each Ag⁺ gains only one electron: Mg + 2Ag⁺ → Mg²⁺ + 2Ag.

Displacement and reacting masses

Worked example: mass of copper from iron

Mg + CuSO₄ → MgSO₄ + Cu. How much copper forms from 1.2 g of magnesium with excess copper sulfate? (Ar: Mg = 24, Cu = 63.5)
moles of Mg = 1.2 ÷ 24 = 0.050 mol
ratio Mg : Cu = 1 : 1, so 0.050 mol Cu forms
mass of Cu = 0.050 × 63.5 = 3.175 g

Answer: 3.2 g (2 s.f., matching the data)

Common mistakes

• Thinking a less reactive metal can displace a more reactive one. Copper does nothing in zinc sulfate solution.
• Mixing up oxidation and reduction with electrons. Use OIL RIG: the metal atom loses electrons, so it is oxidised.
• Leaving the spectator ions in an ionic equation, or forgetting to balance the charges.
• Putting electrons on the wrong side: oxidation half equations have e− on the right; reduction half equations have e− on the left.

Revision questions

Try these, then check your answers below each one.

Question 1. Which mixture would produce a displacement reaction? (Choose from: magnesium + iron(II) sulfate solution, copper + zinc sulfate solution, iron + magnesium nitrate solution, silver + copper(II) sulfate solution.)

Answer: magnesium + iron(II) sulfate solution

Working: Magnesium is above iron, so it displaces iron. In the other mixtures the added metal is less reactive.

Question 2. True or false? In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, zinc is oxidised.

Answer: True

Working: True. Zinc atoms lose electrons to form Zn²⁺ ions – oxidation is loss of electrons.

Question 3. Magnesium reacts with copper(II) sulfate solution. Name the spectator ion.

Answer: sulfate

Working: The sulfate ion, SO₄²⁻, does not change, so it is left out of the ionic equation: Mg + Cu²⁺ → Mg²⁺ + Cu.

Related lessons

• Previous lesson: C5.1 The Reactivity Series
• Next lesson: C5.3 Extracting Metals
• In this chapter: C5.4 Salts from Metals
• In this chapter: C5.5–6 Salts from Insoluble Bases and Making More Salts
• In this chapter: C5.7 Neutralisation and the PH Scale

Continue in the full course

This lesson is section C5.2 (AGCSC2202) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.