Chemical changes: displacement reactions and the reactivity series

What you'll learn

This guide covers displacement reactions and the reactivity series as specified in the AQA GCSE Chemistry specification. You'll learn how to predict whether displacement reactions occur, use experimental evidence to construct a reactivity series, and apply this knowledge to extract metals from their ores. These concepts are fundamental to understanding metal chemistry and frequently appear in both Paper 1 and Paper 2.

Key terms and definitions

Displacement reaction — a reaction where a more reactive element takes the place of a less reactive element in a compound

Reactivity series — a list of elements (usually metals) arranged in order of their reactivity, from most reactive to least reactive

Oxidation — the loss of electrons from a substance; also the gain of oxygen or loss of hydrogen

Reduction — the gain of electrons by a substance; also the loss of oxygen or gain of hydrogen

Redox reaction — a reaction involving both reduction and oxidation occurring simultaneously

Ore — a naturally occurring rock containing sufficient metal compound to make extraction economically worthwhile

Spectator ions — ions present in a reaction mixture that do not participate in the reaction

Ionic equation — a chemical equation showing only the particles that actually change during a reaction, omitting spectator ions

Core concepts

The reactivity series of metals

The reactivity series arranges metals in order of their tendency to form positive ions (lose electrons). The order you must know for AQA GCSE is:

Most reactive:

• Potassium (K)
• Sodium (Na)
• Lithium (Li)
• Calcium (Ca)
• Magnesium (Mg)
• Aluminium (Al)
• Carbon (C) — included because it's used in metal extraction
• Zinc (Zn)
• Iron (Fe)
• Hydrogen (H) — included for reference with acid reactions
• Copper (Cu)
• Silver (Ag)
• Gold (Au)

Least reactive

A useful mnemonic is: "Please Send Lions, Cats May Attack Children Zebras In Hot Countries Selling Gold"

The position in the reactivity series determines:

• Whether a metal will displace another from a compound
• How vigorously metals react with water and acids
• The method required to extract a metal from its ore

Displacement reactions with metal compounds

A more reactive metal will displace a less reactive metal from its compound. This occurs because the more reactive metal has a greater tendency to form positive ions.

General pattern: More reactive metal + compound of less reactive metal → compound of more reactive metal + less reactive metal

Example reactions:

Magnesium displacing copper:

magnesium + copper sulfate → magnesium sulfate + copper
Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s)

Zinc displacing lead:

zinc + lead nitrate → zinc nitrate + lead
Zn(s) + Pb(NO₃)₂(aq) → Zn(NO₃)₂(aq) + Pb(s)

Observable changes:

• Temperature increase (displacement reactions are exothermic)
• Colour change in solution
• Formation of solid metal on the surface of the more reactive metal
• Disappearance of the more reactive metal

When copper sulfate solution (blue) reacts with magnesium, the solution becomes colourless as copper ions are removed and replaced with magnesium ions. Brown copper metal forms on the magnesium surface.

No reaction occurs when: A less reactive metal is added to a compound of a more reactive metal. For example:

copper + magnesium sulfate → no reaction

Ionic equations for displacement reactions

Displacement reactions can be represented by ionic equations showing only the species that change. This highlights the electron transfer occurring.

Full equation:

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Full ionic equation:

Zn(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Zn²⁺(aq) + SO₄²⁻(aq) + Cu(s)

The sulfate ions (SO₄²⁻) are spectator ions — they don't change.

Ionic equation (spectator ions removed):

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

This can be split into half equations:

Oxidation (loss of electrons):

Zn(s) → Zn²⁺(aq) + 2e⁻

Reduction (gain of electrons):

Cu²⁺(aq) + 2e⁻ → Cu(s)

The zinc atoms lose electrons (are oxidised) and the copper ions gain electrons (are reduced). This is a redox reaction.

Experimental determination of reactivity series

You can establish the order of reactivity by conducting displacement experiments systematically.

Method:

1. Place samples of different metal compounds in solution (e.g., in test tubes or wells on a spotting tile)
2. Add a small piece or powder of various metals to each solution
3. Record observations (temperature change, colour change, metal coating)
4. A reaction indicates the added metal is more reactive than the metal in the compound

Example results table:

Conclusions:

• Magnesium displaces both zinc and copper → magnesium is most reactive
• Zinc displaces copper but not magnesium → zinc is middle reactivity
• Copper displaces neither → copper is least reactive

Order: Magnesium > Zinc > Copper

Metal extraction and the reactivity series

The position of a metal in the reactivity series determines the extraction method required.

Metals above carbon (K, Na, Li, Ca, Mg, Al):

• Too reactive to be extracted by heating with carbon
• Extracted by electrolysis of molten compounds
• Requires large amounts of energy
• Expensive process

Metals below carbon (Zn, Fe, Cu):

• Extracted by reduction with carbon in a blast furnace or similar process
• Carbon displaces these less reactive metals from their oxides
• Less expensive than electrolysis
• Used industrially for iron and zinc

Example — iron extraction:

iron oxide + carbon → iron + carbon dioxide
2Fe₂O₃ + 3C → 4Fe + 3CO₂

The carbon reduces the iron oxide to iron. The iron oxide is oxidised (loses oxygen), while carbon is oxidised (gains oxygen).

Metals below hydrogen (Cu, Ag, Au):

• Very unreactive
• Found native (uncombined) in the Earth's crust, especially gold and silver
• Copper occasionally found native, but usually extracted from ores

Displacement reactions with hydrogen

Metals above hydrogen in the reactivity series will displace hydrogen from acids, producing a salt and hydrogen gas.

General equation:

metal + acid → salt + hydrogen

Examples:

magnesium + hydrochloric acid → magnesium chloride + hydrogen
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

zinc + sulfuric acid → zinc sulfate + hydrogen
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

Metals below hydrogen (copper, silver, gold) do not react with dilute acids because they cannot displace hydrogen.

Ionic equation for metal-acid reactions:

Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)

This shows magnesium displacing hydrogen ions from solution.

Worked examples

Example 1: Predicting displacement reactions

Question: A student adds iron nails to solutions of copper sulfate, magnesium sulfate and zinc sulfate. Predict what will happen in each case and write a balanced symbol equation for any reactions that occur. (6 marks)

Answer:

Iron + copper sulfate:

• Reaction occurs because iron is more reactive than copper ✓
• Brown copper metal forms on the iron nail ✓
• Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) ✓

Iron + magnesium sulfate:

• No reaction occurs ✓
• Iron is less reactive than magnesium, so cannot displace it ✓

Iron + zinc sulfate:

• No reaction occurs because iron is less reactive than zinc ✓

Mark scheme notes: 1 mark for each correct prediction, 1 mark for balanced equation, 1 mark for observation. Full explanations referencing reactivity required for full marks.

Example 2: Writing ionic equations

Question: Magnesium powder is added to copper chloride solution. A reaction occurs.

(a) Write a balanced symbol equation for this reaction. (2 marks)

(b) Write an ionic equation for this reaction. (2 marks)

(c) Identify which species is oxidised and which is reduced. (2 marks)

Answer:

(a) Mg(s) + CuCl₂(aq) → MgCl₂(aq) + Cu(s) ✓✓ (Both formula and balancing required for full marks)

(b) Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s) ✓✓ (State symbols and correct ions required)

(c)

• Magnesium is oxidised ✓ (or: Mg loses electrons / Mg → Mg²⁺ + 2e⁻)
• Copper / copper ions is/are reduced ✓ (or: Cu²⁺ gains electrons / Cu²⁺ + 2e⁻ → Cu)

Example 3: Metal extraction

Question: Explain why aluminium is extracted from its ore by electrolysis, but iron is extracted by heating with carbon. (4 marks)

Answer:

• Aluminium is more reactive than carbon ✓
• So carbon cannot displace aluminium from its compounds / aluminium oxide ✓
• Iron is less reactive than carbon ✓
• So carbon can displace iron from its compounds / iron oxide / reduce iron oxide ✓

Alternative acceptable points:

• Electrolysis requires more energy / is more expensive (linked to reactivity)
• Position in reactivity series determines extraction method

Common mistakes and how to avoid them

• Mistake: Writing that the less reactive metal displaces the more reactive metal.

  • Fix: Always check the reactivity series. The MORE reactive metal takes the place of the LESS reactive metal in the compound.

• Mistake: Including spectator ions in ionic equations.

  • Fix: Identify ions that appear unchanged on both sides of the equation and remove them. Only show particles that change.

• Mistake: Confusing oxidation and reduction in displacement reactions.

  • Fix: Remember OIL RIG (Oxidation Is Loss, Reduction Is Gain — of electrons). The more reactive metal loses electrons (oxidation), while the metal ions in solution gain electrons (reduction).

• Mistake: Stating that copper or silver will react with dilute acids.

  • Fix: Check the reactivity series. Only metals above hydrogen react with dilute acids to displace hydrogen.

• Mistake: Not balancing symbol equations correctly, especially with compound ions like sulfate.

  • Fix: Count atoms systematically. Remember sulfate (SO₄²⁻) and nitrate (NO₃⁻) stay together as units in most reactions.

• Mistake: Giving vague observations like "colour change" without specifying colours.

  • Fix: State the colour before AND after (e.g., "blue solution becomes colourless" or "brown solid forms").

Exam technique for "Chemical changes: displacement reactions and the reactivity series"

• Command word "Predict": State what will happen based on reactivity series knowledge. For full marks, you usually need to state whether a reaction occurs AND give a reason referencing relative reactivity. Example: "A reaction occurs because zinc is more reactive than copper."

• Writing equations: Symbol equations require correct formulae, balancing and (often) state symbols for full marks. Check balancing by counting each type of atom on both sides. For ionic equations, remove spectator ions completely.

• Explaining extraction methods: Link the method explicitly to the metal's position relative to carbon in the reactivity series. State which metal is more/less reactive than carbon, then state the consequence for extraction method.

• Marks allocation: 1-mark questions usually require a simple statement. 2-mark questions typically need two distinct points. For 4-6 mark questions, develop explanations fully with reasoning and use scientific terminology precisely (oxidation, reduction, displacement, reactivity).

Quick revision summary

The reactivity series orders metals by their tendency to form positive ions. More reactive metals displace less reactive metals from compounds in displacement reactions. These are redox reactions where the more reactive metal is oxidised (loses electrons) and metal ions are reduced (gain electrons). Metals more reactive than carbon require electrolysis for extraction; less reactive metals can be extracted by reduction with carbon. Only metals above hydrogen in the series react with dilute acids. Use systematic experiments to determine relative reactivity.