In a displacement reaction, a more reactive element takes the place of a less reactive element in a compound. The key skill at GCSE is using the reactivity series to predict whether a displacement will occur, then explaining the outcome in terms of electron transfer. This topic bridges the reactivity series, ionic equations, and redox chemistry.
What is a displacement reaction?
A displacement reaction occurs when a more reactive element displaces (pushes out) a less reactive element from its salt solution or oxide. The more reactive element takes on the ionic form, whilst the less reactive element is released as a neutral element.
General rule: If metal A is more reactive than metal B, then metal A will displace metal B from a solution of B's salt.
Metal A + Salt of B → Salt of A + Metal B
This only works if A is higher than B in the reactivity series. If A is lower than B, no reaction occurs.
How do you use the reactivity series to predict displacement?
The reactivity series lists metals in order of their reactivity, from most to least reactive:
| Position | Metal | Symbol |
|---|---|---|
| Most reactive | Potassium | K |
| Sodium | Na | |
| Calcium | Ca | |
| Magnesium | Mg | |
| Aluminium | Al | |
| Zinc | Zn | |
| Iron | Fe | |
| Lead | Pb | |
| Copper | Cu | |
| Silver | Ag | |
| Least reactive | Gold | Au |
Predict first: If you add iron filings to copper sulfate solution, what do you expect? Iron (Fe) is above copper (Cu) in the series — so iron should displace copper. Then observe: the blue solution fades as Cu²⁺ ions are removed; a brown/red deposit of copper metal forms on the iron. The gap is confirmed.
What happens in a displacement reaction at the particle level?
At the particle level, displacement reactions involve a transfer of electrons — they are redox reactions (simultaneous reduction and oxidation).
Worked example: Iron displacing copper from copper sulfate solution
Word equation:
iron + copper sulfate → iron sulfate + copper
Symbol equation:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
The sulfate ion (SO₄²⁻) is a spectator ion — it appears on both sides unchanged. Removing spectator ions gives the ionic equation:
Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
Now the electron transfer is clear:
- Iron is oxidised: Fe → Fe²⁺ + 2e⁻ (loses 2 electrons)
- Copper is reduced: Cu²⁺ + 2e⁻ → Cu (gains 2 electrons)
A more reactive metal loses electrons more readily — this is why it can reduce (donate electrons to) the ion of a less reactive metal.
What are some common displacement reaction examples?
| Reaction | More reactive metal | Less reactive metal displaced | Observation |
|---|---|---|---|
| Zinc + copper sulfate | Zinc | Copper | Blue solution fades; grey zinc surface becomes coated in brown copper |
| Magnesium + iron sulfate | Magnesium | Iron | Colourless solution; grey iron deposits on magnesium ribbon |
| Iron + lead nitrate | Iron | Lead | Colourless solution; dull grey lead crystals form |
| Copper + silver nitrate | Copper | Silver | Colourless solution turns blue; silver crystals (grey/white) form on copper |
What about non-metal displacement reactions?
Displacement is not limited to metals. The halogens (Group 7) also displace one another in order of reactivity:
Reactivity of halogens (most → least reactive): F₂ > Cl₂ > Br₂ > I₂
A more reactive halogen will displace a less reactive one from its salt solution:
Example: Chlorine water + potassium bromide solution
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Ionic equation (K⁺ is the spectator ion):
Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)
Observation: the colourless solution turns orange-brown as bromine is displaced.
Example: Chlorine water + potassium iodide solution
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
Observation: solution turns brown/dark; with starch indicator, it turns blue-black as iodine is produced.
Bromine water will displace iodide but NOT chloride — a useful test to work out the order of reactivity experimentally.
How do displacement reactions link to thermite and industrial processes?
Thermite reaction — Aluminium displaces iron from iron(III) oxide. The reaction releases enormous heat and produces molten iron:
2Al(s) + Fe₂O₃(s) → Al₂O₃(s) + 2Fe(l)
This is used industrially to weld railway tracks in situ. The molten iron fills the gap between rail sections, solidifying to form a seamless join.
More broadly, displacement reactions underpin metal extraction. Metals lower in the reactivity series can be extracted from their ores by heating with a more reactive element (most commonly carbon via the blast furnace for iron) — this is displacement at an industrial scale.
Frequently asked questions
How do you predict whether a displacement reaction will occur?
Use the reactivity series. If the metal you are adding is higher in the series than the metal in the compound, a displacement reaction will occur. If it is lower, no reaction takes place. For example, copper is below zinc in the reactivity series, so copper will not displace zinc from zinc sulfate solution — but zinc will displace copper from copper sulfate solution. The phrase "more reactive displaces less reactive" is the simplest summary.
What is an ionic equation and why is it useful?
An ionic equation shows only the species that actually change during a reaction — the ions that react. Spectator ions (those that appear unchanged on both sides) are removed. Ionic equations make redox processes much clearer: you can see exactly which species loses electrons (is oxidised) and which gains electrons (is reduced). For example, Fe + Cu²⁺ → Fe²⁺ + Cu immediately shows that iron atoms give electrons to copper ions, with no spectator sulfate ions to obscure the chemistry.
What is the difference between oxidation and reduction in a displacement reaction?
In a displacement reaction, the more reactive metal is oxidised — it loses electrons and forms positive ions. The less reactive metal ion is reduced — it gains electrons and becomes a neutral atom. The two processes always occur together (you cannot have oxidation without reduction), which is why they are called a redox reaction. A simple memory aid is OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
Can a halogen displacement reaction be used to identify unknowns?
Yes. If you add chlorine water to a solution and observe an orange-brown colour, bromine has been displaced — confirming the solution contained bromide ions. If you add bromine water and observe a brown colour that turns blue-black with starch, iodide ions were present and iodine has been displaced. If you add chlorine water and observe no colour change, the solution may contain chloride ions (since chlorine cannot displace itself). These colour tests are a standard experimental technique for identifying halide ions alongside silver nitrate tests.
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