How to explain displacement reactions of metals

Metal displacement is one of the friendliest topics in SPM Chemistry because a single rule predicts every reaction, and the observations are dramatic and easy to describe. The trick to full marks is explaining not just that a reaction happens, but why, and backing it with the right ionic equation. This guide walks through the displacement of metals so you can answer both the prediction and the explanation.

The one rule that runs the whole topic

A more reactive metal displaces a less reactive metal from its salt solution.

To use it, you only need the reactivity series. If the metal you add is higher (more reactive) than the metal already combined in the salt, a reaction happens: the added metal pushes the other one out. If the added metal is lower (less reactive), nothing happens.

Think of it as a swap. The more reactive metal “wants” to be an ion more strongly, so it takes the place in solution and forces the less reactive metal to come out as a solid.

Step-by-step method

1. Identify the two metals, the solid you add and the metal inside the salt solution.
2. Compare them in the reactivity series.
3. Decide: added metal more reactive → reaction; less reactive → no reaction.
4. Write the full equation, then the ionic equation.
5. Describe the observations, colour changes, a coating of new metal, temperature change.

Worked example: zinc in copper(II) sulfate

Add zinc powder to blue copper(II) sulfate solution.

• Zinc is more reactive than copper, so a reaction happens.
• Full equation: Zn + CuSO₄ → ZnSO₄ + Cu
• Ionic equation: Zn + Cu²⁺ → Zn²⁺ + Cu

Observations to describe:

• The blue colour of the solution fades (Cu²⁺ ions are used up).
• A reddish-brown solid (copper) deposits on the zinc.
• The mixture warms up, displacement is exothermic.

Notice the sulfate ion (SO₄²⁻) does not change, it is a spectator ion, which is why the ionic equation leaves it out.

The redox story behind it

Every metal displacement is a redox reaction, and saying so earns marks. In Zn + Cu²⁺ → Zn²⁺ + Cu:

• Zinc loses electrons: Zn → Zn²⁺ + 2e⁻ (oxidised; zinc is the reducing agent).
• Copper(II) gains electrons: Cu²⁺ + 2e⁻ → Cu (reduced; Cu²⁺ is the oxidising agent).

So the more reactive metal is always the one oxidised, and the ion of the less reactive metal is reduced. Linking displacement to electron transfer shows the examiner you understand the mechanism, not just the pattern.

A second example: iron in copper(II) sulfate

Add an iron nail to copper(II) sulfate solution.

• Iron is more reactive than copper, so it displaces copper.
• Ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu
• The nail becomes coated with a reddish-brown layer of copper, and the blue solution slowly turns the pale green of iron(II) ions.

Contrast that with putting a copper strip into iron(II) sulfate: copper is less reactive than iron, so no reaction occurs and the copper stays unchanged. Being able to explain a “no reaction” case is just as important as explaining a reaction.

Displacement and the heat of displacement

The further apart the two metals are in the reactivity series, the more heat is released. So displacing copper with magnesium releases more heat than displacing it with zinc, because magnesium is further from copper. This links neatly to the “heat of displacement” experiments you may meet in thermochemistry, and you can rehearse the practical by displacing metals from salt solutions.

Common mistakes to avoid

• Predicting a reaction the wrong way round. Only a more reactive metal displaces a less reactive one.
• Forgetting the ionic equation, or leaving the spectator ion in it. Cancel ions that appear unchanged on both sides.
• Describing the wrong colour. Copper(II) solutions are blue; iron(II) solutions are pale green; copper metal is reddish-brown.
• Not stating that the reaction is exothermic when asked about temperature.

How to phrase a full-mark answer

A complete explanation reads like this: “Zinc is more reactive than copper, so zinc displaces copper from copper(II) sulfate. Zinc is oxidised to Zn²⁺ while Cu²⁺ is reduced to copper metal. The blue colour fades, a reddish-brown solid forms, and the mixture warms up.” Predict, give the ionic equation, name the redox change, and describe what you would see.

Practise the pattern until it is instant

Because one rule drives everything, displacement rewards a little drilling: pick pairs of metals, predict, write the ionic equation, and describe the observations. Do ten of these and the topic is yours for both Paper 1 and Paper 2.

If the reactivity series or ionic equations still slow you down, that is a quick win with a teacher. Our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial; see how it works if you would like displacement drilled against real exam questions.