The big idea: Some metals lose electrons far more readily than others. The activity series (also called the reactivity series) is a league table of metals ranked from most reactive at the top to least reactive at the bottom.
The higher a metal sits, the more readily it is oxidised — that is, the more easily it loses electrons to form positive ions.
| Metal | Reactivity | Reaction with cold water / acid |
|---|---|---|
| potassium, K | most reactive | violently with cold water |
| sodium, Na | ↑ | vigorously with cold water |
| calcium, Ca | ↑ | steadily with cold water |
| magnesium, Mg | ↑ | very slowly with water; fast with acid |
| zinc, Zn | ↑ | with acid only |
| iron, Fe | ↑ | slowly with acid |
| (hydrogen, H) | reference | — (the dividing line) |
| copper, Cu | ↓ | no reaction with acid |
| silver, Ag | least reactive | no reaction with acid |
Define your terms: - Oxidation — loss of electrons (a more reactive metal does this easily). - Reduction — gain of electrons. - Reducing agent — a species that donates electrons (and is itself oxidised). A reactive metal is a strong reducing agent.
Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
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A displacement reaction is when a more reactive metal pushes a less reactive metal out of its compound (usually a solution of its salt). The more reactive metal takes the place of the less reactive one.
The rule: A more reactive metal displaces a less reactive metal from a solution of its ions.
If the metal added is higher in the series than the metal in solution, a reaction happens. If it is lower, nothing happens.
Zinc and copper(II) sulfate: Zinc is above copper, so zinc displaces copper:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
The blue colour of the Cu²⁺(aq) fades and a reddish-brown copper coating forms on the zinc.
Worked example — splitting into half-equations
Show, using half-equations, why the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s) is a redox reaction.
Solution
- Oxidation (electron loss) — zinc, the more reactive metal, loses electrons:
- Reduction (electron gain) — copper(II) ions gain those electrons:
- Electrons are transferred from Zn to Cu²⁺, so this is a redox reaction: Zn is oxidised (reducing agent), Cu²⁺ is reduced (oxidising agent).
Final answer
Zn is oxidised (loses 2e⁻) and Cu²⁺ is reduced (gains 2e⁻) — electron transfer, so it is redox.
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The same series predicts how a metal reacts with dilute acid and with water. The more reactive the metal, the more vigorous the reaction — and the further down you go, the less happens.
Metal + acid: Any metal above hydrogen in the series reacts with a dilute acid to give a salt + hydrogen gas:
metal + acid → salt + H2
For example: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g). Metals below hydrogen (copper, silver) do not react with dilute acid.
Metal + water: The most reactive metals react even with cold water to give a metal hydroxide + hydrogen:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Less reactive metals (zinc, iron) react only slowly, and copper/silver not at all.
More reactive (top of series)
- easily oxidised — loses electrons readily
- reacts with cold water
- vigorous reaction with acid
- displaces most other metals from solution
Less reactive (bottom of series)
- hard to oxidise — holds onto its electrons
- no reaction with water or acid
- is displaced by metals above it
- found as the metal in nature (e.g. gold, silver)
How this is tested: The activity series shows up two ways.
- Paper 1A (MCQ): given the series, predict which displacement is spontaneous (or which metal + acid reacts). - Paper 2: deduce an order of reactivity from a set of displacement results, or discuss how to compare two metals' reactivity in the lab using the metals and their salt solutions.
The marking points are the rule (more reactive displaces less reactive) applied with a reason, not just a yes/no answer.
Score the reasoning: Every prediction needs the rule as the reason: 'X reacts because X is above Y in the series / X is more reactive than Y'. State the observation too (e.g. colour change, metal coating) where the question asks for evidence.
IB-style question — order three metals by reactivity
Three metals P, Q and R were each added to solutions of the metal sulfates P²⁺(aq), Q²⁺(aq) and R²⁺(aq). The results are shown below. Deduce the order of the three metals from most to least reactive, and justify your order. [3]
How to score the marks
- Read the table — a metal that reacts with another metal's solution is the more reactive of the two.
- Mark 1 — Q is most reactive. Q displaces both P and R (Q reacts with P²⁺ and R²⁺), so Q is more reactive than both.
- Mark 2 — P beats R. P reacts with R²⁺ but R does not react with P²⁺, so P is more reactive than R.
- Mark 3 — the order. Combining these: Q > P > R (most → least reactive).
Final answer
Q > P > R. Q displaces both others (most reactive); P displaces R but not Q; R displaces neither (least reactive).
| Metal added ↓ / Solution → | P²⁺(aq) | Q²⁺(aq) | R²⁺(aq) |
|---|---|---|---|
| P | — | no reaction | reaction |
| Q | reaction | — | reaction |
| R | no reaction | no reaction | — |