Reactivity series

Cambridge IGCSE Chemistry 0620 · Core and Extended

Syllabus points

• State the order of the reactivity series as potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold.
• Describe the reactions, if any, of these metals with cold water, with steam and with dilute hydrochloric acid.
• Explain the apparent unreactivity of aluminium in terms of its oxide layer.
• Deduce an order of reactivity from a given set of experimental results.
• Extended only: explain the relative reactivities of metals in terms of their tendency to form positive ions, illustrated by displacement reactions.

The series

From most reactive at the top:

1. Potassium
2. Sodium
3. Calcium
4. Magnesium
5. Aluminium
6. Carbon
7. Zinc
8. Iron
9. Hydrogen
10. Copper
11. Silver
12. Gold

Carbon and hydrogen are non-metals, put into the list as markers. Carbon's place decides which metals can be extracted by reduction with carbon, and hydrogen's place decides which metals react with dilute acids.

A mnemonic that keeps the order straight: Please Stop Calling Me A Careless Zebra Instead, Have Copper Silver Gold.

The three reactions

Read the cut-offs out of the table, because that is what multiple choice questions test:

• Cold water: only potassium, sodium and calcium react properly. Magnesium reacts extremely slowly.
• Steam: magnesium, aluminium, zinc and iron all react, and the ones above them are too dangerous to try.
• Dilute acid: every metal above hydrogen reacts. Copper, silver and gold do not react at all.

The products differ and the difference is examined:

• Metal + cold water → metal hydroxide + hydrogen. 2Na + 2H₂O → 2NaOH + H₂
• Metal + steam → metal oxide + hydrogen. Mg + H₂O → MgO + H₂
• Metal + dilute acid → salt + hydrogen. Zn + 2HCl → ZnCl₂ + H₂

Aluminium, the awkward one

Aluminium sits above zinc and iron, so it should be the more vigorous of the three. In practice a strip of aluminium foil dropped into dilute hydrochloric acid does nothing for a minute or so before it starts to fizz.

The reason is a thin, hard, impermeable layer of aluminium oxide that forms on the surface the moment it meets air. Air and water cannot get through it, so the metal underneath is protected. Once the acid has eaten through the layer, the reaction proceeds at the rate the reactivity series predicts.

So aluminium's unreactivity is apparent, not real, and the mark is for saying which.

Displacement of one metal by another

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

Zn + CuSO₄ → ZnSO₄ + Cu

The observations are the evidence and should be quoted:

• The blue copper(II) sulfate solution fades to colourless.
• A pink-brown deposit of copper appears on the zinc.
• The temperature rises, because displacement is exothermic.
• The grey zinc strip is eaten away.

Iron does the same, more slowly, and the solution goes from blue to pale green because iron(II) sulfate is formed. Copper added to zinc sulfate solution gives no reaction at all, because copper is the less reactive of the two.

Worked example. 6.5 g of zinc is added to excess copper(II) sulfate solution. The relative atomic mass of zinc is 65 and of copper is 64.

moles of zinc = 6.5 / 65 = 0.10 mol

One mole of zinc displaces one mole of copper, so

mass of copper deposited = 0.10 × 64 = 6.4 g

Extended only: reactivity as the tendency to form ions

A metal is more reactive when it loses electrons more readily to form a positive ion. That single sentence is the explanation the Extended paper wants, and every observation above follows from it.

In the zinc and copper reaction, the two half equations are:

Zn → Zn²⁺ + 2e⁻

Cu²⁺ + 2e⁻ → Cu

Zinc gives up its electrons more readily than copper does, so the electrons pass from the zinc atoms to the copper ions. The reaction is redox: zinc is oxidised, copper is reduced, and zinc is acting as the reducing agent.

The same reasoning explains why metals above hydrogen react with acids. Magnesium loses electrons more readily than hydrogen holds on to them, so Mg → Mg²⁺ + 2e⁻ and 2H⁺ + 2e⁻ → H₂. Copper does not lose electrons readily enough, so nothing happens.

Deducing an order from results

Cambridge gives a grid of experiments and asks for the order. Work by pairs.

Suppose metals W, X, Y and Z give these results:

• X displaces Y from Y sulfate solution, so X is above Y.
• W does not react with dilute acid, so W is below hydrogen and below everything that does.
• Z reacts with cold water, so Z is the most reactive of the four.
• Y displaces W, so Y is above W.

Putting the pairs together: Z, then X, then Y, then W.

Two rules keep the deduction honest. A reaction shows the added metal is the more reactive; no reaction shows it is the less reactive. And a metal that reacts with cold water outranks one that only reacts with steam, which in turn outranks one that only reacts with acid.

What else the series predicts

• Ease of extraction, in 9.6. Metals below carbon are extracted by reduction with carbon; those above it need electrolysis.
• Stability of compounds. The more reactive the metal, the more stable its compounds, and the harder they are to decompose by heating.
• Corrosion and sacrificial protection, in 9.5. A metal higher in the series corrodes in preference to one lower down.

Common mistakes

• Leaving carbon and hydrogen out of the order, and then being unable to answer the extraction question.
• Saying copper reacts with dilute hydrochloric acid to give hydrogen.
• Writing a hydroxide as the product of a metal with steam, or an oxide as the product with cold water.
• Saying aluminium is less reactive than zinc, rather than that its oxide layer makes it appear so.
• Predicting that copper displaces zinc from zinc sulfate solution.
• Explaining reactivity by "it reacts more" rather than by the tendency to lose electrons and form positive ions.
• Describing a displacement without the colour change, which is where the observation mark sits.