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How do we order metals by how reactive they are, and how does that order predict displacement reactions?

Place metals in order of reactivity from their reactions with water, acids and oxygen, and use the reactivity series to predict displacement reactions

A focused answer to the O-Level Combined Science outcome on the reactivity series. Ordering metals from reactions with water, acid and oxygen, and using the series to predict displacement reactions and competition for oxygen.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to order metals by reactivity using their reactions with water (or steam), dilute acid and oxygen, and then to use that order to predict displacement reactions. The key idea is simple: a more reactive metal will displace a less reactive one from its compound, and the evidence for the order comes from how vigorously each metal reacts.

The answer

The reactivity series

Arranging common metals from most to least reactive gives the reactivity series. A useful section, most reactive first, is:

K>Na>Ca>Mg>Al>Zn>Fe>Cu>Ag\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Cu} > \text{Ag}

Carbon and hydrogen are often included as references, because they help explain extraction and reactions with acid.

Reactions with water and steam

  • The most reactive metals (potassium, sodium, calcium) react with cold water, releasing hydrogen.
  • Less reactive metals (magnesium, zinc, iron) react only with steam, giving the metal oxide and hydrogen.
  • Unreactive metals (copper, silver) do not react with water or steam.

The more vigorous the reaction, the higher the metal sits in the series.

Reactions with dilute acid

Metals above hydrogen react with dilute acid to give a salt and hydrogen; the more reactive the metal, the faster the fizzing. Copper and silver, below hydrogen, do not react with dilute acid. This gives another way to rank metals.

Reactions with oxygen

Reactive metals tarnish or burn readily in air, forming oxides; sodium must be stored under oil to keep oxygen away. Unreactive metals such as gold and silver resist oxidation, which is why they stay shiny. The ease of forming the oxide follows the same order.

Displacement reactions

A more reactive metal displaces a less reactive metal from its compound (in solution or in the solid oxide). For example, zinc displaces copper from copper(II) sulfate:

Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}

The more reactive metal "wins" the negative ion and the less reactive metal is left as the free metal. No reaction occurs if the added metal is less reactive than the one in the compound.

Examples in context

Example 1. Sacrificial protection of ships and pipes. Blocks of a more reactive metal such as zinc or magnesium are attached to steel hulls and underground pipes. The reactive metal corrodes in preference to the iron, protecting it, a direct use of the reactivity series to stop rusting.

Example 2. The thermit reaction for welding rails. Aluminium, being more reactive than iron, displaces iron from iron(III) oxide in a vigorous reaction that produces molten iron used to weld railway tracks. The reaction works because aluminium is higher in the reactivity series than iron.

Try this

Q1. Place calcium, copper and iron in order of reactivity, most reactive first. [2 marks]

  • Cue. Calcium > iron > copper.

Q2. State what you would see if a piece of zinc were placed in copper(II) sulfate solution. [2 marks]

  • Cue. A brown coating of copper forms on the zinc and the blue colour of the solution fades.

Q3. Explain why copper does not react with dilute hydrochloric acid. [2 marks]

  • Cue. Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksAn iron nail is placed in blue copper(II) sulfate solution. After some time the nail is coated with a brown solid and the blue colour fades. Explain what has happened, write the equation for the reaction, and state what this shows about the reactivity of iron and copper.
Show worked answer →

Iron is more reactive than copper, so iron displaces copper from copper(II) sulfate solution. The iron goes into solution as iron ions, and copper metal is deposited (the brown solid coating the nail). The blue colour fades because the blue copper(II) ions are being removed from solution.

Equation: Fe+CuSO4→FeSO4+Cu\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}.

This shows iron is more reactive than copper, because a more reactive metal displaces a less reactive metal from its salt solution.

Markers reward iron displacing copper, the brown solid being copper and the fading colour being loss of copper ions, the balanced equation, and the conclusion that iron is more reactive than copper.

Original3 marksDescribe how the reactions of sodium, magnesium and copper with water (or steam) can be used to place these three metals in order of reactivity, most reactive first.
Show worked answer →

Sodium reacts vigorously with cold water, fizzing and moving on the surface, releasing hydrogen. Magnesium reacts very slowly with cold water but reacts readily with steam to give hydrogen. Copper does not react with cold water or with steam.

The more vigorous the reaction with water, the more reactive the metal. So the order, most reactive first, is sodium, then magnesium, then copper.

Markers reward sodium reacting with cold water, magnesium reacting only with steam, copper not reacting, and the correct order sodium > magnesium > copper.

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