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9.4.2 Reactivity, displacement and the oxide layer of aluminium

Reactivity and positive ions

Definition

Reactivity of a metal

The tendency of the atoms of a metal to lose electrons and form positive ions.

  1. When a metal reacts, its atoms lose electrons from their outer shell and become positive ions.
    1. Magnesium atoms each lose two electrons: Mg→Mg2++2e−\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}Mg→Mg2++2e−.
    2. Copper atoms also lose two electrons: Cu→Cu2++2e−\mathrm{Cu \rightarrow Cu^{2+} + 2e^-}Cu→Cu2++2e−.
  2. A more reactive metal has a greater tendency to lose electrons and form positive ions.
  3. Magnesium loses its electrons far more readily than copper, so magnesium is much higher in the reactivity series.
  4. The ions of a less reactive metal have a greater tendency to gain electrons and turn back into metal atoms.
  5. This difference between two metals is what drives a displacement reaction.

Displacement reactions

Definition

Displacement reaction

A reaction in which a more reactive element takes the place of a less reactive element in a compound.

  1. In a metal displacement reaction, a more reactive metal is placed in a solution that contains the aqueous ions of a less reactive metal.
  2. Atoms of the more reactive metal lose electrons and pass into solution as positive ions.
  3. Ions of the less reactive metal gain these electrons and come out of solution as a coating of solid metal.
  4. Zinc displaces copper from copper(II) sulfate solution: Zn(s)+CuSO4(aq)→ZnSO4(aq)+Cu(s)\mathrm{Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)}Zn(s)+CuSO4​(aq)→ZnSO4​(aq)+Cu(s).
  5. The sulfate ions are unchanged, so they are spectator ions and are left out of the ionic equation: Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)\mathrm{Zn(s) + Cu^{2+}(aq) \rightarrow Zn^{2+}(aq) + Cu(s)}Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s).
  6. During this reaction the blue colour of the solution fades, a red-brown coating of copper forms on the zinc, and the temperature rises.
  7. Zinc atoms lose electrons, so zinc is oxidised, while copper(II) ions gain electrons, so they are reduced.
  8. A displacement reaction is therefore a redox reaction.
  9. If the less reactive metal is placed in a solution of the more reactive metal's ions, no reaction happens, so copper in zinc sulfate solution shows no change.
  10. The further apart two metals are in the reactivity series, the larger the temperature rise in their displacement reaction usually is.

Predicting displacement for five metals

  1. The order of magnesium, zinc, iron, copper and silver, from most to least reactive, is Mg>Zn>Fe>Cu>Ag\mathrm{Mg > Zn > Fe > Cu > Ag}Mg>Zn>Fe>Cu>Ag.
  2. Each metal displaces the ions of every metal below it and none of the ions of the metals above it.
    1. Magnesium displaces zinc, iron(II), copper(II) and silver ions.
    2. Zinc displaces iron(II), copper(II) and silver ions.
    3. Iron displaces copper(II) and silver ions.
    4. Copper displaces silver ions only.
    5. Silver displaces none of these ions.
  3. Iron in copper(II) sulfate solution turns the blue solution pale green and becomes coated with brown copper: Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s)\mathrm{Fe(s) + Cu^{2+}(aq) \rightarrow Fe^{2+}(aq) + Cu(s)}Fe(s)+Cu2+(aq)→Fe2+(aq)+Cu(s).
  4. Copper in silver nitrate solution grows shiny grey crystals of silver, while the colourless solution slowly turns blue: Cu(s)+2Ag+(aq)→Cu2+(aq)+2Ag(s)\mathrm{Cu(s) + 2Ag^+(aq) \rightarrow Cu^{2+}(aq) + 2Ag(s)}Cu(s)+2Ag+(aq)→Cu2+(aq)+2Ag(s).
  5. Magnesium in iron(II) sulfate solution becomes coated with dark grey iron as the pale green colour fades: Mg(s)+Fe2+(aq)→Mg2+(aq)+Fe(s)\mathrm{Mg(s) + Fe^{2+}(aq) \rightarrow Mg^{2+}(aq) + Fe(s)}Mg(s)+Fe2+(aq)→Mg2+(aq)+Fe(s).
Practical

Displacement reactions on a spotting tile

  • Aim: to place magnesium, zinc, iron and copper in order of reactivity by testing each metal in solutions of the other metals' ions.
  • Apparatus: a white spotting tile, small pieces of magnesium ribbon, zinc foil, iron wire and copper foil, emery paper, forceps, dropping pipettes, a hand lens, and dilute solutions of magnesium sulfate, zinc sulfate, iron(II) sulfate, copper(II) sulfate and silver nitrate.
  • Method, setting up:
    • Draw a grid on paper with the four metals along the top and the five solutions down the side, and stand the spotting tile on it so that each well matches one combination.
    • Rub each piece of metal with emery paper, so that a surface coating does not stop a reaction from starting.
    • Use a clean dropping pipette for each solution and half fill the matching wells.
  • Method, testing:
    • Use forceps to place one piece of metal into each well, leaving out the wells where a metal would meet its own ions.
    • Leave the tile undisturbed for about five minutes.
    • Look closely at each piece through the hand lens.
    • Record a tick where a coating forms on the metal or the solution changes colour, and a cross where there is no change.
  • Method, processing the results:
    • Count the number of ticks for each metal.
    • Rank the metals, because the metal with the most ticks displaces the most metals and so is the most reactive.
  • Results: magnesium reacts in all four of the other solutions, zinc reacts in three, iron reacts in two and copper reacts only in silver nitrate.
  • Conclusion: the order from most to least reactive is magnesium, zinc, iron, copper, with silver below copper.
  • Watch out: a metal that has not been cleaned may give a false cross, and a very thin coating can be missed unless every piece is checked with the hand lens.
  • Safety: wear eye protection and gloves, because silver nitrate stains skin black and copper(II) sulfate is harmful, and wash your hands at the end.
Example

Deducing an order from a displacement grid

  • Metal P becomes coated with a new metal in solutions of the ions of Q and of R.
  • Metal Q becomes coated only in the solution of the ions of R.
  • Metal R shows no change in any of the solutions.
  • P displaces both Q and R, so P is the most reactive.
  • Q displaces R but not P, so Q comes second.
  • R displaces neither of the others, so it is the least reactive, and the order is P, Q, R.

The oxide layer on aluminium

  1. Aluminium is high in the reactivity series, just below magnesium, yet in everyday use it often seems unreactive.
  2. An aluminium saucepan does not fizz when water is boiled in it, and aluminium foil placed in copper(II) sulfate solution shows little or no change.
  3. This is because aluminium is always covered by a very thin, tough layer of aluminium oxide, Al2O3\mathrm{Al_2O_3}Al2​O3​.
  4. The layer forms as soon as fresh aluminium meets oxygen in the air: 4Al(s)+3O2(g)→2Al2O3(s)\mathrm{4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)}4Al(s)+3O2​(g)→2Al2​O3​(s).
  5. The oxide layer is unreactive, it sticks firmly to the metal and it has no gaps, so water, acids and aqueous ions cannot reach the aluminium underneath.
  6. If the surface is scratched, a new layer of oxide forms almost at once, so the protection repairs itself.
  7. When the oxide layer is removed, the aluminium underneath reacts as vigorously as its position in the series predicts.
    1. In dilute hydrochloric acid, aluminium reacts slowly at first and then fizzes vigorously once the acid has dissolved the oxide layer.
  8. The apparent unreactivity of aluminium is caused by its oxide layer, not by a low tendency to form positive ions.
  9. The same oxide layer explains why aluminium resists corrosion and can be used for food containers.
Exam technique

Explaining displacement and aluminium

  • In a displacement explanation, state that the more reactive metal loses electrons more readily, or has a greater tendency to form positive ions.
  • Give the observations a question asks for, such as the colour change of the solution and the colour of the metal coating.
  • For aluminium, state that it is covered by an oxide layer that stops water or acid reaching the metal underneath.
  • Write ionic equations with state symbols and leave out the spectator ions.
Common Mistake
  • Do not describe aluminium as an unreactive metal, because its high reactivity is hidden by the oxide layer.
  • Do not include the sulfate or nitrate ion in the ionic equation for a displacement reaction, because it is a spectator ion.
  • Do not swap the roles, because the more reactive metal is oxidised and the ions of the less reactive metal are reduced.
Self review
  • How is the reactivity of a metal linked to its tendency to form positive ions?
  • What do you see when zinc is added to copper(II) sulfate solution?
  • Write the ionic equation for copper displacing silver from silver nitrate solution.
  • Which of magnesium, zinc, iron, copper and silver can displace iron(II) ions from solution?
  • Why does aluminium appear unreactive even though it is high in the reactivity series?
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9.4.2 Reactivity, displacement and the oxide layer of aluminium Revision Guide

  1. IGCSE
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  3. /9.4.2 Reactivity, displacement and the oxide layer of aluminium

Revision notes for CIE IGCSE Chemistry 9.4.2 Reactivity, displacement and the oxide layer of aluminium: explanations and worked examples.

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