Extraction of metals

O Level / IGCSE Chemistry · Metals. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

How a metal is extracted from its ore depends on its position in the reactivity series. Metals below carbon, such as zinc and iron, are obtained by heating their oxides with carbon, which removes the oxygen (reduction). Metals above carbon, such as aluminium, hold on to oxygen too strongly and need electrolysis, which costs more. Very unreactive metals such as gold are found uncombined.

Iron is made from hematite, Fe2O3, in the blast furnace. Coke, limestone and hot air are used with the ore, and carbon monoxide does the reducing.

Aluminium is made by electrolysing aluminium oxide, from bauxite, dissolved in molten cryolite. The cryolite lowers the working temperature to about 950 °C, which saves energy.

Rules to remember

Common mistake

Mixing up the jobs of the raw materials in the blast furnace. Coke burns to give the heat and makes carbon monoxide, carbon monoxide reduces the iron(III) oxide, and limestone removes the sandy impurities as slag.

Worked example

What mass of iron can be obtained from 320 kg of iron(III) oxide, Fe2O3? (O 16, Fe 56)

  1. Mr of Fe2O3 = (2 × 56) + (3 × 16) = 112 + 48 = 160.
  2. Fe2O3 + 3CO → 2Fe + 3CO2, so 160 g of Fe2O3 gives 2 × 56 = 112 g of iron.
  3. Mass of iron = 320 × 112 ÷ 160 = 224 kg.

Answer: 224 kg of iron.

Practice questions

Try each one, then open the answer.

1. Why is aluminium extracted by electrolysis rather than by heating its oxide with carbon?

  1. A
    carbon is less reactive than aluminium, so it cannot remove the oxygen
  2. B
    the oxide layer on aluminium stops carbon from reacting with it
  3. C
    electrolysis is always cheaper than heating an oxide with carbon
  4. D
    aluminium oxide is insoluble in water, so carbon cannot reach the metal
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Answer: A. Only metals below carbon in the reactivity series can be extracted by carbon reduction. Aluminium is above carbon, so its molten oxide (in cryolite) is electrolysed.

2. Which half-equation shows the reaction at the cathode during the extraction of aluminium?

  1. A
    Al → Al3+ + 3e−
  2. B
    2O2− → O2 + 4e−
  3. C
    Al3+ + 3e− → Al
  4. D
    O2 + 4e− → 2O2−
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Answer: C. At the negative cathode, aluminium ions gain electrons (reduction) to form molten aluminium. Oxide ions lose electrons at the anode.

3. Which equation shows the reaction that provides the heat in the blast furnace?

  1. A
    CO2 + C → 2CO
  2. B
    C + O2 → CO2
  3. C
    CaCO3 → CaO + CO2
  4. D
    Fe2O3 + 3CO → 2Fe + 3CO2
Show answer

Answer: B. Coke (carbon) burns in the hot air blast in a strongly exothermic reaction. The CO2 formed then reacts with more coke to make CO.

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