Most metals are found as compounds in ores, and the way to get the metal out depends on how tightly it holds on to the oxygen. Metals below carbon can be reduced by carbon; metals above carbon need the far more expensive route of electrolysis.
An ore is a rock that contains enough of a metal compound to make extraction worthwhile. The method of extraction depends on the metal's position in the reactivity series: metals below carbon (such as zinc, iron and copper) can be extracted by reduction of their oxides with carbon or carbon monoxide; metals above carbon (such as aluminium and sodium) must be extracted by electrolysis of a molten compound. Very unreactive metals such as gold are found uncombined.
Fig. 1: the reactivity series decides the extraction method
Section 01
Reduction by carbon.
Carbon is more reactive than metals below it, so it can take the oxygen from their oxides.
Reduction: the metal oxide loses oxygen. Carbon is oxidised to carbon dioxide or carbon monoxide.
Zinc: 2ZnO + C → 2Zn + CO2 (zinc oxide comes from roasting the ore zinc blende, ZnS).
Iron: Fe2O3 + 3CO → 2Fe + 3CO2 in the blast furnace.
Copper: 2CuO + C → 2Cu + CO2.
Why carbon? It is cheap (coke from coal) and also provides the heat when it burns.
Stage 1 · Learn
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What is an ore?
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Which metal must be extracted by electrolysis?
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Which metal can be extracted by heating its oxide with carbon?
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Why is gold found as the uncombined element?
Section 02
Electrolysis for reactive metals.
Metals above carbon hold their oxygen too strongly for carbon to remove it.
Electrolysis: the ore is melted (or dissolved in a molten solvent) and a current splits it; the metal forms at the cathode.
Aluminium from bauxite (aluminium oxide); sodium from molten sodium chloride.
Cost: electrolysis needs large amounts of electricity and high temperatures, so these metals are more expensive to extract.
Native metals: gold and silver are so unreactive that they are found as the element.
Examiner trap
Aluminium is not extracted with carbon, because aluminium is above carbon in the reactivity series. And the metal is reduced during extraction: whether by carbon or at the cathode, the metal ion gains electrons or the oxide loses oxygen.
Stage 2 · Exam
Exam-style questions
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In 2CuO + C → 2Cu + CO2, which substance is reduced?
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Why can magnesium not be extracted by heating magnesium oxide with carbon?
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Why are metals extracted by electrolysis generally more expensive?
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Titanium is between aluminium and zinc in the reactivity series, but titanium oxide is reduced using sodium in industry. Which statement explains why sodium works?
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