Most metals are extracted from ore found in the Earth's crust. An ore is a rock that contains enough of a metal or a metal compound to make extracting the metal worthwhile.
The extraction method used depends upon the metal's position in the reactivity series. In principle, any metal could be extracted from its compounds using electrolysis. However, large amounts of electrical energy are needed to do this, so electrolysis is expensive.
If a metal is less reactive than carbon, it can be extracted from its compounds by heating with carbon. Copper is an example of this. Copper mostly occurs as sulfide ores, which are heated in air to convert them to copper(II) oxide. Molten copper can be produced from copper oxide by heating with carbon:
Copper oxide + carbon → copper + carbon dioxide
2CuO(s) + C(s) → 2Cu(l) + CO2(g)
The table summarises the extraction methods used for different metals.
Although an unreactive metal is found as the metal itself, chemical reactions are often needed to remove other elements that might contaminate it.
Iron(III) oxide is reduced to molten iron when it reacts with carbon. One of the products is carbon monoxide:
iron(III) oxide + carbon → iron + carbon monoxide
Fe2O3(s) + 3C(s) → 2Fe(l) + 3CO(g)
This method of extraction works because carbon is more reactive than iron, so it can displace iron from iron compounds. Extracting a metal by heating with carbon is cheaper than using electrolysis.
In the reaction of iron(III) oxide with carbon, state which substance is reduced and which substance is oxidised.
Carbon gains oxygen, so it is oxidised. Iron(III) oxide loses oxygen, so it is reduced.