Choosing a method of extraction
Most metals are found combined with other elements in ores. Extraction means getting the pure metal from its ore. The method used depends on the metal's position in the reactivity series. The more reactive a metal, the more strongly it holds on to oxygen, so the harder it is to extract.
Key idea
Metals below carbon (e.g. iron, zinc, copper) can be extracted by heating the ore with carbon, which reduces the metal oxide. Metals above carbon (e.g. aluminium) must be extracted by electrolysis. Very unreactive metals such as gold are found native (uncombined).
Reduction with carbon
In the blast furnace, iron(III) oxide is reduced by carbon monoxide. Limestone (calcium carbonate) decomposes and removes sandy impurities as molten slag.
Fe2O3 + 3CO → 2Fe + 3CO2
Electrolysis of reactive metals
Aluminium is more reactive than carbon, so carbon cannot remove its oxygen. Molten aluminium oxide is electrolysed: aluminium forms at the cathode (negative electrode) and oxygen at the anode.
Worked example
Reduce 160 g of Fe2O3 (Mr = 160) completely. Moles of Fe2O3 = 160 ÷ 160 = 1 mol. Each mole gives 2 mol of Fe, so 2 mol Fe are made. Mass of iron = 2 × 56 = 112 g.
Remember
- Reduction = removal of oxygen (gain of electrons).
- Carbon reduction is cheaper; electrolysis uses lots of electricity.
- Reactivity order (extraction): above carbon → electrolysis; below carbon → carbon reduction.