Metals and Extraction
This domain tests the reactivity series and how it controls extraction, plus the properties and protection of metals. You must link a metal's position in the series to the method used to obtain it.
The reactivity series and extraction
Potassium, sodium, calcium, magnesium and aluminium are more reactive than carbon, so they are extracted by electrolysis of their molten compounds. Zinc, iron, lead and copper are less reactive than carbon, so their oxides can be reduced with carbon or carbon monoxide. Gold and platinum occur native because they are very unreactive.
Iron and aluminium
- Iron is extracted in a blast furnace from haematite (Fe₂O₃) using coke and limestone. Coke is oxidised to carbon monoxide, which reduces the iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂.
- Aluminium is extracted by electrolysis of alumina dissolved in molten cryolite. Cryolite lowers the operating temperature and saves energy.
Alloys and corrosion
An alloy is a mixture of a metal with other elements; the different-sized atoms distort the layers, making the alloy harder than the pure metal. Rusting needs both oxygen and water. Prevention methods include painting, oiling, plastic coating, galvanising and sacrificial protection with a more reactive metal such as zinc or magnesium.
Worked example
Explain why aluminium cannot be extracted by heating its oxide with carbon. Aluminium is above carbon in the reactivity series, so aluminium has a stronger attraction for oxygen than carbon does. Carbon cannot remove oxygen from aluminium oxide; the reaction is not feasible. Electrolysis of molten alumina is needed instead, using cryolite to dissolve the alumina and lower the melting point.
Traps that cost marks
- Saying iron is extracted by electrolysis; it is reduced by carbon monoxide.
- Confusing haematite with the product; haematite is the ore, iron is the product.
- Saying rusting needs only air; water is also required.
- Forgetting that galvanising protects by both a barrier and sacrificial action.