Metals and extraction
Metals and extraction tests the reactivity series, industrial extraction, alloys and corrosion.
Reactivity and extraction
The reactivity series places metals in order: potassium, sodium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver and gold. Metals above carbon are extracted by electrolysis of their molten compounds; metals below carbon can be reduced with carbon. Gold and platinum occur native because they are very unreactive.
Iron and aluminium
In the blast furnace, hematite (Fe2O3) is reduced by carbon monoxide: Fe2O3 + 3CO → 2Fe + 3CO2. Limestone removes acidic impurities as slag. Aluminium is extracted from purified bauxite (aluminium oxide) by electrolysis in molten cryolite. Cryolite lowers the melting point and saves energy. At the cathode, Al3+ + 3e− → Al; at the carbon anodes, oxide ions are oxidised to oxygen, which burns the anodes.
Corrosion
Rusting requires both oxygen and water. Iron rusts to hydrated iron(III) oxide. Painting, oiling, galvanising and alloying exclude water or oxygen. Sacrificial protection uses a more reactive metal such as zinc or magnesium, which corrodes instead of iron.
Worked example
Explain why aluminium cannot be extracted from aluminium oxide by heating with carbon. Aluminium is above carbon in the reactivity series, so it has a greater tendency to form compounds. Carbon is not reactive enough to reduce aluminium oxide; the reaction is not feasible under normal furnace conditions. Electrolysis of the molten oxide in cryolite is therefore used.
Traps
Iron ore is not pure iron; it is a compound such as Fe2O3. Cryolite is not the electrolyte; it is a solvent that lowers the melting point. Rusting is not simply iron + oxygen: water is also needed. Galvanising protects iron by both a barrier and sacrificial action.