Reactivity decides the method
Most metals occur combined in ores as oxides, sulfides or carbonates. Extraction always involves reduction of the metal ion to the metal atom. The reactivity series decides which reducing method works.
- Very reactive metals (K, Na, Ca, Mg, Al) are extracted by electrolysis of the molten compound, because they hold their ions too strongly for chemical reduction.
- Moderately reactive metals (Zn, Fe, Sn, Pb) are reduced by heating with carbon (or carbon monoxide).
- Unreactive metals (Ag, Au) occur native or need only gentle heating.
Worked example
In the blast furnace, iron(III) oxide is reduced by carbon monoxide:
Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)
Iron is reduced (Fe goes from +3 to 0) while carbon is oxidised (from +2 in CO to +4 in CO2), so this is a redox reaction.
Aluminium by electrolysis
Aluminium is too reactive to reduce with carbon, so purified alumina (Al2O3) is dissolved in molten cryolite and electrolysed. At the cathode: Al3+ + 3e− → Al.
The full reactivity series, in decreasing order, is K, Na, Ca, Mg, Al, (C), Zn, Fe, Sn, Pb, (H), Cu, Ag, Au. Carbon sits between aluminium and zinc, which is why zinc and every metal below it can be reduced by carbon, while metals above carbon need electrolysis. Some sulfide ores are first roasted in air to convert them into oxides before reduction. Recycling metals is important because it saves the very large amount of energy needed to extract them from their ores again.
Remember
- Extraction = reduction of the metal ion.
- Above carbon in the series → electrolysis.
- Below carbon → reduction by carbon/CO.
- Reactivity high = harder (more energy) to extract.