Electrolysis and extracting metals
Chemistry · GCSE · The School
Splitting with electricity
ELECTROLYSIS uses an electric current to break a compound into its elements. The compound must be molten or dissolved so its ions can move. Positive ions travel to the negative electrode (the CATHODE) and gain electrons; negative ions travel to the positive electrode (the ANODE) and lose them.
What forms where
At the cathode, metal forms — unless the metal is more reactive than hydrogen and the solution is aqueous, in which case hydrogen is produced instead. At the anode, a halogen forms if one is present; otherwise oxygen. Working out the products means asking what ions are present and which is discharged more readily.
Why the method depends on reactivity
Metals LESS reactive than carbon — iron, zinc, copper — can be extracted by heating their ore with carbon, which is cheaper. Metals MORE reactive than carbon — aluminium, sodium, magnesium — cannot, because carbon will not displace them, so electrolysis is required. That is why aluminium remained rare and expensive until electrolysis was industrialised.
Iron sits BELOW carbon in the reactivity series, so carbon can take the oxygen from it: iron oxide + carbon → iron + carbon dioxide, in a blast furnace, and cheaply. Aluminium sits ABOVE carbon, so carbon cannot reduce it — the aluminium holds its oxygen more strongly. It must be electrolysed, which is why aluminium was once more precious than gold and is still made where electricity is cheapest.
Half equations, and which way the electrons go
Reduction happens at the cathode, where positive ions gain electrons; oxidation at the anode, where negative ions lose them. Writing the half equation with electrons on the correct side is the mark. The mnemonic is only useful if you can also say what is physically happening at each electrode.
In solution, water competes
Electrolysing a solution is not the same as electrolysing a molten compound, because water provides hydrogen and hydroxide ions too. At the cathode, the less reactive of the metal and hydrogen is produced — so sodium chloride solution gives hydrogen, not sodium. That rule explains most of the surprising products.