Electrolysis uses electricity to break down an ionic compound that is molten or dissolved, so its ions can move. The liquid is the electrolyte. Positive ions (cations) move to the negative electrode (cathode) and gain electrons: they are reduced. Negative ions (anions) move to the positive electrode (anode) and lose electrons: they are oxidised. Molten lead bromide gives lead at the cathode and bromine at the anode.
In aqueous solutions, water also provides H⁺ and OH⁻ ions. At the cathode, hydrogen is produced if the metal is more reactive than hydrogen; otherwise the metal forms. At the anode, oxygen is produced unless the solution contains halide ions, in which case the halogen forms. Aluminium is extracted by electrolysis of aluminium oxide dissolved in molten cryolite, which lowers the melting point and saves energy; the carbon anodes burn away and must be replaced.
Worked example
What forms at each electrode in the electrolysis of sodium chloride solution?
- Cathode: sodium is more reactive than hydrogen, so hydrogen gas forms.
- Anode: chloride ions are present, so chlorine gas forms.
Answer:Hydrogen at the cathode, chlorine at the anode (sodium hydroxide stays in solution).
Key words
- electrolyte
- the molten or dissolved ionic compound
- cathode
- negative electrode; cations are reduced here
- anode
- positive electrode; anions are oxidised here
- half equation
- shows electrons gained or lost at an electrode
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NGSS-referenced: HS-PS1-2