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Electrolysis
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Electrolysis uses electricity to split ionic compounds. It is how we get aluminium, chlorine and hydrogen.
The set-up
The electrolyte is an ionic compound that is molten or dissolved, so its ions can move.
Positive ions (cations) move to the negative electrode, the cathode.
Negative ions (anions) move to the positive electrode, the anode.
Positive ions (cations) move to the negative electrode, the cathode.
Negative ions (anions) move to the positive electrode, the anode.
Molten compounds
Molten compounds only contain two ions, so it is simple.
Molten lead bromide: lead forms at the cathode and bromine at the anode.
Molten lead bromide: lead forms at the cathode and bromine at the anode.
Solutions in water
Water adds H+ and OH− ions.
Cathode: hydrogen forms, unless the metal is less reactive than hydrogen (such as copper).
Anode: oxygen forms, unless a halide (chloride, bromide, iodide) is present.
Cathode: hydrogen forms, unless the metal is less reactive than hydrogen (such as copper).
Anode: oxygen forms, unless a halide (chloride, bromide, iodide) is present.
Sodium chloride solution is electrolysed. What forms at each electrode?
- Cathode: sodium is more reactive than hydrogen, so hydrogen forms.
- Anode: chloride is a halide, so chlorine forms.
- Sodium and hydroxide ions stay in solution.
Answer: Hydrogen at the cathode, chlorine at the anode
Cations go to the cathode, anions go to the anode. In solutions, hydrogen forms at the cathode unless the metal is less reactive, and oxygen at the anode unless a halide is present.
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