Electrolysis of Aqueous Solutions — AQA GCSE Chemistry
Electrolysing a solution is more complex than a molten compound because water also provides ions.
Why solutions are different
In an aqueous solution, the water also produces a small number of H⁺ and OH⁻ ions, as well as the ions from the dissolved compound. This means there is a "competition" at each electrode, decided by simple rules.
The rules
At the cathode (negative):
- Hydrogen is produced unless the metal ions are from a metal less reactive than hydrogen (e.g. copper, silver), in which case the metal is deposited.
At the anode (positive):
- A halogen (chlorine, bromine, iodine) is produced if a halide ion is present.
- Otherwise, oxygen is produced (from OH⁻ ions).
Examples
- Copper sulfate solution (inert electrodes): copper at the cathode (less reactive than H), oxygen at the anode.
- Sodium chloride solution (brine): hydrogen at the cathode (Na is more reactive than H), chlorine at the anode (halide present).
Required practical link
You can electrolyse solutions such as copper sulfate and sodium chloride using inert electrodes and identify the products formed at each electrode.
Exam tips
- Remember water adds H⁺ and OH⁻ to the mix.
- Cathode: hydrogen unless the metal is less reactive than hydrogen (then metal).
- Anode: halogen if a halide is present, otherwise oxygen.
- Practise predicting the products for given solutions.