4.14 Electrolysis of aqueous solutions
How water provides hydrogen and hydroxide ions in solution, the rules for predicting the products at each electrode when solutions are electrolysed, and examples including sodium chloride and copper sulfate solutions.
AQA
Edexcel
OCR
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Ions from water
Key terms
- Aqueous solution: a solution in which water is the solvent, shown by (aq).
When an ionic compound is dissolved in water, the solution contains its ions and a small number of \mathrm{H^+} and \mathrm{OH^-} ions from water. At each electrode, only one type of ion is discharged, so you need rules to decide which.
At the cathode (−)
- If the metal is more reactive than hydrogen (such as sodium, potassium, magnesium), hydrogen gas is produced.
- If the metal is less reactive than hydrogen (such as copper, silver), the metal is deposited.
At the anode (+)
- If halide ions (\mathrm{Cl^-}, \mathrm{Br^-}, \mathrm{I^-}) are present, the halogen is produced.
- If not (for example with sulfate or nitrate ions), oxygen is produced from the hydroxide ions.
Examples
| Solution | At the cathode | At the anode | Left in solution |
|---|---|---|---|
| sodium chloride (brine) | hydrogen | chlorine | sodium hydroxide |
| copper chloride | copper | chlorine | (solution fades) |
| copper sulfate | copper | oxygen | sulfuric acid |
| sodium sulfate | hydrogen | oxygen | sodium sulfate |
| dilute sulfuric acid | hydrogen | oxygen | sulfuric acid |
Brine
Electrolysing sodium chloride solution (brine) is a very important industrial process. It produces three useful chemicals:
- hydrogen (for making margarine and ammonia)
- chlorine (for bleach, PVC and sterilising water)
- sodium hydroxide (for soap and paper), which is left in the solution
Test yourself on this lesson
5 exam questions · 10 marks · with mark schemes and model answers
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