Electrolysis of aqueous solutions

A solution in water contains the ions of the compound and H⁺ and OH⁻ ions from the water. Only one product forms at each electrode.

At the cathode (−):

• If the metal is more reactive than hydrogen, hydrogen gas forms.
• If the metal is less reactive than hydrogen (e.g. copper, silver), the metal forms.

At the anode (+):

• If the solution contains halide ions (Cl⁻, Br⁻, I⁻), the halogen forms.
• Otherwise (e.g. sulfate or nitrate ions), oxygen forms.

Tests for the gases: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; chlorine bleaches damp litmus paper white.

When sodium chloride solution is electrolysed, the Na⁺ and OH⁻ ions are left behind, forming sodium hydroxide solution.

Cathode (reduction — ions gain electrons):

• Hydrogen: 2H⁺ + 2e⁻ → H₂
• Copper: Cu²⁺ + 2e⁻ → Cu

Anode (oxidation — ions lose electrons):

• Chlorine: 2Cl⁻ → Cl₂ + 2e⁻
• Oxygen: 4OH⁻ → O₂ + 2H₂O + 4e⁻

Check that atoms and charges balance. In the oxygen equation, 4OH⁻ has a total charge of 4−, matching the 4e⁻ on the right.

1. Pour about 50 cm³ of copper(II) sulfate solution into a beaker.
2. Place two graphite electrodes in the solution, not touching, held by a lid.
3. Connect them to a low-voltage DC supply (about 4 V) and switch on.
4. Watch each electrode. Collect any gas in a test tube held over the electrode, then test it.
5. Record what forms at each electrode. Repeat with other solutions, such as sodium chloride.

Hazard: chlorine is toxic — work in a well-ventilated room and only run the electrolysis for a short time.

Better results: use clean electrodes and keep the voltage the same for every solution, so the results can be compared fairly.