Electrolysis of Sodium ChlorideOxford AQA IGCSE Chemistry: Revision notes

Electrolysing concentrated sodium chloride solution (brine) produces three useful products:

• Hydrogen gas at the cathode
• Chlorine gas at the anode
• Sodium hydroxide solution remaining in the electrolyte

This is different from electrolysing molten sodium chloride, which simply gives sodium metal and chlorine gas — the presence of water changes the products completely.

Sodium chloride solution contains Na⁺ and H⁺ ions (H⁺ from water), both attracted to the cathode. Because sodium is far more reactive than hydrogen, the H⁺ ions are discharged in preference, producing hydrogen gas.

Half-equation: 2H⁺ + 2e⁻ → H₂

The solution also contains Cl⁻ and OH⁻ ions, both attracted to the anode. In concentrated brine, the chloride ions are present in high enough concentration to be discharged in preference to hydroxide ions, producing chlorine gas.

Half-equation: 2Cl⁻ → Cl₂ + 2e⁻

Once the H⁺ and Cl⁻ ions have been discharged and removed as gases, the ions that remain in the solution are Na⁺ and OH⁻. Together, these make sodium hydroxide solution, which is why the process is a valuable source of NaOH as well as hydrogen and chlorine gas.

All three products of brine electrolysis have major industrial uses:

• Sodium hydroxide — used to make soap (and other cleaning products)
• Chlorine — used to make bleach and PVC plastics
• Hydrogen — used as a fuel and in the manufacture of other chemicals

This makes electrolysis of brine one of the most economically important industrial processes in the chemical industry.