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1. States of Matter

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Text lesson

3.4.7 Electrolysis of Sodium Chloride — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

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3.4.7 Electrolysis of Sodium Chloride: Describe the electrolysis of concentrated sodium chloride solution, predict the products and explain how they are used.

3.4.7 Electrolysis of Sodium Chloride" - Dr Khaled Dabour Academy

Brine is the electrolyte used industrially, and it contains four different ions.

Brine
  • Brine is a concentrated aqueous solution of sodium chloride.
Ions present
  • Brine contains Na⁺ and Cl⁻ from the salt, and H⁺ and OH⁻ from the water.
Cathode choice
  • Hydrogen ions are discharged at the cathode, because sodium is above hydrogen in the reactivity series.
Anode choice
  • Chloride ions are discharged at the anode because the solution is concentrated, which produces chlorine gas.

The electrode reactions remove hydrogen and chlorine, leaving sodium hydroxide in solution.

1

At the negative electrode, hydrogen ions gain electrons: 2H⁺(aq) + 2e⁻ → H₂(g).
2

At the positive electrode, chloride ions lose electrons: 2Cl⁻(aq) → Cl₂(g) + 2e⁻.
3

The sodium ions and hydroxide ions remain in solution, which forms sodium hydroxide solution as the electrolysis continues.
4

Overall, the electrolysis of brine produces hydrogen, chlorine and sodium hydroxide solution.
Uses of the Products
Chlorine
  • Chlorine is used to make bleach and a range of plastics.
Hydrogen
  • Hydrogen is used to make margarine by hardening vegetable oils.
Sodium hydroxide
  • Sodium hydroxide is used to make soap and detergents in industry.
Why it matters
  • Electrolysis of one cheap raw material gives three useful products.
Product Used to make
Chlorine Bleach and plastics
Hydrogen Margarine
Sodium hydroxide Soap and detergents

Question: Write the half-equations for the electrolysis of concentrated sodium chloride solution and name the products.

  1. Cathode: H⁺ ions are discharged because sodium is more reactive than hydrogen: 2H⁺ + 2e⁻ → H₂.
  2. Anode: Cl⁻ ions are discharged in concentrated solution: 2Cl⁻ → Cl₂ + 2e⁻.
  3. The remaining Na⁺ and OH⁻ ions form sodium hydroxide solution.

Answer: Cathode: hydrogen gas from 2H⁺ + 2e⁻ → H₂; anode: chlorine gas from 2Cl⁻ → Cl₂ + 2e⁻; sodium hydroxide solution is left behind.

3.4.7 – Key Points for the Exam

  • Brine: A concentrated solution of sodium chloride in water.
  • Discharge: The reaction of an ion at an electrode, gaining or losing electrons to form an element.
  • Bleach: A chemical that removes colour, made in part from chlorine.
  • Sodium hydroxide: An alkali, NaOH, formed by the electrolysis of brine.

Common Mistakes

  • Stating that sodium metal forms at the cathode in aqueous solution; hydrogen is formed instead.
  • Writing that oxygen forms at the anode in brine; chlorine is produced because chloride ions are present.
  • Forgetting that water supplies the H⁺ and OH⁻ ions.
  • Leaving out the sodium hydroxide that remains in solution as one of the products.
Part of 3. Stoichiometry — see every lesson and quiz in this unit.
YOUR STUDY PATH

3.4.7 Electrolysis of Sodium Chloride

Igcse Chemistry / 3. Stoichiometry

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