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3.4.7Stoichiometry

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

5 min read 5 parts Lesson 4 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين المول والحسابات الكيميائية - Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو
Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.
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Part 1 of 5Brine and Its Ions

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

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

Part 2 of 5Reactions and Products

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.

Part 3 of 5Uses of the Products

Uses of the Products
Chlorine
  • Chlorine Chlorine is used to make bleach and a range of plastics.
Hydrogen
  • Hydrogen Hydrogen is used to make margarine by hardening vegetable oils.
Sodium hydroxide
  • Sodium hydroxide Sodium hydroxide is used to make soap and detergents in industry.
Why it matters
  • Why it matters Electrolysis of one cheap raw material gives three useful products.
ProductUsed to make
ChlorineBleach and plastics
HydrogenMargarine
Sodium hydroxideSoap and detergents

Part 4 of 5Worked Example

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

  1. CathodeH⁺ ions are discharged because sodium is more reactive than hydrogen: 2H⁺ + 2e⁻ → H₂.
  2. AnodeCl⁻ 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.

Part 5 of 5Exam Checklist & Tips

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.

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