Electrolysis of Aqueous Solutions

Learning Objectives

• Describe the reactions during electrolysis of an aqueous solution.
• Predict the products at electrodes during electrolysis.
• For higher tier students: Write half equations for reactions at the cathode and anode.

Key Concepts

Aqueous Solutions

• Definition: Aqueous means dissolved in water.
• Water Molecule Splitting:
  • Produces hydrogen ion (H⁺) and hydroxide ion (OH⁻).

Electrolysis of Sodium Chloride Solution

• Ions in Solution:
  • Sodium ion (Na⁺)
  • Chloride ion (Cl⁻)
  • Plus ions from water: H⁺ and OH⁻

Reactions at the Electrodes

Cathode (Negative Electrode)

• Attracted Ions: Na⁺ and H⁺
• Reactivity Series:
  • Hydrogen is produced if metal is more reactive than hydrogen.
  • Sodium is more reactive than hydrogen, hence hydrogen gas is produced at the cathode.

Anode (Positive Electrode)

• Attracted Ions: Cl⁻ and OH⁻
• Halide Rule:
  • Halogens are produced at the anode if the solution contains halide ions (e.g., Cl⁻).
  • Chlorine gas is produced at the anode when chloride ions are present.

Writing Half Equations (Higher Tier)

Cathode Reaction

• Hydrogen Ion Reduction:
  • H⁺ gains one electron to form a hydrogen atom.
  • Equation: 2H⁺ + 2e⁻ → H₂
  • Hydrogen atoms pair to form H₂.

Anode Reaction

• Chloride Ion Oxidation:
  • Cl⁻ loses one electron to form a chlorine atom.
  • Equation: 2Cl⁻ → Cl₂ + 2e⁻
  • Chlorine atoms pair to form Cl₂.

Study Resources

• Vision workbook contains questions on electrolysis of aqueous solutions.