When ionic compounds are dissolved in water, water also breaks into H⁺ and OH⁻ ions. This means there are more ions present, so more possible reactions at the electrodes.

What is an aqueous solution?

• Aqueous means dissolved in water
• The solution contains ions from the ionic compound and from water:
  • Water breaks down into H⁺ and OH⁻ ions
  • So the electrolyte now contains:
    • Positive ions: metal ions and H⁺
    • Negative ions: non-metal ions and OH⁻

What happens at each electrode?

Cathode (negative electrode):

• Cations (positive ions) are attracted
• Either H⁺ or metal ions are discharged
• The less reactive one is discharged (gains electrons)

💡 If the metal is more reactive than hydrogen, hydrogen gas is produced instead

Anode (positive electrode):

• Anions (negative ions) are attracted
• Usually OH⁻ or halide ions (Cl⁻, Br⁻, I⁻)
• If halide ions are present → halogen forms
• If not → oxygen gas is formed from OH⁻

Example 1: Electrolysis of sodium chloride solution (NaCl (aq))

• Cathode: H⁺ is discharged (Na is more reactive) → hydrogen gas
• Anode: Cl⁻ is discharged → chlorine gas

Products: Hydrogen and chlorine gases

Example 2: Electrolysis of copper(II) sulfate solution (CuSO₄ (aq))

• Cathode: Cu²⁺ is discharged (less reactive than H⁺) → copper metal
• Anode: No halide ions, so OH⁻ is discharged → oxygen gas

Products: Copper and oxygen

General rules to remember