Electrolysis of Aqueous Solutions HL Only

Full Notes:

This page builds on the basics of electrolysis outlined in R3.2.8.

What Makes Aqueous Electrolysis Different?

The ionic compound is dissolved in water and is aqueous (aq). H⁺(aq) and OH⁻(aq) ions from water are also present due to the natural ionisation of water:

• 2H₂O + 2e⁻ → H₂ + 2OH⁻
• 2H₂O → O₂ + 4H⁺ + 4e⁻

As a result, water may compete with the ions from the compound at the electrodes for oxidation and reduction.

We can use standard electrode potentials (E° values) or reactivity trends to predict which species is discharged.

• At the cathode:
  • H⁺(aq) ions are reduced to H₂(g) if the metal is more reactive than hydrogen (lower E° value).
  • The metal ion is reduced if it is less reactive than hydrogen (higher E° value).
• At the anode:
  • Generally, OH⁻(aq) is oxidised to form O₂ gas.
  • If halide ions (e.g. Cl⁻, Br⁻) are present, they are oxidised instead.
  • Ions like SO₄²⁻ and NO₃⁻ do not get oxidised.

Example: Electrolysis of Aqueous NaCl

This system contains:

• Na⁺ and H₂O (possible reduction at the cathode)
• Cl⁻ and H₂O (possible oxidation at the anode)

At the Cathode (Reduction)

• Na⁺ + e⁻ → Na (E° = –2.71 V)
• 2H₂O + 2e⁻ → H₂ + 2OH⁻ (E° = –0.83 V)

Water is reduced, not sodium (because –0.83 V is more positive than –2.71 for Na⁺ reduction):

2H₂O + 2e⁻ → H₂ + 2OH⁻

At the Anode (Oxidation)

• 2Cl⁻ → Cl₂ + 2e⁻ (E° = +1.36 V)
• 2H₂O → O₂ + 4H⁺ + 4e⁻ (E° = +1.23 V)

Even though water has a slightly lower reduction potential, Cl⁻ is preferentially oxidised in concentrated solutions of NaCl (due to kinetic factors):

2Cl⁻ → Cl₂ + 2e⁻

Another Example: Electrolysis of Aqueous CuSO₄

CuSO₄(aq) contains:

• Cu²⁺ and SO₄²⁻ from the salt
• H₂O, which contributes H⁺ and OH⁻ ions

Cathode: Possible Reductions

• Cu²⁺ + 2e⁻ → Cu(s), E° = +0.34 V
• 2H₂O + 2e⁻ → H₂ + 2OH⁻, E° = –0.83 V

Copper is reduced, because it has a much more positive E° value than hydrogen gas:

Cu²⁺(aq) + 2e⁻ → Cu(s) (copper metal deposited)

Anode: Possible Oxidations

• S₂O₈²⁻ + 2e⁻ → 2SO₄²⁻, E° = +2.01 V
• O₂ + 4H⁺ + 4e⁻ → 2H₂O, E° = +1.23 V

Since oxidation is the reverse of reduction, the species with the lower E° for reduction is easier to oxidise. Water (E° = +1.23 V) is oxidised more readily than S₂O₈²⁻.

2H₂O → O₂(g) + 4H⁺ + 4e⁻

Effect of Concentration

If halide concentration is very low (e.g., dilute NaCl), OH⁻ from water may be oxidised instead.

If the metal ion concentration is very low, hydrogen gas may form instead of metal at the cathode.

Summary

• In aqueous electrolysis, water introduces H⁺ and OH⁻ ions that compete with the compound’s ions.
• Which species is discharged depends on E° values, ion concentration, and electrode material.
• At the cathode, the less reactive species is reduced.
• At the anode, OH⁻ is usually oxidised unless halides are present.
• Concentration influences which ion is preferentially discharged.