Electrolysis: predicting products
For a molten ionic compound the products are simply the elements: cation discharged at the cathode, anion at the anode. For an aqueous solution, water itself can be discharged, so we need rules.
Molten ionic compounds — straightforward
The compound contains only one type of cation and one type of anion. Each goes to its electrode.
Examples:
- Molten NaCl → Na at cathode, Cl₂ at anode.
- Molten Al₂O₃ → Al at cathode, O₂ at anode.
- Molten KBr → K at cathode, Br₂ at anode.
Aqueous solutions — extra ions to consider
A salt solution contains the salt's ions and H⁺ + OH⁻ from water (slight self-ionisation).
At the cathode (reduction)
The less reactive of the two cations is preferentially discharged.
- If the metal cation is less reactive than hydrogen (e.g. Cu²⁺, Ag⁺, Au⁺): metal is deposited.
- If the metal cation is more reactive than hydrogen (e.g. Na⁺, K⁺, Mg²⁺, Al³⁺): hydrogen gas is given off (from water/H⁺).
At the anode (oxidation)
- If the solution contains a halide (Cl⁻, Br⁻, I⁻): halogen gas (Cl₂, Br₂, I₂) is given off.
- If the anion is sulfate, nitrate, phosphate (or any oxoanion): oxygen gas is given off (from OH⁻/water).
✦Worked example— Worked example: aqueous CuCl₂
- Cathode: Cu²⁺ vs H⁺. Cu less reactive than H → Cu deposited (pink-brown coating).
- Anode: Cl⁻ vs OH⁻. Halide present → Cl₂ gas.
Overall: CuCl₂(aq) → Cu(s) + Cl₂(g).
✦Worked example— Worked example: aqueous NaCl (brine — industrially important)
- Cathode: Na⁺ vs H⁺. Na more reactive than H → H₂ gas at cathode.
- Anode: Cl⁻ vs OH⁻. Halide present → Cl₂ gas at anode.
Solution becomes NaOH. So electrolysis of brine produces three valuable products: H₂, Cl₂, NaOH.
✦Worked example— Worked example: aqueous Na₂SO₄
- Cathode: Na⁺ vs H⁺ → H₂ gas.
- Anode: SO₄²⁻ vs OH⁻ → O₂ gas (no halide).
Net effect: water decomposed; Na₂SO₄ stays in solution. Useful for splitting water.
✦Worked example— Worked example: aqueous CuSO₄ (with inert electrodes)
- Cathode: Cu²⁺ vs H⁺ → Cu deposited.
- Anode: SO₄²⁻ vs OH⁻ → O₂ gas.
The blue colour fades as Cu²⁺ is removed.
⚠Common mistakes
- Forgetting water has its own ions in aqueous electrolysis.
- Saying Na is deposited from aqueous NaCl — it isn't, because Na is more reactive than H.
- Forgetting halide preference — Cl⁻ wins over OH⁻ even though OH⁻ is sometimes thermodynamically easier to oxidise (a kinetic/concentration effect at GCSE).
- Mixing up products in different examples. Practice with a table.
Links
Builds on C4.9. Sets up C4.11 (Al extraction by electrolysis) and C4.12 (half-equations HT).
AI-generated · claude-opus-4-7 · v3-deep-chemistry