Electrolysis in industry — Grade 12 Electrochemistry

Electrolytic cells — a quick reminder

An electrolytic cell converts electrical energy to chemical energy; the reaction is non-spontaneous.
Cathode: reduction, connected to the negative terminal. Anode: oxidation, connected to the positive terminal.
Use the Table of Standard Reduction Potentials to predict which species is reduced or oxidised.

The chlor-alkali industry (membrane cell)

Concentrated brine (NaCl(aq)) is electrolysed to make chlorine, hydrogen and sodium hydroxide.

📖 Half-reactions

Anode (oxidation): 2Cl⁻(aq) → Cl₂(g) + 2e⁻

Cathode (reduction): 2H₂O(ℓ) + 2e⁻ → H₂(g) + 2OH⁻(aq)

Overall: 2NaCl(aq) + 2H₂O(ℓ) → Cl₂(g) + H₂(g) + 2NaOH(aq)

Water is reduced at the cathode rather than Na⁺, because water is a stronger oxidising agent than Na⁺.
The ion-selective membrane lets only Na⁺ ions pass from the anode compartment to the cathode compartment, where they join OH⁻ to form NaOH.
The membrane keeps Cl₂ away from H₂ (an explosive mixture) and from NaOH (which would react with Cl₂ to form bleach), so the products stay pure.
Uses: Cl₂ — water purification, PVC; NaOH — soap, paper, aluminium processing; H₂ — fuel, margarine, ammonia.

Copper refining and electroplating

✏️ Refining copper

1Anode: impure copper. Cathode: a thin sheet of pure copper. Electrolyte: CuSO₄(aq).

2Anode: Cu(s) → Cu²⁺(aq) + 2e⁻ — the anode gets smaller.

3Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s) — pure copper builds up.

4Unreactive impurities such as Ag, Au and Pt do not oxidise; they fall to the bottom as anode sludge, which is sold. More reactive metals (Zn, Fe) oxidise but stay in solution as ions because they are weaker oxidising agents than Cu²⁺.

5The concentration of Cu²⁺ in the electrolyte stays constant.

Electroplating uses the same design: the object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal (for example silver plating with Ag anode and AgNO₃ electrolyte).

Extraction of aluminium

Bauxite is purified to aluminium oxide (alumina), Al₂O₃.
Al₂O₃ melts at over 2 000 °C, so it is dissolved in molten cryolite (Na₃AlF₆), which lowers the operating temperature to about 1 000 °C and saves energy.
Cathode (carbon lining): Al³⁺ + 3e⁻ → Al(ℓ) — molten aluminium is tapped from the bottom.
Anode (carbon blocks): 2O²⁻ → O₂(g) + 4e⁻. The hot oxygen reacts with the carbon anodes to form CO₂, so the anodes burn away and must be replaced regularly.

Environmental impact: the process uses huge amounts of electricity (in South Africa mostly from coal power, releasing CO₂ and SO₂); CO₂ is also released at the anodes; fluoride emissions come from the cryolite; bauxite mining damages land. Recycling aluminium uses about 5% of the energy needed for extraction.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (4 marks)

Write the half-reactions at the anode and the cathode in the membrane cell, and the overall reaction.

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Anode: 2Cl⁻ → Cl₂ + 2e⁻ ✓
Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻ ✓
Overall: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH ✓✓ (balancing ✓)

Question 2 · (3 marks)

State TWO functions of the membrane in the chlor-alkali cell.

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Allows only Na⁺ ions to move to the cathode compartment ✓. Keeps Cl₂ and H₂ apart / prevents Cl₂ reacting with NaOH ✓, so the NaOH produced is pure ✓.

Question 3 · (4 marks)

In copper refining: (a) which electrode is impure copper? (b) Write the reduction half-reaction. (c) Explain why silver collects under the anode.

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(a) The anode ✓
(b) Cu²⁺ + 2e⁻ → Cu ✓
(c) Silver is a weaker reducing agent than copper ✓, so it is not oxidised and falls off as anode sludge ✓.

Question 4 · (4 marks)

Give TWO reasons why cryolite is used in aluminium extraction, and ONE reason why the carbon anodes must be replaced.

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Lowers the melting point/operating temperature of the alumina ✓, which saves energy/costs ✓ (also improves conductivity). Oxygen formed at the anode reacts with the carbon to form CO₂, so the anodes are used up ✓✓.