Electrolysis (C4.3)

What is electrolysis?

Electrolysis is the decomposition of a substance (the electrolyte) using electricity. The electrolyte must be an ionic compound that is either molten (liquid) or dissolved in water (aqueous solution) — ions must be free to move.

• Electrolyte: ionic compound in solution or molten state
• Electrodes: conductors that carry current into/from the electrolyte
• Anode: positive electrode — anions (negative ions) are attracted here and are oxidised (lose electrons)
• Cathode: negative electrode — cations (positive ions) are attracted here and are reduced (gain electrons)

Memory aid: OA (Oxidation at Anode), RC (Reduction at Cathode)

Electrolysis of molten compounds

Pure molten ionic compound — only ions from that compound present.

Molten lead bromide (PbBr₂):

• At cathode: Pb²⁺ + 2e⁻ → Pb (lead deposited)
• At anode: 2Br⁻ → Br₂ + 2e⁻ (bromine gas released)

Electrolysis of aqueous solutions

Water is also present, so water can be oxidised or reduced competing with the ions.

Electrolysis of dilute sulfuric acid / water:

• Cathode: 2H⁺ + 2e⁻ → H₂ (hydrogen gas)
• Anode: 4OH⁻ → 2H₂O + O₂ + 4e⁻ (oxygen gas)

Electrolysis of copper sulfate solution with copper electrodes:

• Cathode: Cu²⁺ + 2e⁻ → Cu (copper deposited on cathode)
• Anode: Cu → Cu²⁺ + 2e⁻ (copper anode dissolves)
• Solution concentration stays constant — used in copper refining and electroplating.

Electrolysis of brine (NaCl solution):

• Cathode: 2H⁺ + 2e⁻ → H₂ (hydrogen collected)
• Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine collected)
• Remaining solution: NaOH (sodium hydroxide)
• Products: hydrogen (fuel), chlorine (bleach/PVC/disinfectant), NaOH (soap/paper/cleaning products)

Extraction of aluminium

Aluminium ore (bauxite → purified to Al₂O₃, alumina). Al₂O₃ melts at ~2 000°C — too expensive to melt alone. Dissolved in molten cryolite to lower melting point to ~900°C.

• Cathode: Al³⁺ + 3e⁻ → Al (liquid aluminium sinks to bottom)
• Anode: 2O²⁻ → O₂ + 4e⁻ (oxygen attacks carbon anode → CO₂ → anodes must be regularly replaced)

High energy cost → aluminium is expensive; worth recycling (95% energy saving vs new).

Common exam errors

1. Saying ions "move towards" electrodes of the same charge — opposites attract: cations (+) go to cathode (−); anions (−) go to anode (+).
2. Forgetting that in aqueous electrolysis, water competes — H⁺ and OH⁻ from water are present.
3. Saying the anode gains electrons — oxidation at anode = electrons are removed from ions and flow away from the anode.