Knowledge Organiser: Electrolysis of Aluminium
Key Terms
- Electrolysis — decomposition using electricity
- Cathode — negative electrode (reduction)
- Anode — positive electrode (oxidation)
- Cryolite — solvent that lowers melting point
- Bauxite — aluminium ore (contains Al₂O₃)
- Half equation — equation showing electron transfer
Must-Know Facts
- Al is too reactive for carbon reduction
- Cryolite lowers mp from 2072°C to ~950°C
- Cathode: Al³⁺ + 3e⁻ → Al (reduction)
- Anode: 2O²⁻ → O₂ + 4e⁻ (oxidation)
- Anodes burn away: C + O₂ → CO₂
- Recycling saves 95% of energy
- Overall: 2Al₂O₃ → 4Al + 3O₂
Key Equations
- Al³⁺ + 3e⁻ → Al (cathode — reduction)
- 2O²⁻ → O₂ + 4e⁻ (anode — oxidation)
- 2Al₂O₃ → 4Al + 3O₂ (overall equation)
- C + O₂ → CO₂ (anode burning away)
Common Mistakes
- Saying aluminium forms at the anode: Aluminium is a positive ion (cation) so it moves to the CATHODE (negative) — metals always form at the cathode
- Forgetting why anodes need replacing: The carbon anodes react with oxygen produced at the anode to form CO₂ — they burn away and must be regularly replaced
- Saying cryolite is used to purify aluminium: Cryolite is a solvent that dissolves aluminium oxide and lowers its melting point — it does not purify the product
- Not balancing the half-equations for charge: Al³⁺ + 3e⁻ → Al — the 3 electrons balance the 3+ charge on aluminium
Memory Aid
- PANIC = Positive Anode, Negative Is Cathode
- Use this to remember which electrode is which: the Positive electrode is the Anode, the Negative Is the Cathode.
- CATions go to the CAThode — both start with "CAT". ANions go to the ANode — both start with "AN".
- OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). At the anode = oxidation (loss). At the cathode = reduction (gain).
- For the anode burning away: "The ANODE gets ATE — it reacts with oxygen and burns away as CO₂!"