Knowledge Organiser: Electrolysis of Molten Compounds
Key Terms
- Cathode — negative electrode (reduction)
- Anode — positive electrode (oxidation)
- Cation — positive ion, moves to cathode
- Anion — negative ion, moves to anode
- Molten — melted; ions free to move
- Half equation — shows electron transfer at one electrode
Must-Know Facts
- Solid ionic compounds do NOT conduct electricity
- Molten compounds conduct — ions are free
- Cathode (−): cations gain electrons → METAL
- Anode (+): anions lose electrons → NON-METAL
- PbBr₂: Pb at cathode, Br₂ at anode
- NaCl: Na at cathode, Cl₂ at anode
- Ions carry current inside electrolyte (not electrons)
Key Equations
- Pb²⁺ + 2e⁻ → Pb (cathode — reduction of lead bromide)
- 2Br⁻ → Br₂ + 2e⁻ (anode — oxidation of lead bromide)
- Na⁺ + e⁻ → Na (cathode — reduction of sodium chloride)
- 2Cl⁻ → Cl₂ + 2e⁻ (anode — oxidation of sodium chloride)
Common Mistakes
- Saying solid ionic compounds conduct: Ions are fixed in the lattice when solid — only molten or dissolved ionic compounds allow ions to move and carry charge
- Getting electrode products reversed: OARC — Oxidation at Anode (non-metal), Reduction at Cathode (metal) — never swap these
- Saying electrons flow through the electrolyte: Electrons flow through the external circuit (wires); inside the electrolyte, ions carry the current by moving toward electrodes
- Forgetting to balance half-equations for charge: 2Br⁻ → Br₂ + 2e⁻ — the 2 electrons balance the 2 negative charges lost from bromide ions
Memory Aid
- CATions go to the CAThode, ANions go to the ANode
- Both "CATion" and "CAThode" start with "CAT". Both "ANion" and "ANode" start with "AN". This makes it easy to remember which ion goes where.
- OIL RIG — Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)
- At the Anode = Oxidation (A and O are both vowels!)
- At the Cathode = Reduction (both contain the letter C!)
- For predicting molten products: "Metal at the Minus, Non-metal at the Plus" — cathode is negative (minus) → metal; anode is positive (plus) → non-metal.