Knowledge Organiser: Electrolysis of Aqueous Solutions
Key Terms
- Aqueous solution — contains ions from salt + H⁺/OH⁻ from water
- Preferential discharge — one ion reacts rather than another
- Brine — concentrated NaCl solution
- Cathode — negative electrode (reduction)
- Anode — positive electrode (oxidation)
Must-Know Facts
- 4 ions always present in aqueous solution
- Cathode: metal above H → H₂; below H → metal
- Anode: halide present → halogen; no halide → O₂
- Brine → H₂ + Cl₂ + NaOH
- 2H⁺ + 2e⁻ → H₂ (cathode half equation)
- 4OH⁻ → O₂ + 2H₂O + 4e⁻ (anode, no halide)
- Gas tests: H₂ = squeaky pop; Cl₂ = bleaches litmus
Key Equations
- 2H⁺ + 2e⁻ → H₂ (cathode — when metal is above H in reactivity)
- 4OH⁻ → O₂ + 2H₂O + 4e⁻ (anode — when no halide present)
- 2Cl⁻ → Cl₂ + 2e⁻ (anode — when halide present in solution)
- Brine: cathode = H₂; anode = Cl₂; solution = NaOH
Common Mistakes
- Forgetting water contributes H⁺ and OH⁻ ions: In aqueous solutions, 4 ions compete at each electrode — from the dissolved salt AND from water ionisation
- Getting cathode product wrong for reactive metals: Na, K, Ca, Mg are above H in the reactivity series — H₂ forms at the cathode, NOT the metal
- Saying oxygen forms at the anode when halide is present: When a halide (Cl⁻, Br⁻, I⁻) is present in solution, the halogen is preferentially discharged — not oxygen
- Confusing products of brine electrolysis: Brine gives three products — hydrogen at cathode, chlorine at anode, and sodium hydroxide solution remaining
Memory Aid
- For the CATHODE rule: "If the metal is ABOVE hydrogen, HYDROGEN forms — it takes its place!"
- Think of it as hydrogen "jumping the queue" above reactive metals — because the metal is so reactive (so keen to stay ionic), hydrogen gets the electrons instead.
- For the ANODE rule: "HALIDE = Halogen at the anode. No halide = no halogen = OXYGEN instead."
- Remember the three halides: Cl⁻, Br⁻, I⁻ — if any of these are present in solution, a halogen gas forms at the anode.
- Brine products mnemonic: "3 Hs from Brine — Hydrogen, cHlorine, Hydroxide (sodium)" — three useful products from one reaction!