This topic summary covers Knowledge Organiser: Electrolysis of Aqueous Solutions within Electrolysis of Aqueous Solutions for GCSE Chemistry. Revise Electrolysis of Aqueous Solutions in Electrolysis for GCSE Chemistry with 21 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 13 of 13 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
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
Practice questions for Electrolysis of Aqueous Solutions
When sodium chloride (NaCl) is dissolved in water, which four types of ion are present in the solution?
Describe the three products formed when concentrated brine is electrolysed, and state where each is produced.
Quick recall flashcards
State the anode rule for aqueous electrolysis.
If a halide ion (Cl⁻, Br⁻, or I⁻) is present → the halogen gas forms.
If NO halide is present → oxygen gas forms from the OH⁻ ions.
Example: Cl⁻ present → Cl₂ forms. SO₄²⁻ present (no halide) → O₂ forms.
State the cathode rule for aqueous electrolysis.
If the metal is MORE reactive than hydrogen (above H in reactivity series) → hydrogen gas forms.
If the metal is LESS reactive than hydrogen (below H) → the metal deposits.
Examples: Na above H → H₂ forms. Cu below H → Cu deposits.