The Water Complication
Part of Electrolysis of Aqueous Solutions — GCSE Chemistry
This introduction covers The Water Complication within Electrolysis of Aqueous Solutions for GCSE Chemistry. Revise Electrolysis of Aqueous Solutions in Electrolysis for GCSE Chemistry with 20 exam-style questions and 0 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 2 of 12 in this topic. Use this introduction to connect the idea to the wider topic before moving on to questions and flashcards.
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Section 2 of 12
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20 questions
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📖 The Water Complication
Aqueous electrolysis is like a talent show audition! At each electrode, ions compete to react. At the cathode, it's H⁺ vs. metal ions — hydrogen often wins if the metal is reactive. At the anode, it's OH⁻ vs. halide ions — halogens win if concentrated. The reactivity series is your judge's scorecard!
The Four Ions in Aqueous NaCl:
From water: H⁺ and OH⁻
At the cathode, both Na⁺ and H⁺ want to gain electrons. Who wins?
At the anode, both Cl⁻ and OH⁻ want to lose electrons. Who wins?
THE CATHODE RULE — Use the Reactivity Series:
- If the metal is MORE reactive than hydrogen (above H in the series) → HYDROGEN gas forms
- If the metal is LESS reactive than hydrogen (below H in the series) → The METAL forms
Why? Very reactive metals like sodium hold onto their electrons incredibly tightly — they form stable ions and don't want to change back to atoms. Hydrogen ions are easier to reduce, so they get the electrons instead. But less reactive metals like copper are happy to accept electrons and become metal atoms.
THE ANODE RULE — Check for Halides:
- If a halide ion (Cl⁻, Br⁻, or I⁻) is present → The HALOGEN gas forms
- If NO halide present (e.g., sulfate SO₄²⁻ or nitrate NO₃⁻) → OXYGEN gas forms from OH⁻
Why? Halide ions are easier to oxidise than hydroxide ions — they lose their electrons more readily. But if there's no halide, the OH⁻ ions from water get oxidised instead, releasing oxygen gas.
Let's Apply These Rules:
• Cathode: Cu is BELOW H → Copper metal forms ✓
• Anode: SO₄²⁻ is NOT a halide → Oxygen gas forms ✓
Sodium Chloride (NaCl) Solution:
• Cathode: Na is ABOVE H → Hydrogen gas forms ✓
• Anode: Cl⁻ IS a halide → Chlorine gas forms ✓
The Chlor-Alkali Industry:
Electrolysis of brine (concentrated NaCl solution) is hugely important industrially. It produces three valuable products:
- Hydrogen (cathode) — used as fuel and in making ammonia
- Chlorine (anode) — used to make bleach, PVC plastic, and kill bacteria in water
- Sodium hydroxide (left in solution) — used in soap, paper, and cleaning products