ElectrolysisComparison

The Two Decision Rules — Learn These!

Part of Electrolysis of Aqueous Solutions · GCSE GCSE Chemistry revision

This comparison covers The Two Decision Rules — Learn These! 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 4 of 13 in this topic. Use this comparison to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 4 of 13

Practice

21 questions

Recall

15 flashcards

🎯 The Two Decision Rules — Learn These!

Electrode	Condition	Product	Half equation
CATHODE (−)	Metal above hydrogen in reactivity series (K, Na, Li, Ca, Mg, Al)	Hydrogen gas forms	`2H⁺ + 2e⁻ → H₂`
CATHODE (−)	Metal below hydrogen in reactivity series (Cu, Ag, Au, Pt)	Metal deposits	`Cu²⁺ + 2e⁻ → Cu`
ANODE (+)	Halide ion present (Cl⁻, Br⁻, I⁻)	Halogen gas forms	`2Cl⁻ → Cl₂ + 2e⁻`
ANODE (+)	No halide present (SO₄²⁻, NO₃⁻, etc.)	Oxygen gas forms	`4OH⁻ → O₂ + 2H₂O + 4e⁻`

Keep building this topic

Read this section alongside the surrounding pages in Electrolysis of Aqueous Solutions. That gives you the full topic sequence instead of a single isolated revision point.

How It Works: Why Water Creates Competing Ions Key Definitions The Water Complication Electrolysis of Brine — Click to Explore

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?

A. Na⁺, Cl⁻, H⁺ and OH⁻
B. Na⁺, Cl⁻, H₂O and OH⁻
C. Na⁺, Cl⁻ only
D. Na⁺, Cl⁻, H₂ and O²⁻

1 markfoundation

Describe the three products formed when concentrated brine is electrolysed, and state where each is produced.

3 marksstandard

Quick Recall Flashcards

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.

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.

21 questions on Electrolysis of Aqueous Solutions — practise free

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How It Works: Why Water Creates Competing Ions

Key Definitions