The Ion Arena: Electrolysis of Aqueous NaCl
Four ions enter. Two win. What happens to the losers?
🎯 The Story
Imagine an arena with four competitors floating in the solution. When you switch on the electricity, the race begins. Only the fastest — and most strategic — ions get discharged at the electrodes. The others? They stay behind, watching.
From NaCl dissolving:
Na⁺ (sodium ions) + Cl⁻ (chloride ions)
From Water (H₂O) itself:
H⁺ (hydrogen ions) + OH⁻ (hydroxide ions)
⚡ Step 1: The Discharge Selector
Think like an examiner. At each electrode, which ion gets discharged? Choose below.
⊖ CATHODE (negative electrode)
Attracts positive ions (cations). Which positive ion is discharged?
⊕ ANODE (positive electrode)
Attracts negative ions (anions). Which negative ion is discharged?
🔬 Step 2: The Ion Arena — Live Simulation
Switch on the current to see the ions migrate and gases form at the electrodes.
📋 Observation Log
⊖ Cathode Observation
Colourless gas bubbles
Hydrogen gas (H₂) is produced. Test: lighted splint → "squeaky pop"
2H⁺(aq) + 2e⁻ → H₂(g) [Reduction]
⊕ Anode Observation
Pale green gas, pungent smell
Chlorine gas (Cl₂) is produced. Test: damp litmus → bleaches white
2Cl⁻(aq) → Cl₂(g) + 2e⁻ [Oxidation]
🏆 Step 3: The Leftovers
H⁺ was discharged at the cathode. Cl⁻ was discharged at the anode. What remains in the solution?
⚠️ Examiner Traps
"Sodium is discharged at the cathode"
Wrong. Sodium is too reactive — it prefers to remain as Na⁺(aq). H⁺ is always preferentially discharged over Group 1 metals.
"Oxygen is produced at the anode" (in concentrated solution)
Only if dilute! In concentrated NaCl, Cl⁻ wins because it's in excess. This is a key distinction for Paper 4.
Forgetting state symbols in electrode equations
Always write: 2H⁺(aq) + 2e⁻ → H₂(g) and 2Cl⁻(aq) → Cl₂(g) + 2e⁻ — the (g) is essential for full marks.
📝 Keyword Summary
Electrolyte
Substance that conducts electricity when dissolved or molten
Cathode
Negative electrode — site of reduction
Anode
Positive electrode — site of oxidation
Discharge
Ions lose/gain electrons and become atoms/molecules
Selective discharge
More concentrated or less reactive ion discharged first
Chlor-Alkali
Industrial process producing H₂, Cl₂, NaOH simultaneously
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