Electrolysis of Aqueous Sodium Chloride
Cambridge IGCSE Chemistry (0620) · 15 Questions · 35 Marks Total
Interactive Cell Diagram
Animated: H₂ at cathode, Cl₂ at anode, NaOH remains
Section A — Ion Identification & Half-Equations
List all four ions present in concentrated aqueous sodium chloride solution.
💡 Think: which ions come from NaCl and which come from H₂O?
At which electrode does oxidation occur during the electrolysis of aqueous sodium chloride? Define oxidation in terms of electron transfer.
💡 Remember OIL RIG — Oxidation Is Loss of electrons.
At which electrode does reduction occur? Define reduction in terms of electron transfer.
💡 Remember OIL RIG — Reduction Is Gain of electrons.
Write the half-equation for the reaction at the anode during the electrolysis of concentrated aqueous NaCl.
💡 Chloride ions are discharged. They lose electrons to form Cl₂ gas.
Write the half-equation for the reaction at the cathode during the electrolysis of concentrated aqueous NaCl.
💡 H⁺ ions from water are discharged at the cathode, not Na⁺.
Section B — Products & Industrial Uses
Name the gas produced at the cathode during the electrolysis of concentrated aqueous sodium chloride.
💡 The less reactive ion is discharged.
Name the gas produced at the anode during the electrolysis of concentrated aqueous sodium chloride.
💡 The more concentrated halide ion is discharged in preference to OH⁻.
Name the solution remaining in the electrolytic cell after prolonged electrolysis of concentrated aqueous sodium chloride.
💡 Na⁺ and OH⁻ ions are left behind in solution.
State TWO large-scale industrial uses of chlorine produced from the electrolysis of brine.
💡 Think: water treatment, plastics, and medicine.
State ONE large-scale use of hydrogen and ONE use of sodium hydroxide produced from the electrolysis of brine.
💡 H₂ → fuel/margarine. NaOH → soap/paper.
Section C — Explanation & Test Questions
Explain why hydrogen gas is produced at the cathode rather than sodium during the electrolysis of concentrated aqueous sodium chloride.
💡 Compare reactivity. Less reactive ion is discharged preferentially.
Explain why chlorine gas is produced at the anode rather than oxygen during the electrolysis of CONCENTRATED aqueous sodium chloride.
💡 Both Cl⁻ and OH⁻ are present. Concentration is the deciding factor here.
State the test for chlorine gas. Include the reagent used, the observation, and the explanation for the result.
💡 Cl₂ is a powerful oxidising agent — it destroys the dye in litmus.
State the test for hydrogen gas. Include the observation and the name of the sound produced.
💡 Hydrogen is flammable — a burning splint is the test.
The diagram below represents an electrolytic cell containing concentrated aqueous sodium chloride. (a) Identify the electrode labelled X where chlorine is produced. (b) State the colour and physical state of chlorine at room temperature. (c) Write the ionic equation for the overall reaction during electrolysis of brine. (d) Explain what would happen to the product at the anode if DILUTE NaCl was used instead of concentrated NaCl.
💡 Combine your knowledge of electrode identity, chlorine properties, and concentration effects.
⚡ Quick Reference — Key Facts
Cathode product
H₂ (hydrogen gas)
Anode product
Cl₂ (chlorine gas)
Remaining solution
NaOH(aq)
Why H₂ not Na?
H less reactive than Na
Why Cl₂ not O₂?
Concentrated Cl⁻ preferred
Test for Cl₂
Bleaches damp litmus
Test for H₂
Squeaky pop (lighted splint)
All 4 ions in brine
Na⁺, Cl⁻, H⁺, OH⁻
Cambridge IGCSE Chemistry 0620 · Electrolysis of Brine (4.1.4) · All answers aligned to official mark schemes
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