A student carries out electrolysis of brine (concentrated sodium chloride solution) using inert electrodes. Describe what is produced at each electrode and explain why these products form.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Brine is a concentrated solution of sodium chloride in water. It is an important industrial feedstock and its electrolysis produces useful products at both electrodes.
Model answer (4 marks)
At the anode (positive electrode) chloride ions (Cl⁻) are oxidised to give chlorine gas:
Cl⁻ → ½Cl₂ + e⁻
At the cathode (negative electrode) hydrogen ions from water are reduced to give hydrogen gas:
2H⁺ + 2e⁻ → H₂
These products form because chloride ions are attracted to the positive electrode and lose electrons (oxidation), while hydrogen ions are attracted to the negative electrode and gain electrons (reduction). Sodium ions remain in solution because they are not involved in the electrode reactions.
Cl⁻ → ½Cl₂ + e⁻
At the cathode (negative electrode) hydrogen ions from water are reduced to give hydrogen gas:
2H⁺ + 2e⁻ → H₂
These products form because chloride ions are attracted to the positive electrode and lose electrons (oxidation), while hydrogen ions are attracted to the negative electrode and gain electrons (reduction). Sodium ions remain in solution because they are not involved in the electrode reactions.
Examiner tips
- Use the correct electrode names (anode/cathode) and the corresponding ion charges.
- Show the half‑reactions to demonstrate oxidation and reduction.
- Explain the movement of ions under the electric field.
- Mention that Na⁺ stays in solution.
Common mistakes
- Confusing the charges of ions or the direction of electron flow.
- Writing the wrong gas at the wrong electrode.
- Forgetting to state that Na⁺ remains in solution.
Mark scheme (4 marks)
- Chlorine gas is produced at the anode (positive electrode)
- Chloride ions are negatively charged (anions) and move to the anode where they lose electrons (oxidation), forming chlorine gas
- Hydrogen gas is produced at the cathode (negative electrode)
- Hydrogen ions (H⁺) from water are positively charged (cations) and move to the cathode where they gain electrons (reduction), forming hydrogen gas, because sodium is more reactive than hydrogen
Key terms in this question
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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