Question
During the electrolysis of concentrated aqueous NaCl solution, identify the products formed at the anode and cathode. Explain why Na metal is NOT deposited at the cathode, even though Na⁺ ions are present in solution.
(NCERT Class 12, Electrochemistry)
Solution — Step by Step
Aqueous NaCl contains: Na⁺, Cl⁻ (from NaCl dissociation), and H⁺, OH⁻ (from water’s autoionization). At the cathode, both Na⁺ and H⁺ compete for reduction. At the anode, both Cl⁻ and OH⁻ compete for oxidation.
Compare standard reduction potentials:
The species with the less negative (higher) reduction potential gets reduced preferentially. Water wins. So H₂ gas is liberated at the cathode — not sodium metal.
For oxidation (reverse the sign logic — the species with lower oxidation potential discharges first):
By standard potentials, water should be oxidized preferentially. But with concentrated NaCl, the overpotential effect and high Cl⁻ concentration cause Cl₂ gas to be liberated at the anode instead. This is an important exception.
| Electrode | Product | Reaction |
|---|---|---|
| Cathode | H₂ gas | |
| Anode | Cl₂ gas |
The solution near the cathode becomes alkaline (NaOH accumulates). This is the basis of the chlor-alkali process.
Why This Works
The key principle is selective discharge: when multiple ions compete at an electrode, the one requiring less energy gets discharged first. At the cathode, reducing water to H₂ needs far less energy than reducing Na⁺ to Na metal (a difference of nearly 1.9 V).
At the anode, the situation is trickier. Standard potentials favour water oxidation, but in practice, the overpotential for O₂ evolution on the anode surface is high, and the high concentration of Cl⁻ ions tips the balance. This is why concentration matters — with dilute NaCl, you’d get O₂ at the anode instead of Cl₂.
Alternative Method
Think of it from an energy perspective: depositing Na metal from aqueous solution would require V, but the water molecules sitting right next to the cathode happily accept electrons at just V. The cell takes the path of least resistance.
This is a very common CBSE board question (2-3 marks) and also appears in NEET. The chlor-alkali process — producing NaOH, Cl₂, and H₂ simultaneously — is asked both as a reaction-based and application-based question.
Common Mistake
Many students write “Na is deposited at the cathode” because they see Na⁺ ions in the solution. This is wrong in aqueous medium. Na metal can only be obtained by electrolysis of molten NaCl (Downs process), where water is absent and Na⁺ has no competition. Always check whether the electrolyte is molten or aqueous before predicting products.