Comprehension:
Electrolysis
In the electrolysis process, the nature of material being electrolyzed, and the electrodes nature are the important factors in predicting the products of electrolysis. The electrode material, whether acting as a source or sink or itself, is not inert will also decides the nature of the product of the electrolysis. The standard electrode potentials of the various oxidizing and reducing species present in the electrolytic mixture also decides the product of electrolysis. The voltage applied also decides the product of electrolysis since some of the reactions which are feasible are so slow kinetically that they are not seem to take place at lower voltage. The quantitative aspects of electrolysis was described by Faraday law of electrolysis.
During the electrolysis of the aqueous NaCl, which of the following is not observed?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → pH of the solution remains same.
In aqueous NaCl, both the ions of the salt and water molecules can react at the electrodes. The reaction with the more favourable potential (allowing for overpotential) takes place.
At the cathode, \( \ce{H^{+}} \) from water (E° = 0.00 V) is reduced in preference to \( \ce{Na^{+}} \) (E° = −2.71 V); the net cathode reaction is:
\( \displaystyle \ce{2H2O(l) + 2e^- -> H2(g) + 2OH^-(aq)} \)
At the anode, oxidation of water has the lower E° (1.23 V) than \( \ce{Cl^{-}} \) (1.36 V), but because of the overpotential of oxygen, chlorine is liberated:
\( \displaystyle \ce{2Cl^-(aq) -> Cl2(g) + 2e^-} \)
The net reaction produces NaOH in the solution:
\( \displaystyle \ce{2NaCl(aq) + 2H2O(l) -> 2NaOH(aq) + H2(g) + Cl2(g)} \)
Option (1) Overpotential of oxygen is considered for the electrode reaction. — Observed during the electrolysis of aqueous NaCl. The overpotential of oxygen explains why \( \ce{Cl2} \), not \( \ce{O2} \), is liberated at the anode.
Option (2) pH of the solution remains same. — Not observed. \( \ce{OH^{-}} \) ions build up, so the solution becomes alkaline and its pH increases.
Option (3) Hydrogen gas is produced at cathode. — Observed during the electrolysis of aqueous NaCl. Water is reduced in preference to \( \ce{Na^{+}} \).
Option (4) Chlorine gas is produced at anode — Observed during the electrolysis of aqueous NaCl. \( \ce{Cl^{-}} \) ions are oxidised to \( \ce{Cl2} \).
NCERT (2026-27 edition):
Products of Electrolysis – “… During the electrolysis of aqueous sodium chloride solution, the products are NaOH, Cl2 and H2. In this case besides Na+ and Cl– ions we also have H+ and OH– ions along with the solvent molecules, H2O.”