JEE Main 2022 · 24 June · evening · Single correct · +4 / −1
In the industrial production of which of the following, molecular hydrogen is obtained as a byproduct?
- (a) NaOH
- (b) NaCl
- (c) Na metal
- (d) Na₂CO₃
Answer and solution
Answer (a) NaOH
Solution
Chemistry ma’am · 3 steps
Electrolyse brine and water gets reduced at the cathode, so hydrogen bubbles off beside the NaOH.
- Chlor-alkali process. NaOH is made industrially by electrolysing brine, a concentrated aqueous solution of NaCl. At the anode, chloride ions are oxidised: 2Cl⁻ → Cl₂ + 2e⁻.
- Water wins the cathode. At the cathode, water is reduced more easily than Na⁺. So hydrogen gas forms there, and the OH⁻ left behind pairs with Na⁺ to give NaOH.
2NaCl + 2H₂O → 2NaOH + Cl₂ ↑ + H₂ ↑
Product Industrial method Byproduct NaOH Electrolysis of aqueous brine (Castner–Kellner or membrane cell) Cl₂ and H₂ NaCl Evaporation of sea water No H₂ Na metal Electrolysis of molten NaCl (Downs cell), no water present Cl₂ only Na₂CO₃ Solvay (ammonia–soda) process CaCl₂, no H₂ - So. Molecular hydrogen is a byproduct only in the manufacture of NaOH, option (a).
The idea to keep. Ask whether water is being electrolysed. If it is, water is reduced at the cathode and H₂ comes off. In a molten salt there is no water, so no H₂.
Answer from the NTA final answer key. Solution by PrepWiser.