Common salt is an important raw material for various chemicals of daily use. State in brief the method of preparation of (i) Sodium hydroxide, and (ii) Sodium hydrogen carbonate from common salt. Write balanced chemical equations of the reactions that occur.
Generated by claude-sonnet-4-6 · 2026-06-15 06:44 · grounding rag
Model Answer
(i) Sodium Hydroxide:
An aqueous solution of sodium chloride (brine) is electrolysed. This is called the chlor-alkali process. NaOH is formed near the cathode, Cl₂ at the anode, and H₂ at the cathode.
$$2\text{NaCl(aq)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{Cl}_2\text{(g)} + \text{H}_2\text{(g)}$$
(ii) Sodium Hydrogen Carbonate:
NaCl solution is treated with water, CO₂, and NH₃. The less soluble NaHCO₃ is precipitated.
$$\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3$$
Source: Acids, Bases and Salts, Section 2.4.3
---
Explanation
- Examiners expect the name of the process (chlor-alkali) for NaOH — this is a common 1-mark component.
- Both balanced equations must be written correctly; missing state symbols may cost half a mark.
- For NaHCO₃, mention all four reactants (NaCl, H₂O, CO₂, NH₃) — students often forget NH₃.
- Keep descriptions brief; the equation carries most of the marks.