Question

Starting with sodium chloride how would proceed to prepare:-
(i) sodium hydroxide
(ii) sodium carbonate ?

Answer 1

(i) Sodium hydroxide is prepared by the electrolysis of sodium chloride solution in castner-kellne cell. A brine solution (1 % NaCl solution) is electrolysed Using a mercury cathode and a carbon anode.
NaCl ➝ Na⁺ + Cl^–
H₂O ➝ H⁺ + OH^–
At cathode Na⁺ + e^– ➝ Na
Na + Hg ➝ Na – Hg (amalgam)
At anode Cl^– ➝ Cl + e^–
Cl + Cl ➝ Cl₂
NaOH ➝ Na⁺ + OH^–
H₂O ➝ H⁺ + OH^–
Sodium and hydrogen ion migrate towards cathode while OH^– and Hg migrate towards anode.
At Cathode, 2H⁺ + 2e^– ➝ H₂
At anode, OH^– ➝ OH + e^–
Na-Hg + OH ➝ NaOH + Hg
Over all reaction

(ii) Sodium Carbonate is prepared from sodium chloride by solvay process. Common salt and lime stone are used as raw materials in this process. It is formed in following steps

Step I:- Carbon dioxide passes through a concentrated aqueous solution of sodium chloride and ammonia.

The carbon dioxide required for this reaction is produced by heating (calcination) of the lime stone at 950-1100^o C.
CaCO₃ ➝ CaO + CO₂

Step II:- The sodium bi carbonate (NaHCO₃) that precipitates out in above step is filtered out from the hot ammonium chloride (NH₄Cl) solution and the solution is then reacted with the quick lime (CaO) formed in above reaction.
2NH₄Cl + CaO ➝ 2NH₃ + CaCl₂ + H₂O
CaO makes a strong basic solution. The ammonia formed in this reaction is recycled back to the initial brine solution of first reaction.

Step III:- Sodium bicarbonate that precipitate out in step I. It is converted to the product (sodium carbonate) by calcination at 160°-230°C.