Starting with sodium chloride how would you proceed to prepare (i) sodium metal, (ii) sodium hydroxide, (iii) sodium peroxide, (iv) sodium carbonate?

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Solution

(i) Preparation of sodium metal:
Electrolysis of fused mixture of 40% NaCl and 60% $C a C l_{2}$ in Down cell at 873K using graphite anode and iron cathode gives sodium metal at anode. At cathode, chlorine is obtained.
$N a^{+} (m e l t) + e^{-} \to N a (l)$
$2 C l^{-} (m e l t) \to C l_{2} + 2 e^{-}$
(ii) Preparation of sodium hydroxide:
Electrolysis of sodium chloride (brine) in Castner Kellner cell, using carbon anode and mercury cathode gives sodium hydroxide. At cathode, sodium metal is discharged which combines with mercury to form sodium amalgam. At anode, chlorine gas is obtained.
$N a^{+} + e^{-} \to N a$
$2 N a + H g \to N a H g (a m a l g a m)$
$2 C l^{-} \to C l_{2} + 2 e^{-}$
Sodium amalgam, on reaction with water produces NaOH and hydrogen gas.
$2 N a H g + 2 H_{2} O \to 2 N a O H + 2 H g + H_{2}$
(iii) Preparation of sodium peroxide:
Sodium is heated in excess of oxygen to form sodium peroxide. Initially, sodium oxide is obtained which reacts with more oxygen to form sodium peroxide.
$4 N a + O_{2} \to 2 N a_{2} O$
$2 N a_{2} O + O_{2} \to N a_{2} O_{2}$
(iv) Preparation of sodium carbonate:
Sodium carbonate is obtained by Solvay process of ammonia soda process.
In this process, carbon dioxide is passed through a brine solution (containing about 28 % NaCl) which is saturated with ammonia to form sodium carbonate.
$2 N H_{3} + H_{2} O + C O_{2} \to (N H_{4})_{2} C O_{3}$
$(N H_{4})_{2} C O_{3} + H_{2} O + C O_{2} \to 2 N H_{4} H C O_{3}$
$N H_{4} H C O_{3} + N a C l \to N a H C O_{3} ↓ + N H_{4} C l$
The precipitate of sodium bicarbonate is filtered, dried and ignited to form sodium carbonate.
$2 N a H C O_{3} h e a t - - \to N a_{2} C O_{3} + C O_{2} + H_{2} O$
The carbon dioxide required for the reaction can be obtained by heating limestone (calcium carbonate) to 1300 K in a lime klin. Lime dissolves in water to form calcium hydroxide which is then transferred to the ammonia recovery tower.
$C a C O_{3} h e a t - - \to C a O + C O_{2}$
$C a O + H_{2} O \to C a (O H)_{2}$
Ammonia required for the process can be prepared by heating ammonium chloride with calcium hydroxide.
$2 N H_{4} C l + C a (O H)_{2} 2 N H_{3} + C a C l_{2} + H_{2} O$
Hence, the only byproduct of the reaction is calcium chloride.

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