Question

Starting with sodium chloride how would proceed to prepare-
1. sodium hydroxide
2. sodium carbonate?

Answer 1

1. Sodium hydroxide is prepared by the electrolysis of sodium chloride solution in castner-kellne cell. A brine solution ($1$ % $NaCI$ solution) is electrolysed Using a mercury cathode and a carbon anode.

$NaCl\to N{a}^{+}+C{l}^{-}$

$H_{2}O\to H^{+}+O{H}^{-}$

At cathode $N{a}^{+}+e^{-}\to Na$

$Na+Hg\to Na-Hg$ (amalgam)

At anode $C{l}^{-}\to Cl+e^{-}$

$Cl+Cl\to C{l}_2$

$NaOH\to N{a}^{+}+O{H}^{-}$

$H_{2}O\to H^{+}+O{H}^{-}$

Sodium and hydrogen ion migrate towards cathode while $O{H}^{-}$ and $Hg$ migrate towards anode.

At Cathode, $2H^{+}+2e^{-}\to H_2$

At anode, $O{H}^{-}\to OH+e^{-}$

$Na-Hg+OH\to NaOH+Hg$

Over all reaction

$\underset{Sodiumchloride}{2NaCl}+2H_{2}O⟶\underset{sodiumhudroxide}{2NaOH}+C{l}_2+H_2$

(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 s formed in following steps-

Step 1- Carbon dioxide passes through a concentrated aqueous solution of sodium chloride and amoonia.

$\underset{sodiumchloride}{NaCl}+C{O}_2+N{H}_3+H_{2}O⟶NaHC{O}_3+N{H}_{4}Cl$

The cabon dioxide required for this reaction is produced by heating (calcination) of the lime stone at ${95}^{o}C.$

$CaC{O}_3\to CaO+C{O}_2$

Step 2- The sodium bi carbonate $\left(NaHC{O}_3\right)$ that precipitates out in above step is filtered out from the hot ammonium chloride $\left(N{H}_{4}Cl\right)$ solution and the solution is then reacted with the quick lime $\left(CaO\right)$ formed in above reaction.

$2N{H}_{4}Cl+CaO\to 2N{H}_3+CaC{l}_2+H_{2}O$

$CaO$ makes a strong basic solution. The ammonia formed in this reaction is recycled back to the initial brine solution of fist reaction.

step 3- Sodium bicarbonate that percipitate out in step 1. It is converted to the product (sodium carbonate) by calcination at ${16}^o-{23}^{o}C.$

$2NaHC{O}_3⟶\underset{sodiumcarbonate}{N{a}_{2}C{O}_3}+H_{2}O+C{O}_2$