Discuss the various reactions that occur in the Solvay process.

Solution:

Solvay process is used to prepare sodium carbonate.

When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, sodium hydrogen carbonate is formed. This sodium hydrogen carbonate is then converted to sodium carbonate.

Step 1: Brine solution is saturated with ammonia.

$2 \mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2} \longrightarrow\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CO}_{3}$

This ammoniated brine is filtered to remove any impurity.

Step 2: Carbon dioxide is reacted with this ammoniated brine to result in the formation of insoluble sodium hydrogen carbonate.

$\mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2} \rightarrow \mathrm{NH}_{4} \mathrm{HCO}_{3}$

 

$\mathrm{NaCl}+\mathrm{NH}_{4} \mathrm{HCO}_{3} \rightarrow \mathrm{NaHCO}_{3}+\mathrm{NH}_{4} \mathrm{Cl}$

Step 3: The solution containing crystals of $\mathrm{NaHCO}_{3}$ is filtered to obtain $\mathrm{NaHCO}_{3}$.

Step 4: $\mathrm{NaHCO}_{3}$ is heated strongly to convert it into $\mathrm{NaHCO}_{3}$.

$2 \mathrm{NaHCO}_{3} \longrightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}$

Step 5: To recover ammonia, the filtrate (after removing $\mathrm{NaHCO}_{3}$ ) is mixed with $\mathrm{Ca}(\mathrm{OH})_{2}$ and heated.

$\mathrm{Ca}(\mathrm{OH})_{2}+2 \mathrm{NH}_{4} \mathrm{Cl} \rightarrow 2 \mathrm{NH}_{3}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{CaCl}_{2}$

The overall reaction taking place in Solvay process is $2 \mathrm{NaCl}+\mathrm{CaCO}_{3} \longrightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{CaCl}_{2}$