Question 10.12:

Discuss the various reactions that occur in the Solvay process.

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.
$2{\mathrm{NH}}_{3}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{CO}}_{2}\to {\left({\mathrm{NH}}_{4}\right)}_{2}{\mathrm{CO}}_{3}$
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.
${\mathrm{NH}}_{3}+{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{CO}}_{2}\to {\mathrm{NH}}_{4}{\mathrm{HCO}}_{3}\phantom{\rule{0ex}{0ex}}\mathrm{NaCl}+{\mathrm{NH}}_{4}{\mathrm{HCO}}_{3}\to {\mathrm{NH}}_{4}{\mathrm{HCO}}_{3}↓+{\mathrm{NH}}_{4}\mathrm{Cl}$
Step 3: The solution containing crystals of NaHCO3 is filtered to obtain NaHCO3.
Step 4: NaHCO3 is heated strongly to convert it into NaHCO3.
$2{\mathrm{NaHCO}}_{3}\to {\mathrm{Na}}_{2}{\mathrm{CO}}_{3}+{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}$
Step 5: To recover ammonia, the filtrate (after removing NaHCO3) is mixed with Ca(OH)2 and heated.
$\mathrm{Ca}{\left(\mathrm{OH}\right)}_{2}+2{\mathrm{NH}}_{4}\mathrm{Cl}\to {\mathrm{LNH}}_{3}+2{\mathrm{H}}_{2}\mathrm{O}+{\mathrm{CaCl}}_{2}$
The overall reaction taking place in Solvay process is
$2\mathrm{NaCl}+{\mathrm{CaCO}}_{3}\to {\mathrm{Na}}_{2}{\mathrm{CO}}_{3}+{\mathrm{CaCl}}_{2}$