Compounds of Sodium
The industrially important compound sodium carbonate (Na2CO3.10H2O) is manufactured by the ammonia-soda (also called Solvay) process (Fig.14.2). The main raw materials needed are limestone (CaCO3) and sodium chloride. Limestone is heated to obtain quick-lime (CaO) and carbon dioxide (kiln used for heating limestone is not shown in Fig.14.2).
|CaCO3 CaO + CO2 (i)
Carbon dioxide is passed up the carbonation tower, fitted with perforated plates, down which ammoniated sodium chloride (also containing carbon dioxide) solution trickles. The following reactions take place in the tower.
|NH3 + H2O + CO2 NH4HCO3
NaCl + NH4HCO3 NH4Cl + NaHCO3 (iii)
Sodium hydrogen Carbonate
Solvay (Ammonia Soda) Process for the Manufacture of Sodium Carbonate
Sodium hydrogen carbonate being less soluble, is filtered off and heated to obtain sodium carbonate.
|2NaHCO3Na2CO3 + H2O + CO2 (iv)
Carbon dioxide produced in reaction (iv) is also used in the tower. Ammonia is generated by reacting quicklime [from reaction (i)] with ammonium chloride [from reaction (iii)].
|Ca(OH)2 + 2NH4Cl 2NH3 + H2O + CaCl2
Thus, the actual by-product of the process is calcium chloride.
Sodium carbonate forms white crystals with composition Na2CO3.10H2O. It is commonly called washing soda and is used for softening water and for washing purposes in the laboratory. It is also used in the manufacture of soap and glass, in paper-making and in the preparation of many other sodium compounds. Na2CO3.H2O is called soda ash or efflorescent crystal of sodium carbonate.