Important compounds of alkali metals:
Sodium carbonate is one of the important inorganic compounds used in industries. It is prepared by Solvay process. In this process, ammonia is converted into ammonium carbonate which then converted to ammonium bicarbonate by passing excess carbon dioxide in a sodium chloride solution saturated with ammonia. The ammonium bicarbonate thus formed reacts with the sodium chloride to give sodium bicarbonate and ammonium chloride. As sodium bicarbonate has poor solubility, it gets precipitated. The sodium bicarbonate is isolated and is heated to give sodium carbonate. The equations involved in this process are,
2NH3 + H2O + CO2 → (NH4)2CO3
(NH4)2CO3 + H2O + CO2 → 2 NH4HCO3
2 NH4HCO3 + NaCl → NH4Cl + NaHCO3
2 NaHCO3 → Na2CO3 + CO2 + H2O
The ammonia used in this process can be recovered by treating the resultant ammonium chloride solution with calcium hydroxide. Calcium chloride is formed as a by-product.
Sodium carbonate, commonly known as washing soda, crystallises as decahydrate which is white in colour. It is soluble in water and forms an alkaline solution. Upon heating, it looses the water of crystallisation to form monohydrate. Above 373 K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.
Na2CO3·10H2O → Na2CO3·H2O + 9H2O
Na2CO3·H2O → Na2CO3 + H2O
i. Sodium carbonate known as washing soda is used heavily for laundering
ii. It is an important laboratory reagent used in the qualitative analysis and in volumetric analysis.
iii. It is also used in water treatment to convert the hard water to soft water
iv. It is used in the manufacturing of glass, paper, paint etc...
Sodium chloride is isolated by evaporation from sea water which contains 2.7 to 2.9% by mass. Approximately 50 lakh tons of salt are produced annually in India by solar evaporation. Crude sodium chloride can be obtained by crystallisation of brine solution which contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities. Pure sodium chloride can be obtained from crude salt as follows. Firstly removal of insoluble impurities by filtration from the crude salt solution with minimum amount of water. Sodium chloride can be crystallised by passing HCl gas into this solution. Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution.
Sodium chloride melts at 1081K. It has a solubility of 36.0 g in 100 g of water at 273 K. The solubility does not increase appreciably with increase in temperature.
i) It is used as a common salt or table salt for domestic purpose.
ii) It is used for the preparation of many inorganic compounds such as NaOH and Na2CO3
Sodium hydroxide is prepared commercially by the electrolysis of brine solution in Castner-Kellner cell using a mercury cathode and a carbon anode. Sodium metal is discharged at the cathode and combines with mercury to form sodium amalgam. Chlorine gas is evolved at the anode. The sodium amalgam thus obtained is treated with water to give sodium hydroxide.
At cathode : Na+ + e– → Na(amalgam)
At anode : Cl– → ½ Cl2 ↑+ e–
Sodium hydroxide is a white, translucent and deliquescent solid, that dissolves in water to give a strong alkaline solution. It melts at 591 K. The sodium hydroxide solution at the surface reacts with the CO2 in the atmosphere to form Na2CO3
· Sodium hydroxide is used as a laboratory reagent
· It is also used in the purification of bauxite and petroleum refining
· It is used in the textile industries for mercerising cotton fabrics
· It is used in the manufacture of soap, paper, artificial silk and a number of chemicals
Sodium hydrogen carbonate or sodium bicarbonate is used in backing cakes pastries etc. It is called so because it decomposes on heating to generate bubbles of carbon dioxide, leaving holes in cakes or pastries and making them light and fluffy. This compound is prepared by saturating a solution of sodium carbonate with carbon dioxide. The white crystalline powder of sodium bicarbonate, being less soluble, precipitated out.
· Primarily used as an ingredient in backing.
· Sodium hydrogen carbonate is a mild antiseptic for skin infections.
· It is also used in fire extinguishers.