Preparation of Ammonia Chemistry Notes
Preparation of Ammonia :
Ammonia can be prepared by the following methods.
→ Laboratory Method : Ammonia can be prepared in laboratory by heating ammonium chloride with slaked lime.
In this procese evolved ammonia is collected in a jar by the downward displacement of air.
→ By heating ammonium salts with strong base Ammonia can be prepared by heating ammonium salts with strong base like NaOH, KOH etc.
→ By heating ammonium salts : On heating certain ammonium salts ammonia is evolved.
→ By hydrolysis of urea : On hydrolysis of urea, first of all ammonium carbonate is formed which on further hydrolysis produced ammonia.
→ By the action of water on metal nitrides : Metal nitrides like aluminium nitride or magnesium nitride react with water and evolve ammonia.
→ Industrial preparation of Ammonia (Haber’s Process) On commerical scale ammonia can be manufactured by Haber’s process from dinitrogen and dihydrogen as follows
→ The favourable conditions for good yield of Ammonia : According to Le-Chatelier’s principle
- High concentration of reactants : On increasing the concentration of reactants i.e., H2 and N2 the concentration of ammonia will be increased
- Low temperature : As the reaction is exothermic hence, low temperature is the favourable condition for good yield of ammonia. Hence, optimum temperature i.e., 700 K is necessary to increase the yield.
- High pressure : High pressure ie, about 200 atm to 900 atm is needed for good yield of ammonia.
- Catalyst : Small amount of catalyst is sufficient to increase the rate of production. For this purpose finally divided iron oxide as catalyst with traces of K2O and Al2O3 is used. Earlier iron was used as catalyst with molybdenum acting as promotor.
→ Method : In preparation of ammonia, dihydrogen gas is obtained by the electrolysis of water. Dinitrogen gas is obtained by the fractional distillation of liquified air. These two gases are purified and then dried. Now the mixture of dinitrogen and dihydrogen gas in the ratio of 1:3 is compressed to 200 atmospheric pressure and allowed to enter in the catalyst chamber packed with the catalyst and the promoter.
→ The chamber is now heated to about 673 K to start the reaction. The formation of ammonia is an exothermie process so the evolved heat is consumed in further heating the reacting mixture. Hence no heating is required. The gases obtained from the catalyst chamber are the mixture of dihydrogen, dinitrogen and ammonia.
→ In mixture ammonia is about 15 to 20 %. The gaseous mixture (H2 + N2 + NH3) is now passed through a condensing chamber where only ammonia is condensed and gets liquified. This liquified ammonia is collected. The rest gases ie, dihydrogen and dinitrogen are recycled.
→ Drying of ammonia : Ammonia gas, prepared either from laboratory method or from Haber’s process, is generally moist. This ammonia gas is dried by passing it through quick lime (Cao). Ammonia gas can not be dried by concentrated sulphuric acid, anhydrous calcium chloride or phosphorus pentaoxide as it reacts with them as given here.