7.3 Ammonia

 NEETprep Audio Note:  

On a small scale ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide.

2NH4Cl + Ca(OH)2 → 2NH3 + 2H2O + CaCl2

(NH4)2 SO4 + 2NaOH → 2NH3 + 2H2O + Na2SO4

On a large scale, ammonia is manufactured by Haber’s process.

In accordance with Le Chatelier’s principle, high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of 200 × 105 Pa (about 200 atm), a temperature of ~ 700 K and the use of a catalyst such as iron oxide with small amounts of K2O and Al2O3 to increase the rate of attainment of equilibrium. The flow chart for the production of ammonia is shown in Fig. 7.1. Earlier, iron was used as a catalyst with molybdenum as a promoter.

Properties

 NEETprep Audio Note:  

Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH– ions.

NH3(g) + H2O(l)   ⇋   NH4+ (aq) + OH– (aq)

It forms ammonium salts with acids, e.g., NH4Cl, (NH4)2 SO4, etc. As a weak base, it precipitates the hydroxides (hydrated oxides in case of some metals) of many metals from their salt solutions. For example,

 NEETprep Audio Note:  

Cu2+ (aq) + 4 NH3(aq)  ⇋   [Cu(NH3)4]2+(aq)

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