3. Ammonia

Ammonia

Preparation :

(i)   Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous orgainc matter e.g., urea.

NH₂CONH₂ + 2 H₂O —®  (NH₄)₂CO₃   2 NH₃ + H₂O  + CO₂

(ii)  On a small scale ammonia is obtaned from ammonia salts which decompose when treated with caustic soda or lime.

2 NH₄Cl + Ca (OH)₂ —® 2 NH₃ + 2 H₂O + CaCl₂          

(NH₄)₂ SO₄ + 2 NaOH —® 2 NH₃ + 2 H₂O + Na₂SO₄

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

  N₂ (g) + 3 H₂ (g) —® 2 NH₃ (g) ;      D_f H–  = – 46.1 kJ mol^–1

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.

(iv)   By hydrolysis of metal nitrides like AIN or Mg₃N₂         

AIN + NaOH + H₂O —® NaAIO₂ + NH₃ 

For drying, dehydrating agents like H₂SO₄ , P₂O₅ or CaCl₂ can not be used as these react with NH₃.

2NH₃ + H₂SO₄ —® (NH₄)2SO₄ ; 6NH₃ + P₂O₅ + 3H₂O —® 2(NH₄)₃PO₄ 

 CaCl₂ + 8NH₃  —® CaCl₂ 8NH₃ (Adduct)

So quicklime (CaO) is used for drying of NH₃ .

CaO + H₂O —® Ca(OH)₂

Properties :

Physical properties : Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are 198.4 and 239.7 K respectively. In the solid and liquid states , it is associated through hydrogen bonds and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass. The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons as shown in the structure. 

Ammonia gas is highly soluble in water. Its aqueous solution is weakly basic due to the formation of OH^– ions.  NH₃ (g) + H₂O (1)  NH₄⁺ (aq) + OH^– (aq)

Chemical properties :

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

 2 FeCl₃ (aq) + 3 NH₄OH (aq) —® Fe₂O₃ . xH₂O (s) + 3 NH₄Cl (aq)

                                                                                (brown ppt)

(ii) The presence of lone pair of electrons on the nitrogen atoms of the ammonia molecule makes it a Lewis base. It donates the electrons pair and forms linkage with metal ions forming complex compounds.

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

(blue)                                            (deep blue)

Ag+ (aq) + Cl^–1 (aq)  AgCl (s)

(colourless)                      (white ppt)

 AgCl (s) + 2 NH₃ (aq)  —® [Ag (NH₃)₂]Cl (aq)

(white ppt)                          (colourless)

Test of ammonia/ammonium salts :

When NH₃ gas is passed into the colourless solution of Nessler’s reagent a brown precipitate or coloration is formed. This is a test for NH₃ gas.

2K₂HgI₄ + 3KOH + NH₃ —® H₂N·HgO·HgI ↓(brown) + 7KI + 2H₂O

Uses :

1. Liquid ammonia is used as a refrigerant.

2. For the production of ammonium fertilizers such as ammonium sulphate, ammonium phosphate, ammonium nitrate, urea etc.

3. For removing grease because NH₄OH dissolves grease.

4. For manufacture of HNO₃ by the Ostwald process.

5. As a laboratory reagent.

6. In the production of artificial rayon, silk, nylon etc