p - Block Elements Part - 1d | Class 12th

ü Ammonia:

Ø  Preparation: In laboratory, ammonia is obtained by treating ammonium salts with caustic soda (NaOH) or slaked lime.

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

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

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

N₂(g) + 3H₂(g) → 2NH₃(g)

In accordance with Le Chatelier’s principle, high pressure of about 200 atm, a temperature of about 773 K and the catalyst such as iron oxide with small amounts of K₂O and Al₂O₃ are employed to increase the rate of this reaction.

Ø  Properties:

·   Ammonia is a colourless gas with pungent smell.

·   It is highly soluble in water because of its ability to form inter molecular hydrogen bond with water.

·   Liquid ammonia has high melting and boiling points because of inter molecular hydrogen bonding.

·       The ammonia molecule has a trigonal pyramidal geometry. It has three bond pairs and one lone pair of electrons.

·       Its aqueous solution is weakly basic due to the formation of OH^– ions.

NH₃(g) + H₂O(l) → NH₄⁺ (aq) + OH^– (aq)

·       As a weak base, it precipitates the hydroxides of many metals from their salt solutions. For example,

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

ZnSO₄ (aq)+ 2NH₄OH (aq) → Zn(OH)₂ (s) + (NH₄)₂SO₄ (aq)

·       The presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule makes it a Lewis base. It donates the electron pair and forms complex compounds with Cu²⁺, Ag⁺ etc. So, it is used for the detection of these metal ions.

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

(blue)                               (deep blue)

Ag⁺ ( aq) + Cl− (aq) → AgCl (s)

(colourless)                    (white ppt)

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

(white ppt)                      (colourless)

 

Ø  Uses: Ammonia is used

i)    To produce various nitrogenous fertilizers (ammonium nitrate, urea, ammonium phosphate and ammonium sulphate)

ii)   In the manufacture of nitric acid.

iii)    liquid ammonia is used as a refrigerant.

iv)    As a laboratory reagent.

v)  In the production of artificial rayon, silk, nylon etc.

vi)    For manufacture of HNO₃ by the Ostwald process.