Ammonia (NH₃)

Ammonia is produced in the atmosphere by decay of urea,

NH₂CONH₂ + 2H₂O → (NH₄)₂CO₃ ↔ 2NH₃ + H₂O + CO₂

Preparation of ammonia

a. Lab method

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

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

b. Haber’s process

Ammonia is prepared at high pressure (200 atm) and 700 K, in the presence of catalysts.

N₂ (g) + 3H₂ (g)  2NH₃ (g); Δ_fH = – 46.1 kJ mol^–1

The optimum conditions for manufacturing ammonia are:

i. Pressure (around 200 × 10⁵ Pa)

ii. Temperature (700 K)

iii. Catalyst such as iron oxide with small amounts of Al₂O₃ and K₂O

Properties of ammonia

(i) Physical properties of ammonia

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 as in the case of water and that accounts for its higher melting and boiling points than expected on the basis of its molecular mass.

(ii) Geometry of ammonia molecule

The molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bond pairs and one lone pair of electrons.

(iii) Solubility of ammonia

It is extremely soluble in water due to H–bonding. Its aqueous solution is weakly basic due to the formation of OH^–ions.

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

(iv) Reaction of ammonia with acids

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

Due to presence of a lone pair of electrons on the nitrogen atom of the ammonia molecule, it behaves as a Lewis base. It donates the electron pair and forms linkage with metal ions. Formation of complex compounds is used for detection of metal ions such as Cu²⁺, Ag⁺:

(v) Reaction of ammonia with chlorine

When NH₃ is in excess, N₂ is the main product

8NH₃ + 3Cl₂ → 6NH₄CI + N₂

When Cl₂ is in excess, NCl₃ is the main product

NH₃ + 3Cl₂ → NCl₃ + 3HCl

Uses of ammonia

i. Ammonia is used to produce various nitrogenous fertilizers (ammonium nitrate, urea, ammonium phosphate and ammonium sulphate) and in the manufacture of some inorganic nitrogen compounds, like nitric acid.

ii. Liquid ammonia is also used as a refrigerant.