Ammonia

Ammonia (NH₃)

Preparation:

Ammonia is formed by the hydrolysis of urea.

NH₂CONH₂ +H₂O → 2NH₃ +CO₂

Ammonia is prepared in the laboratory by heating an ammonium salt with a base.

2 NH₄ ⁺ + OH⁻ → 2NH₃ + H₂O

2NH₄Cl + CaO → CaCl₂ + 2NH₃ + H₂O

It can also be prepared by heating a metal nitrides such as magnesium nitride with water.

Mg₃ N₂ + 6H₂O → 3Mg(OH)₂ + 2NH₃

It is industrially manufactured by passing nitrogen and hydrogen over iron catalyst (a small amount of K₂O and Al₂O₃ is also used to increase the rate of attainment of equilibrium) at 750 K at 200 atm pressure. In the actual process the hydrogen required is obtained from water gas and nitrogen from fractional distillation of liquid air.

Properties

Ammonia is a pungent smelling gas and is lighter than air. It can readily liquefied by at about 9 atmospheric pressure. The liquid boils at -38.4°C and freezes at -77° C. Liquid ammonia resembles water in its physical properties. i.e. it is highly associated through strong hydrogen bonding. Ammonia is extremely soluble in water (702 Volume in 1 Volume of water) at 20°C and 760mm pressure.

At low temperatures two soluble hydrate NH₃.H₂O and 2NH₃.H₂O are isolated. In these molecules ammonia and water are linked by hydrogen bonds. In aqueous solutions also ammonia may be hydrated in a similar manner and we call the same as (NH₃.H₂O)

NH₃ + H₂ON  ↔  H₄⁺ OH

The dielectric constant of ammonia is considerably high to make it a fairly good ionising solvent like water.

2NH₃  ↔ NH₄⁺ +  NH₂^-

K_−50C = [NH₄⁺ ][NH₂⁻ ] = 10⁻³⁰

2H₂O ↔ H₃O⁺ + OH⁻

K_{23 C} = [H₃O⁺ ][OH⁻ ] = 10⁻¹⁴

Chemical Properties

Action of heat: Above 500°C ammonia decomposes into its elements. The decomposition may be accelerated by metallic catalysts like Nickel, Iron. Almost complete dissociation occurs on continuous sparking.

2NH₃  → ^{> 5000 C} →N₂ + 3H₂

Reaction with air/oxygen: Ammonia does not burn in air but burns freely in free oxygen with a yellowish flame to give nitrogen steam.

4NH₃ + 3O₂ ↔ N₂ + 6H₂O

In presence of catalyst like platinum, it burns to produce nitric oxide. This process is used for the manufacture of nitric acid and is known as ostwalds process.

4NH₃ + 5O₂ ↔ 4NO + 6H₂O

Reducing property: Ammonia acts as a reducing agent. It reduces the metal oxides to metal when passed over heated metallic oxide.

3PbO + 2NH₃ → 3Pb + N₂ + 3H₂O

Reaction with acids: When treated with acids it forms ammonium salts. This reaction shows that the affinity of ammonia for proton is greater than that of water.

Reaction with chlorine and chlorides: Ammonia reacts with chlorine and chlorides to give ammonium chloride as a final product. The reactions are different under different conditions as given below.

With excess ammonia

2 NH₃ + 3 Cl₂ →N₂ + 6 HCl

6 HCl + 6 NH₃ → 6 NH₄Cl

With excess of chlorine ammonia reacts to give nitrogen trichloride, an explosive substance.

2NH₃ + 6Cl₂ → 2NCl₃ + 6 HCl

2NH₃ (g) + HCl(g) →NH₄Cl (s)

Formation of amides and nitrides: With strong electro positive metals such as sodium, ammonia forms amides while it forms nitrides with metals like magnesium.

2Na + 2NH₃ → 2NaNH₂ + H₂

3Mg + 2NH₃ → Mg₃N₂ + 3H₂

With metallic salts: Ammonia reacts with metallic salts to give metal hydroxides (in case of Fe) or forming complexes (in case Cu)

Fe³⁺ + 3NH₄⁺ →^{3 OH−} →Fe(OH)₃ + 3NH₄⁺

Cu²⁺ + 4NH₃ → [Cu(NH₃ )₄ ]²⁺ etraamminecopper(II)ion (a coordinattion complex)

Formation of amines: Ammonia forms ammonated compounds by ion dipole attraction. Eg. [CaCl₂.8NH₃]. In this, the negative ends of ammonia dipole is attracted to Ca²⁺ ion.

It can also act as a ligand and form coordination compounds such as [Co(NH₃)₆]³⁺, [Ag(NH₃)²]⁺.

For example when excess ammonia is added to aqueous solution copper sulphate a deep blue colour compound [Cu(NH₃)₄ ]²⁺ is formed.

Structure of ammonia

Ammonia molecule is pyramidal in shape N-H bond distance is 1.016 Å and H-H bond distance is 1.645 Å with a bond angle 107 °. The structure of ammonia may be regarded as a tetrahedral with one lone pair of electrons in one tetrahedral position hence it has a pyramidal shape as shown in the figure.