Hydrides of nitrogen

Ammonia

Ammonia is the most important compound of nitrogen. It can be manufactured by Haber’s process. In this process, a mixture of N₂ and H₂ in the ratio of 1 : 3 is passed over heated Fe at 650 –800K as catalyst and Mo as promotor, N₂ + 3H₂ ⇌ 2NH₃, ΔH = – 93.6 mol^–1 This is a reversible exothermic reaction.

Ammonia is prepared in the laboratory by heating ammonium salt (NH₄Cl) with a strong alkali like NaOH

NH₄Cl + NaOH → NH₃ + H₂O + NaCl Ammonia can be dried by passing over quick lime (CaO). However, it cannot be dried with dehydrating agents such as conc. H₂SO₄, P₂O₅ and anhydrous CaCl₂ because ammonia reacts with these compounds.

NH₃ is a colourless gas with a characteristic pungent smell called ammonical smell. It is highly soluble in water and its solution is basic in nature, NH₃ + H₂O ⇌ NH₄⁺ + OH^– NH₃ is expected to have a tetrahedral geometry, but the lone pair distorts its geometry and the molecule has pyramidal geometry with N – H bond length of 101.7 pm and a bond angle of 107.5^o. Liquid ammonia is widely used as a refrigerant due to its high heat of vaporization.

Hydrazine, (NH₂ – NH₂)

Hydrazine is prepared commercially by boiling aqueous ammonia or urea with sodium hypochloride in the presence of glue or gelatin.

                   2NH₃ + NaOCl → N₂H₄  +  NaCl  + H₂O

The resulting solution is concentrated and anhydrous hydrazine may be obtained by further distillation over barium oxide. Alternatively, the hydrazine present in the resulting solution is precipitated as sparingly soluble crystalline hydrazine sulphate on treatment with sulphuric acid, NH₂NH₂ + H₂SO₄ → N₂H₄.H₂SO₄.

The precipitate is removed and treated with an alkali when hydrazine hydroxide H₂N.NH₃OH is obtained. This is distilled under reduced pressure, over barium oxide to liberate free hydrazine.

H₂N.NH₃OH  + BaO   NH₂NH₂ + Ba(OH)₂

Physical properties : Anhydrous hydrazine is a colourless fuming liquid (m.p. 2⁰C and b.p. 114⁰C )  soluble in water in all proportions. It is also soluble in alcohol. It is strongly hygroscopic.

Chemical properties : It behaves as a diacid base, Thus with hydrochlorides it forms hydrazine monochloride H₂N.NH₃Cl and hydrazine dichloride ClH₃N.NH₃Cl.

(i)      Hydrazine burns in air with the evolution of heat.

N₂H₄ + O₂ → N₂ + 2H₂O      ΔH = – 622 kJ

The alkyl derivative of hydrazine are used as rocket fuels.

(ii)     It reduces Fehling’s solution to red cuprous oxide and iodates to iodides.

4Cu²⁺ + NH₂NH₄ → 4Cu⁺ + 4H⁺ + N₂ ;  2lO₃^– + 3NH₂NH₂ → 2l^– + 6H₂O +  3N₂

Uses : Hydrazine is used as a rocket fuel. It is also used as a reagent in organic chemistry.