Nitrogen and Its Compounds

Dinitrogen (N₂)

Preparation of dinitrogen (N₂)

Dinitrogen is produced commercially by the liquefaction and fractional distillation of air. Liquid dinitrogen (b.p. 77.2 K) distils out first leaving behind liquid oxygen (b.p. 90 K).

Laboratory method

i. In the laboratory, dinitrogen is prepared by treating an aqueous solution of ammonium chloride with sodium nitrite.

NH₄Cl (aq) + NaNO₂ (aq) → N₂ (g) + 2H₂O (l) + NaCl (aq)

Small amounts of NO and HNO₃ are also formed in this reaction; these impurities can be removed by passing the gas through aqueous sulphuric acid containing potassium dichromate.

ii. It can also be obtained by the thermal decomposition of ammonium dichromate.

(NH₄)₂Cr₂O₇ N₂ + 4H₂O + Cr₂O₃

iii. Very pure nitrogen can be obtained by the thermal decomposition of sodium or barium azide.

Ba(N₃)₂  Ba + 3N₂

Physical properties of N₂

Dinitrogen is a colourless, odourless, tasteless and non-toxic gas. It has two stable isotopes: ¹⁴N and ¹⁵N. It has a very low solubility in water (23.2 cm³ per litre of water at 273 K and 1 bar pressure) and low freezing and boiling points.

Chemical properties of N₂

Reaction of dinitrogen with metals

Dinitrogen is quite inert at room temperature because of the high bond enthalpy of N≡N bond. Reactivity increases rapidly with rise in temperature. At higher temperatures, it directly combines with some metals to form ionic nitrides.

6Li + N₂ 2Li₃N

3Mg + N₂ Mg₃N₂

Reaction of dinitrogen with dihydrogen

It combines with hydrogen at about 700 K in the presence of a catalyst to form ammonia (Haber’s Process).

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

Reaction of dinitrogen with dioxygen

Dinitrogen combines with dioxygen only at very high temperature (at about 2000 K) to form nitric oxide, NO.

N₂ + O₂ (g)  2NO (g)

Reaction of dinitrogen with non-metals

At high temperature it reacts with non-metals to form covalent nitrides.

2B + N₂ 2BN

Uses of dinitrogen

Ø N₂ is used in the manufacture of HNO₃, NH₃, CaCN₂ (calcium cyanamide) and other nitrogenous compounds.

Ø It is used for filling electric bulbs.

Ø Liquid dinitrogen is used as a refrigerant to preserve biological materials, food items and in cryosurgery.