2. Dinitrogen

DINITROGEN (N₂) 

Preparation :

(i)     Laboratory method of preparation : 

NH₄Cl (aq) + NaNO₂ —® (aq)  N₂ (g) + 2 H₂O (1) + 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 suplhuric acid containing potassium dichromate.

N₂ is collected by the downward displacement of water.

(ii)    By heating ammonium dichromate :  (NH₄)₂Cr₂O₇  N₂↑ ­ + 4H₂O + Cr₂O₃

(iii) Very pure nitrogen   ;  Ba(N₃)₂  Ba + 3N₂ 

Sodium azide also gives N₂ on heating.     

(iv) Industrial method of preparation :

From liquefied air by fractional distillation : The boiling point of N₂ is –196^oC and that of oxygen is –183^oC and hence they can be separated by distillation using fractional column.

Properties :

(i) N₂ is a colourless, odourless, tastless, non-toxic gas having very low solubility in water (23.2 cm3 per litre water at 273 K and 1 bar pressure). It has two stable isotopes : 14N and 15N. It is neither combustible nor a supporter of combustion.

(ii)   Li, Mg and Al on heating with N₂ form corresponding nitrides.

6Li + N₂  2Li₃N ; 3Mg + N₂  Mg₃N₂ ; 2Al + N₂  2AlN

(iii) Reaction with H₂ : At 200 atm and 773 K, and in the presence of iron catalyst and molybdenum promoter, N₂ combines with H₂ reversibly to form ammonia. The process is called Haber’s Process and is the industrial method of manufacturing ammonia. The reaction is exothermic.

N₂ + 3H₂  —® 2NH₃

(iv)Reaction with oxygen: When air free from CO₂ and moisture is passed over an electric arc at about 2000 K, nitric oxide is formed. This reaction is endothermic.

  N₂ + O₂ —® 2NO

Uses :

1.  For providing an inert atmosphere during many industrial processes where presence of air or O₂ is to be avoided (e.g., in iron and steel industry, inert diluent for reactive chemicals).

2. For manufacture of NH₃ by the Haber’s process.

3. For manufacture of HNO₃ by the Birkeland-Eyde process.

4. For manufacture of industrial chemicals containing nitrogen like calcium cyanamide.

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