Question

(i) How is ammonia gas prepared in laboratory starting from NH₄Cl? State the conditions and balanced equation for the preparation.
(ii) How is gas collected in a gas jar?
(iii) Name the substance used for drying ammonia gas. Why cannot substances such as conc. H₂SO₄, anhydrous calcium chloride and phosphorus pentoxide be used for drying ammonia gas?

Answer 1

(i) Preparation: Ammonium chloride (NH₂Cl) and dry slaked lime [Ca(OH)₂] are used in the preparation of ammonia in the laboratory. Both the compounds are taken in the ratio 2:3 by weight and ground thoroughly before being placed in a flask and heated. The apparatus used for the preparation of ammonia is shown in the following figure:

Conditions: The reactants must be dry and grounded.
A slightly higher ratio of slaked lime is used to counteract any loss of ammonium chloride due to sublimation.
Reaction: $\underset{\mathrm{Ammonium}\mathrm{chloride}}{2{\mathrm{NH}}_4\mathrm{Cl}}+\underset{\mathrm{Slaked}\mathrm{lime}}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2}\stackrel{△}{\to }\underset{\mathrm{Ammonia}}{2{\mathrm{NH}}_3}+{\mathrm{CaCl}}_2+{\mathrm{H}}_2\mathrm{O}$


(ii) Ammonia is lighter than air; therefore, it is collected in the gas jar by downward displacement of air.

(iii) Quick lime, also known as calcium oxide (CaO), is used for drying ammonia. Sulphuric acid, calcium chloride and phosphorus pentoxide cannot be used for drying ammonia.This is due to the reason that ammonia being basic reacts with them.