Physical Properties of Ammonia, Laboratory Preparation of Ammonia and the Haber Process

Ammonia is manufactured from the direct combination of hydrogen and nitrogen by the Haber process. The system is set up primarily to create the condition necessary to shift the equilibrium to the right (i.e. production of ammonia) as it is a reversible reaction.

N₂(g) + 3H₂(g) ↔ 2NH₃(g) + heat

The entire set up is summarised into three stages/chambers:

• The dryer and condenser

• The catalyst chamber

• Absorption tower

The nitrogen is mixed with the hydrogen in a ratio of 1:3 and then dried and condensed. The condensed mixture is delivered into the catalytic chamber containing finely divided iron catalyst heated to a temperature of 500°C and a chamber pressure of 200 atmospheres. Once the nitrogen is catalytically combined with the hydrogen, the product is delivered to the absorption tower with continuous water spray to form ammonia solution.

The ammonia yield in this condition is about 15%. Hence, the unreacted hydrogen and nitrogen are recirculated over the system for further production of ammonia.

Ammonia is prepared in the laboratory by heating a non-volatile base with any ammonium salt. The most common ammonium salt used is ammonium chloride. Ammonium chloride is heated with slaked lime (calcium hydroxide), Ca(OH)₂, to form water, ammonia and calcium chloride, CaCl₂.

Ca(OH)₂(s) + 2NH₄Cl(s) → CaCl₂(s) + 2H₂O(l) + 2NH₃(g)

Ammonia is a very soluble gas that can dissolve in the water product, hence it is essential to dry the ammonia before storage. But the most frequently used drying agents like tetraoxosulphate(VI) acid and calcium chloride are not suitable for drying ammonia since they react with the ammonia.

Teraoxosulphate(VI) acid reacts with ammonia to form ammonium tetraoxosulphate(VI) salt, (NH₄)₂SO₄.

2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(s),

while ammonia react with calcium chloride to form an addition product of calcium chloride ammonia complex

4NH₃(g) + CaCl₂(s) → CaCl₂.4NH₃(s)

Drying of ammonia involves passing the gas through quicklime (calcium oxide), CaO. The quicklime is usually placed in a U-tube or a drying tower connected to an upward delivery system.

You can appreciate the preparation of ammonia more by taking a tour of the video of the laboratory preparation of ammonia.

Ammonia is a highly soluble gas due to the formation of hydrogen bond in aqueous ammonia. one volume of water is capable of dissolving 1200 volume of ammonia to give aqueous ammonia, NH₃.H₂O.

NH₃(g) + H₂O(l) ↔ NH₃.H₂O(aq)

This confers a weakly alkaline properties to aqueous ammonia as it ionizes slightly to ammonium, NH₄⁺ and hydroxide, OH^-, ions.

NH₃.H₂O(aq) ↔ NH₄⁺(aq) + OH^-(aq)

Air is denser than ammonia, i.e. 1.7 times less dense than air. 880 ammonia is a concentrated ammonia (i.e. 35% by mass of ammonia gas) with a density of 0.880gcm^-3.

The formation of hydrogen bond is responsible for the relatively high boiling point of ammonia amounting to -77.7°C compared with other similar compounds.

It is a colourless gas with characteristic choking smell. It is a poisonous alkaline gas that turns moist red litmus paper to blue.