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The following question is based on the preparation of ammonia gas in the laboratory:
Explain why it is not collected over water?

Last updated date: 13th Jun 2024
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Hint: In laboratory production of ammonia gas, ammonia is produced by reaction between ammonium chloride and calcium hydroxide. The method used for industrial preparation of ammonia gas is known as Haber process or Haber-Bosch process. It is the most efficient process and is adopted by industries for the commercial production of ammonia gas.

Complete Step by step answer: The chemical reaction for laboratory preparation of ammonia gas is shown below:

$2N{H_4}Cl + Ca{(OH)_2} \rightleftharpoons CaC{l_2} + 2{H_2}O + 2N{H_3}$
Ammonia is less dense than air or lighter than air. Ammonia gas is highly soluble in water. Hence, ammonia is not collected over water. It should be collected in an apparatus which does not contain moisture at all. Before collection of ammonia in a gas jar, it is passed over a bed of $CaO$ which removes the moisture present in the air.
The ammonia gas is produced by the reaction between nitrogen and hydrogen. It is a reversible and exothermic chemical reaction. Iron is used as a catalyst.
The nitrogen used for preparation of ammonia is sourced from air and hydrogen comes from natural gas and water. During production of ammonia gas, nitrogen and hydrogen gas combines in the ratio of 1:3 by volume. These gases are first heated and then passed over a catalyst under high pressure.

${N_{2(g)}} + 3{H_{2(g)}} \rightleftharpoons 2N{H_{3(g)}}$
The ideal conditions for favouring the forward reaction in ammonia production is high pressure of about 200 atm and high temperature of about 700K. Cooling is very important during this reaction as the reaction is exothermic. Hence, cool reactants that are nitrogen and hydrogen are added in the reactors during the reaction in order to reduce the temperature.

Note: In the Haber process of ammonia production, iron is used as a catalyst. The catalyst used is generally mixed with $A{l_2}{O_3}$or ${K_2}O$ which acts as a promoter. A promoter refers to the substance added to the catalyst to increase its efficiency.
The catalyst added in the Haber process ensures that the reaction is taking place at higher rates.