Classwork Series and Exercises {Chemistry – SS2}: Compound of Nitrogen (Ammonia)

Chemistry SS 2 Week 5

Topic: Compound of Nitrogen (Ammonia)

Introduction

Ammonia was earlier known by the name “alkaline air.” It was identified in 1716 by J. Kunckel. It was prepared in a lab for the first time by Joseph Priestly in 1774. Although Claude Berthelot studied the chemical composition of the gas in 1785, it was Davy who proved in 1800 that it was composed of the elements nitrogen and hydrogen. In nature, ammonia occurs in both free as well as in combined state.

Structure of ammonia

Ammonia is a covalent molecule as is shown by its dot structure. The ammonia molecule is formed due to the overlap of three sp³ hybrid orbitals and orbitals of three hydrogens. The fourth sp³ hybrid orbital is occupied by a lone-pair. This gives a trigonal pyramidal shape to ammonia molecule. The H-N-H bond angle is 107.3°, which is slightly less than the tetrahedral angle of 109°28. This is because the lone pair – bond pair repulsions tend to push the N-H bonds slightly inwards. In liquid and solid states, ammonia is associated through hydrogen bonds.

LABORATORY PREPARATION OF AMMONIA

Apparatus – Round bottomed flask, clump, Bunsen burner, delivery tube, trough, curd board, wire gauze, lime water.

Chemicals – Calcium hydroxide, Ammonium chloride, Calcium oxide.

Procedure

Ammonia is prepared by heating a mixture of calcium hydroxide and ammonium chloride.

Ca(OH)_2(s) + 2NH₄Cl_(s) CaCl_2(s) + 2H₂O_(i) + 2NH_3(g)

The tube in which ammonia is generated is fixed in a slanting position to prevent the water formed from running back and crack the whole tube.

Concentrated sulphuric acid and anhydrous calcium chloride are not used to dry ammonia because they react with it. Ammonia is collected by upward delivery as it is lighter than air.

HABER PROCESS (INDUSTRIAL PREPARATION)

Apparatus – Compressing chamber, catalytic chamber, cooling chamber, delivery tube.
Chemicals – Finely divided iron, Aluminium Oxide, Water, Hydrogen, Nitrogen.

Procedure:

It is manufactured by reacting Nitrogen and hydrogen in the presence of finely divided iron as a catalyst at temperatures 350ºC – 400ºC at a pressure of about 350 atmospheres.

N_2(g) + 3H_2(g) 2NH_3(g)

Aluminium Oxide is added to the catalyst to improve its performance. It makes it more porous and this provides a high surface area to the reaction.
The reaction is reversible hence it is not possible to convert all the reactants into ammonia.
To separate ammonia from the mixture is cooled, only ammonia liquidfies and it is separated.
The uncombined Nitrogen and hydrogen are recycled.
Another way of separation is to pass the mixture into water.
Only ammonia dissolves.

FACTORS AFFECTING THE REACTION (PROCESS)

1. Pressure

High pressure causes a better yield of ammonia because it favours the formation of the smaller products. It also increases the speed of reaction because the reacting molecules collide more often.

1. Temperature

At low temperatures the yield at equilibrium of Ammonia is higher but the reaction is slow. At high temperatures the yield of ammonia is low but the reaction is fast, a temperature of about 500ºC is used. The yield is good but the reaction is still too slow. A catalyst is therefore necessary to speed up the reaction.

1. Catalyst

A catalyst speeds the reaction but does not affect the equilibrium. The catalyst should be finely divided because reaction occurs only at the surface.

Physical Properties of Ammonia

• Ammonia is a colourless gas

• It has a pungent odour with and an alkaline or soapy taste. When inhaled suddenly, it brings tears into the eyes.

• It is lighter than air and is therefore collected by the downward displacement of air.

• It is highly soluble in water: One volume of water dissolves about 1300 volumes of ammonia gas. It is due to its high solubility in water that the gas cannot be collected over water.

• It can be easily liquefied at room temperature by applying a pressure of about 8-10 atmosphere.

• Liquid ammonia boils at 239.6 K (- 33.5°C) under one atmosphere pressure. It has a high latent heat of vapourization (1370 J per gram) and is therefore used in refrigeration plants of ice making machines.

• Liquid ammonia freezes at 195.3 K (-77.8°C) to give a white crystalline solid.

Chemical Properties of Ammonia

Thermal stability: Ammonia is highly stable. However, it can be decomposed into hydrogen and nitrogen by passing over heated metallic catalysts or when electric discharge is passed through it.

Basic character: Ammonia molecule has a strong tendency to donate its lone pair ofelectrons of nitrogen to other molecules. Thus, it acts like a strong Lewis base. In aqueous solutions, NH₃ ionizes in accordance with the reaction.

The equilibrium constant for this reaction at 298 K is 1.8 x 10^-5. Thus, ammonia ionizes to a very small extent in aqueous solution. The aqueous solution of ammonia acts as a weak base due to the presence of OH^– ions therein. Therefore, ammonia turns red litmus blue and reacts with acids to form salts.

For example,

With metal oxides: Ammonia gets oxidized to nitrogen, when passed over heated metal oxides.

With alkali metals: When ammonia is passed over heated sodium or potassium, amides are formed and hydrogen is set free.

Uses of Ammonia

1. It is used in the manufacture of fertilizers e.g. Ammonium sulphate.
2. It is used in softening water.
3. It is used in making nitric acid.
4. It is used in making plastics.
5. Ammonium chloride is used in dry cells.
6. It is used in making explosives.

Test for Ammonia

1. It is the only common alkaline gas known. It changes the dump / wet litmus paper blue.
2. Ammonia forms dense fumes of ammonium chloride when brought into contact with fumes of hydrogen chloride from concentrated hydrochloric acid.