CHEMISTRY

NITROGEN AND ITS COMPOUNDS

Introduction

Laboratory preparation of Nitrogen

Laboratory preparation of Ammonia

NITROGEN AND ITS COMPOUNDS

AMMONIA (NH3)

Ammonia is a covalent compound. It has a molecular mass of 17.

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) CaCl2(s) + 2H2O(i) + 2NH3(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.

  1. 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, Alminium 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.

N2(g) + 3H2(g) 2NH3(g)

Alminium 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.

Properties of Ammonia

  1. Ammonia is a colorless gas with a characteristic pungent smell.

  2. It turns dump / wet red litmus paper blue. It is the only common alkaline gas.

  3. It forms dense white fumes with hydrogen chloride gas

NH3(g) + HCl(g) NH4Cl(s)         


Ammonia diffuses faster and white dense fumes will be formed near hydrogen chloride gas – the white dense fume is ammonia chloride.

  1. Ammonia is very soluble in water. The great solubility of ammonia can be demonstrated using the fountain experiment.

Procedure:

A largely dry, round bottomed flask with ammonia is used. The mouth of the flask is placed under water and a red litmus paper in a trough of gas jar. It is clamped firmly in position. The spring clip at the end of the long glass tube is opened. Water slowly rises up the tube until one drop is at the jet at the top.
The drop dissolves so much ammonia that there is a partial vaccum in the flask.
Water is sucked rapidly up the tube and enters the flask as a fountain. The litmus paper turns blue.

  1. Ammonia burns in a lot of air (oxygen). The flame is yellow green.

4NH3(g) + 3O2(g) 6H2O(i) + 2N2(g)

NB: Glass wool is to spread out the oxygen in order to bring it into greater contact with ammonia.

In presence of a catalyst ammonia reacts with oxygen to form nitrogen monoxide. The monoxide is easily oxidized to dioxide hence if a hot platinum or copper wire is suspended in a beaker of concentrated ammonia and oxygen is bubbled through the solution, redish brown fumes are seen. The fumes later turn white. The brown fumes are due to nitrogen dioxide which turn white as ammonium nitrate is formed.

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

2NO(g) + O2(g) 2NO2(g)

NO2(g) + O2(g) + 2H2O(i) 4HNO3(g)

HNO3(g) + NH3(g) NH4NO3(s)


  1. Ammonia reduces heated copper(II) oxide to copper i.e. copper turns from black to brown.

3 CuO(s) + 2NH3(g) 3 Cu(s) + 3H2O(i) + N2(g)
           

 

  1. Ammonia burns in chlorine forming mist of hydrogen chloride gas. In excess ammonia, dens white fumes of ammonia chloride are formed.

2NH3(g) + 3Cl2(g) N2(g) + 6HCl(g)

HCl(g) + 3 NH3(g) NH4 Cl(s)

 

  1. Ammonia solution (Ammonium hydroxide) contains hydroxyl ions with metal ions precipitates of the hydroxides are formed. Hence a blue precipitate forms when aqueous ammonia is added to copper II sulphate solution. The precipitate dissolves in excess ammonia forming a deep blue solution.

Cu(aq)2+ + 2OH-(aq)Cu(OH)2(s)

Cu(OH)2(s)+ 4NH3(aq) Cu(HN3)2+4 + 2OH-(aq)


Iron(II) is (Fe2+) forms a dirty green precipitate with ammonia insoluble in excess Iron(III) is (Fe3+) forms a brown precipitate insoluble in excess.

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. 

 

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