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Group 15: Nitrogen - Ammonia

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Group 15: Nitrogen - Ammonia - Lesson Summary

Ammonia is found in traces in the atmosphere. The bacterial degradation of nitrogenous organic matter of plants and animals forms ammonia.
 
NH 2CONH 2  +  2H 2O →             (NH 4) 2CO 3     →        2NH 3    +   H 2O   +       CO 2
Urea                Water     Ammonium Carbonate     Ammonia      water       Carbondioxide 
In the laboratory, ammonia is generally prepared by heating an ammonium salt with a base such as caustic soda or slaked lime. Ammonium chloride, when heated with lime or caustic soda, evolves ammonia gas. The gas evolved is dried over quick lime. The dry ammonia gas is then collected by downward displacement of air.
 
 2NH 4Cl                 +   Ca(OH) 2           Δ→         2NH 3↑    +    2H 2O  +     CaCl 2
Ammonium Choride   Slaked Lime                Ammonia        Water   Calcium Chloride
   NH 4Cl                 +   Na OH              Δ→         2NH 3↑    +    2H 2O  +     NaCl 2
Ammonium Choride   Caustic Soda                Ammonia        Water   Sodium Chloride


In place of ammonium chloride, ammonium sulphate can also be used, but not other salts such as ammonium nitrate or ammonium dichromate.

 2NH 4Cl                 +   Ca(OH) 2             Δ→         2NH 3↑    +    2H 2O  +     CaCl 2
Ammonium Choride   Slaked Lime                    Ammonia        Water   Calcium Chloride

 [NH 4] 2SO 4                 +   Ca(OH) 2             Δ→         2NH 3↑    +    2H 2O  +     CaCl 2
Ammonium Sulphate    Slaked Lime                     Ammonia        Water       Calcium Sulphate

NH 4NO 3                     +      Ca(OH) 2                 Δ→       No Ammonia Formed
Ammonium Nitrate         Slaked Lime

[NH4] 2Cr 3O 7                      +      Ca(OH) 2                Δ→       No Ammonia Formed
Ammonium Dichromate         Slaked Lime

On an industrial scale, ammonia is prepared by Haber’s process. The constituents of ammonia –N 2&H 2 combine in a ratio of 1:3.
 
                                            Iron Oxide(Fe 3O 4)
                                              Alumina (Al 2O 3)
1N 2      +       3H 2                               ⇔                                    2NH 3             |  ΔH = -92.2 KJ mol -1
Nitrogen      Hydrogen                 200-300 atm                       Ammonia
                                                    at 700k

The reaction proceeds in the forward direction with a remarkable decrease in volume & the reaction is exothermic. Hence, in accordance with Le Chatlier’s principle, high pressure and low temperature favours the better yield of ammonia. Thus, a high pressure of 200 to 300 atmospheres, an optimum temperature about 700 kelvin and the catalyst iron oxide along with a promoter, alumina , are used to obtain better yields of ammonia.
 
Structure of ammonia:
Ammonia is a covalent molecule. The central atom in ammonia is nitrogen, which undergoes sp3 hybridisation gives 4 sp 3 hybrid orbitals. Among the four electron pairs present in the 4 sp3 hybrid orbitals, one is a lone pair. Due to the presence of one lone pair, the geometry of the molecule is not regular, but distorted. Thus, the molecule acquires trigonal pyramidal shape.  As a consequence, the bond angle is also less than expected, that is, 107 0.
 
         
 

Properties of ammonia:
Ammonia is a colourless gas with a pungent smell. It boils at 239.7 kelvin and freezes at 198.4 kelvin. Ammonia is associated with hydrogen bonding in both its liquid state and solid state. Hence, this factor accounts for the high boiling point and high melting point. Ammonia is highly soluble in water. The aqueous solution of ammonia is alkaline because it produces hydroxyl ions. Ammonia acts as a Lewis base as it has a tendency to donate its lone pair of electrons to a proton or other electron deficient molecules.

Dissociation constant is very small-weak base. It combines with acids to form ammonium salts.
EX: when NH 3passed through a solution of  HCl, it formsNH 4Cl.
 
An aqueous solution of ammonia precipitates hydroxides of metals from their aqueous salt solutions.
 
Equation 1:
                 FeCl 3(aq)    +   3NH 4OH(aq)      →          Fe(OH) 3               +           3NH 4Cl(aq)
                                                                          (Reddish Brown ppt.)
Equation 2:
                 ZnSO 4(aq)    +   2NH 4OH(aq)      →         Zn(OH) 2(s)              +           (NH 4)SO 4(aq)
                                                                          (White ppt.)

Ammonia forms complex compounds with transition metal ions such as copper, silver and cadmium. An aqueous solution of ammonia combines with copper sulphate solution to form a deep blue colour. It forms complex compounds by donating its electron pair to the metal ions, thereby forming coordinate bonds.
 
Uses of ammonia:
Liquid ammonia is used as a refrigerant. It is used to manufacture fertilisers such as ammonium sulphate and ammonium phosphate. It is also used in the manufacture of certain industrially important chemicals such as nitric acid, baking soda and washing soda. 

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