Give the preparation of ammonia. Discuss the physical and chemical properties of ammonia. Give the uses of ammonia.

(A) Ammonia is present in small quantities in air and soil where it is formed by the decay of nitrogenous organic matter,e.g.,urea.
$H_{2}N-CO-NH_{2} + 2H_{2}O \rightarrow (NH_{4})_{2}CO_{3} + 2NH_{3} + CO_{2} + H_{2}O$
On a small scale,ammonia is obtained from ammonium salts which decompose when treated with caustic soda or calcium hydroxide.
$2NH_{4}Cl + Ca(OH)_{2} \rightarrow 2NH_{3} + 2H_{2}O + CaCl_{2}$
$(NH_{4})_{2}SO_{4} + 2NaOH \rightarrow 2NH_{3} + 2H_{2}O + Na_{2}SO_{4}$
On a large scale,ammonia is manufactured by Haber's process:
$N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)} ; \Delta_{f}H^{\circ} = -46.1 \ kJ \ mol^{-1}$
In accordance with Le Chatelier's principle,high pressure would favour the formation of ammonia. The optimum conditions for the production of ammonia are a pressure of $200 \times 10^{5} \ Pa$ ($200$ bar),a temperature of $\sim 700 \ K$ and the use of a catalyst such as iron oxide with small amounts of $K_{2}O$ and $Al_{2}O_{3}$ to increase the rate of attainment of equilibrium; earlier,molybdenum was used as a promoter.
$I$. Physical properties: The ammonia molecule is trigonal pyramidal with the nitrogen atom at the apex. It has three bonding electron pairs and one non-bonding electron pair. Ammonia is a colourless gas with a pungent odour. Its freezing and boiling points are $198.4 \ K$ and $239.7 \ K$ respectively. In the solid and liquid states,it is associated through $H$-bonds as in the case of water,which accounts for its higher melting and boiling points than expected on the basis of its molecular mass.
$II$. Chemical properties: Ammonia is highly soluble in water. Its aqueous solution is weakly basic due to the formation of $OH^{-}$ ions.
$\ddot{N}H_{3(g)} + H_{2}O_{(l)} \rightleftharpoons NH_{4(aq)}^{+} + OH^{-}_{(aq)}$
It forms ammonium salts with acids,e.g.,$NH_{4}Cl, (NH_{4})_{2}SO_{4}$,etc. As a weak base,it precipitates hydroxides (hydrated oxides in the case of some metals) of many metals from their salt solutions.
For example:
$ZnSO_{4(aq)} + 2NH_{4}OH_{(aq)} \rightarrow Zn(OH)_{2(s)} + (NH_{4})_{2}SO_{4(aq)}$ (White ppt)
$FeCl_{3(aq)} + NH_{4}OH_{(aq)} \rightarrow Fe_{2}O_{3} \cdot xH_{2}O_{(s)} + NH_{4}Cl_{(aq)}$ (Brown ppt)
Ammonia acts as a Lewis base due to the presence of a lone pair of electrons on the nitrogen atom. It donates the electron pair and forms linkages with metal ions; the formation of such complex compounds finds application in the detection of metal ions such as $Cu^{2+}, Ag^{+}$,etc.
$Cu^{2+}_{(aq)} + 4NH_{3(aq)} \rightleftharpoons [Cu(NH_{3})_{4}]^{2+}_{(aq)}$ (Blue to deep blue)
$Ag^{+}_{(aq)} + Cl^{-}_{(aq)}$ $\rightarrow AgCl_{(s)}$ $\xrightarrow{2NH_{3(aq)}} [Ag(NH_{3})_{2}]Cl_{(aq)}$
Uses: Liquid ammonia is used as a refrigerant due to its large heat of vapourisation. Ammonia is used to produce various nitrogenous fertilizers (ammonium nitrate,ammonium phosphate,and ammonium sulphate) and in the manufacture of some inorganic nitrogen compounds,the most important one being nitric acid.