(i) Ammonia is obtained on a small scale from ammonium salts which evolve it when heated with caustic soda or lime.

`NH_4Cl + NaOH -> NH_3 + NaCl + H_2O`

`2NH_4Cl + undersettext(Slaked lime)(Ca(OH)_2) -> 2NH_3 + CaCl_2 + 2H_2O`

(ii) Ammonia is formed when ammonium chloride is heated with litharge

`2NH_4Cl + PbO -> 2NH_3 + PbCl_2 + H_2O`

(iii) By reacting nitrides with water, ammonia is obtained.

`AlN + 3 H_2O -> Al(OH)_3 + NH_3`

`Mg_3N_2 + 6H_2O -> 3Mg (OH)_2 + 2NH_3`

Manufacture of Ammonia :

(i) `text(Haber's process : Principle :)` Haber process is the most important industrial method of preparing ammonia. This method was discovered by a German chemist Fritz Haber. The method involves the direct combination of nitrogen and hydrogen according to the following reaction.

`N_2 + 3H_2 -> 2NH_3 + 24.0` kcal

The reaction is reversible, exothermic and formation of `NH_3` is followed by a decrease in volume. According to Le Chatelier's principle, the optimum conditions for the greater yield of ammonia are :

(a) `text(High pressure :)` Usually a pressure of `200` atmospheres is applied.

(b) `text(Low temperature :)` The working temperature of `450- 550^oC` is maintained.

(c) `text(Catalyst :)` At low temperature, although the yield of ammonia is more yet the reaction is very slow. In order to speed up the reaction, a catalyst is used. The following catalysts have been proposed for this purpose.

(i) Finely divided iron with some molybdenum as a promotor.

(ii) Finely divided nickel and sodalime deposited over pumice stone.

`text(Raw materials :)` Nitrogen and hydrogen are the chief raw materials. Nitrogen is obtained from air by liquefaction followed by fractional evaporation of liquid air. Hydrogen is obtained by electrolysis of water.

(i) Ammonia is a colourless gas with a characteristic pungent odour. It brings tears into the eyes.

(ii) It is lighter than air.

(iii) It is highly soluble in water . One volume of water dissolves `1300` volume of ammonia at `0^oC` and `1` atmosphere. The high solubility is due to the hydrogen bonding. The solubility of ammonia increase with increase of pressure and decreases with increase of temperature. See fig.

(iv) It can be easily liquefied at room temperature by the application of pressure. The liquid ammonia is colorless and boils at `-33^oC`. It freezes at `-78^oC`. Liquid ammonia has a large heat of vaporization ( `327` cal/g). It is, therefore, used in ice-plants.

(i) `text(Stability :)` It is highly stable. It decomposes into nitrogen and hydrogen at red heat or when electric sparks are passed throughout it.

`2NH_3 ⇋ N_2 + 3H_2`

(ii) `text(Basic nature :)` Ammonia is a Lewis base, accepting proton to form ammonium ion as it has tendency to donate an electron pair. It forms salts with acids. See fig.

`NH_3 +HCl -> undersettext(Thick white fumes)NH_4Cl`

`text(Oxidation :)` It is oxidised to nitrogen when passed over heated `CuO` or `PbO`.

`3CuO + 2NH_3 -> 3Cu + N_2 + 3H_2O`

`3PbO + 2NH_3 -> 3Pb + N_2 + 3H_2O`

Both chlorine and bromine oxidise ammonia.

`2NH_3 + 3Cl_2 -> N_2+ 6 HCl`

`ul(6NH_3 + 6HCl -> 6 NH_4Cl)`

`undersettext(Excess)(8NH_3) + 3Cl_2 -> N_2 + 6NH_4Cl`

When chlorine is in excess an explosive substance nitrogen trichloride is formed.

`NH_3 + 3Cl_2 -> NCl_3 + 3 HCl`

Hypochlorites and hypobromite oxidise ammonia to nitrogen.

`2NH_3 + 3NaClO -> N_2 + 3NaCl + 3H_2O`

The oxidation of ammonia with bleaching powder occurs on warming.

`3CaOCl_2 + 2NH_3 -> 3CaCl_2 + N_2 + 3NaCl + 3H_2O`

Thus, ammonia acts as a reducing agent.

(i) Liquid ammonia is used in refrigeration on account of its large heat of evaporation.

(ii) Ammonia is used in the form of ammonium hydroxide (aq. solution) in the laboratory in qualitative and quantitative analysis.

(iii) Ammonia is used for making artificial silk.

(iv) It is used as a cleansing agent for removing greese.

`ast` `text(Phosphorous :)`

(i) Phosphorous is largely used in match industry.

(ii) White phosphorous is used as a rat poison.