Ammonia (NH3)

Preparation: Ammonia is produced in the body/agriculturally by the hydrolysis (in effect, bacterial or thermal decomposition) of urea: NH2CONH2 + H2O -> 2NH3 + CO2. In the laboratory it is generated by heating an ammonium salt with a base — for example, ammonium chloride with slaked lime: 2NH4Cl + CaO -> CaCl2 + 2NH3 + H2O (the reaction proceeds through the acid-base step NH4+ + OH- -> NH3 + H2O). It can also be obtained by heating a metal nitride, such as magnesium nitride, with water: Mg3N2 + 6H2O -> 3Mg(OH)2 + 2NH3. Industrially, ammonia is manufactured by passing nitrogen and hydrogen (in the ratio 1:3) over an iron catalyst promoted with small amounts of K2O and Al2O3 (to speed the attainment of equilibrium), at about 750 K and 200 atm pressure; the hydrogen feedstock is obtained from water gas and the nitrogen from the fractional distillation of liquid air.

Physical properties: Ammonia is a colourless, pungent-smelling gas, lighter than air, which liquefies readily under about 9 atmospheres of pressure; the liquid boils at -38.4 °C and freezes at -77 °C. Liquid ammonia closely resembles water physically because it too is extensively hydrogen-bonded (associated). Ammonia is extraordinarily soluble in water — about 702 volumes dissolve in 1 volume of water at 20 °C and 760 mm pressure. At low temperature two solid hydrates, NH3.H2O and 2NH3.H2O, can be isolated in which ammonia and water molecules are linked by hydrogen bonds; in aqueous solution the same hydrogen-bonded association persists and is loosely represented as NH3.H2O, which can partially ionise as NH3.H2O <=> NH4+ + OH-. Ammonia's high dielectric constant makes it, like water, a fairly good ionising solvent, and it undergoes self-ionisation/autoprotolysis (2NH3 <=> NH4+ + NH2-) analogous to water's (2H2O <=> H3O+ + OH-), each with its own characteristic equilibrium constant at a given temperature. …