Ammonia (NH3)

Ammonia, NH3, occurs naturally in small amounts in air and soil, formed by the decomposition of nitrogenous organic matter such as urea: NH2CONH2 + 2H2O → (NH4)2CO3, which decomposes to 2NH3 + H2O + CO2. On a laboratory scale, it is prepared by decomposing an ammonium salt with a base such as calcium hydroxide or caustic soda — for example 2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + 2H2O, and (NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O. On an industrial scale, ammonia is made by the direct combination of nitrogen and hydrogen gas in the Haber process: N2(g) + 2H2(g) → 2NH3(g), an exothermic reaction (standard enthalpy of formation ΔfH° = −46.1 kJ/mol) for which high pressure favours product formation; the optimum industrial conditions are about 200×10^5 Pa (200 atm) pressure, a temperature near 700 K, and an iron-oxide catalyst promoted with traces of K2O and Al2O3, under which equilibrium is reached quickly. Ammonia is a colourless gas with a pungent odour, with a freezing point of 198.4 K and a boiling point of 239.7 K — both unusually high for so small a molecule, because in the solid and liquid states NH3 molecules hydrogen-bond to one another, and breaking these intermolecular hydrogen bonds takes extra energy. It is highly soluble in water, and its concentrated aqueous solution is called liquor ammonia; in aqueous solution ammonia forms deep-blue [Cu(NH3)4]2+ with Cu2+, and reacts with AgCl to give the soluble, colourless complex [Ag(NH3)2]Cl. Chemically, aqueous ammonia is basic, since NH3(g) + H2O(l) is in equilibrium with NH4+(aq) + OH-(aq); it reacts with acids to give ammonium salts (NH3 + HCl → NH4Cl; 2NH3 + H2SO4 → (NH4)2SO4); and its aqueous solution precipitates the hydroxides (or hydrated oxides) of many metals out of their salt solutions — for example a white precipitate of Zn(OH)2 from ZnSO4, or a precipitate of hydrated iron(III) oxide from FeCl3. Its most distinctive chemical feature, though, is its ability to act as a ligand: the lone pair of electrons on nitrogen lets ammonia coordinate to transition-metal ions and form complexes, as seen above with copper and silver. Ammonia's aqueous solution also gives a characteristic brown precipitate (Millon's base) wi …