Chlorine

Occurrence: the halogens, being highly reactive, are found only in combined form. Fluorine's principal source is fluorspar (fluorite, CaF2), with cryolite and fluorapatite as other ores; chlorine's main source is sodium chloride from sea water and salt deposits, and bromides and iodides also occur dissolved in sea water. Chlorine itself is highly reactive and hence never occurs free; it is found widely distributed as various metal chlorides, sodium chloride from sea water being by far the most important.

Laboratory preparation: chlorine is prepared by the action of concentrated sulphuric acid on a chloride in the presence of manganese dioxide (4NaCl + MnO2 + 4H2SO4 -> Cl2 + MnCl2 + 4NaHSO4 + 2H2O). It can equally be prepared by oxidising hydrochloric acid with various oxidising agents such as manganese dioxide, lead dioxide, potassium permanganate or potassium dichromate (PbO2 + 4HCl -> PbCl2 + 2H2O + Cl2; MnO2 + 4HCl -> MnCl2 + 2H2O + Cl2; 2KMnO4 + 16HCl -> 2KCl + 2MnCl2 + 8H2O + 5Cl2; K2Cr2O7 + 14HCl -> 2KCl + 2CrCl3 + 7H2O + 3Cl2). Chlorine is also liberated when bleaching powder is treated with a mineral acid (CaOCl2 + 2HCl -> CaCl2 + H2O + Cl2; CaOCl2 + H2SO4 -> CaSO4 + H2O + Cl2).

Manufacture: on an industrial scale, chlorine is manufactured either by the electrolysis of brine (the electrolytic process) or by the oxidation of hydrochloric acid with air (Deacon's process). In the electrolytic process, electrolysis of aqueous sodium chloride generates Na+ and Cl- ions; water is reduced at the cathode to hydrogen gas and hydroxide ion (2H2O + 2e- -> H2 + 2OH-, and the resulting Na+ and OH- give sodium hydroxide), while chloride ion is oxidised at the anode to chlorine gas (2Cl- -> Cl2 + 2e-) — so the process simultaneously yields chlorine, hydrogen and sodium hydroxide (the chlor-alkali process). In Deacon's process, a mixture of air and hydrochloric acid gas is passed up a chamber packed with pumice stone shelves soaked in cuprous chloride catalyst, with hot gases at about 723 K passed through a surrounding jacket (4HCl + O2 --CuCl2 catalyst, 400°C--> 2H2O + 2Cl2); mechanistically this proceeds via cuprous chloride being oxidised to cupric oxychloride, which then reacts with more HCl to regenerate cupric chloride and liberate chlorine, cupric chloride in turn decomposing back to cuprous chloride and chlorine (2Cu2Cl2 + O2 -> 2Cu2OCl2; Cu2OCl2 + 2HCl -> 2CuCl2 + H2O; 2CuCl2 -> Cu2Cl2 + Cl2). The chlorine obtained by Deacon's process is relatively dilute and is mainly used for manufacturing bleaching powder.

Physical properties: chlorine is a greenish-yellow gas with a pungent, irritating odour; it causes headache when inhaled even in small amounts and can be fatal in large quantities. It is about 2.5 times heavier than air, is soluble in water (the solution being called chlorine water, from which greenish-yellow crystals of chlorine hydrate, Cl2.8H2O, separate on cooling), and can be liquefied (boiling point -34.6 °C) or solidified as yellow crystals (melting point -102 °C). …