Chlorine

Chlorine: the essentials

 Brief description: chlorine is a greenish yellow gas which combines directly with nearly all elements. Chlorine is a respiratory irritant. The gas irritates the mucous membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odour, and 1000 ppm is likely to be fatal after a few deep breaths. It was used as a war gas in 1915. It is not found in a free state in nature, but is found commonly as NaCl (solid or seawater).

Image adapted with permission from Prof James Marshall's (U. North Texas, USA) Walking Tour of the elements CD.

Cartoon by Nick D Kim (nearingzero.net, used by permission).

Isolation

Isolation: it is rarely necessary to make chlorine in the laboratory as it is readily available commercially in cylindes. Chlorine is found largely in seawater where it exists as sodium chloride. It is recovered as a reactive, corrosive, pale green chlorine gas from brine (a solution of sodium chloride in water) by electrolyis. Electrolysis of molten salt, NaCl, also succeeds, in which case the other product is sodium metal rather than sodium hydroxide.

Na⁺ + Cl^- + H₂O → Na⁺ + ¹/₂Cl₂ + ¹/₂H₂ + OH^-

In the laboratory under carefully controlled conditions, chlorine can be made by the action of an oxidizing agent such as manganese dioxide, MnO₂, upon concentrated hydrochloric acid - the same reaction used by Scheele in 1774 when discovering chlorine.

MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

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