Chlorine

Chlorine

Chlorine was first isolated in 1774 by Scheele, who obtained it by reacting hydrochloric acid with manganese dioxide. It wasn't until 1810 that Davy established that this gas was in fact an element in its own right, proposing the name chlorine after its colour (Greek chloros = greenish yellow).

Preparation

Chlorine can be prepared in the laboratory by either of two routes:

1. Heating manganese dioxide with concentrated hydrochloric acid:

   $MnO_2 + 4HCl \rightarrow MnCl_2 + Cl_2 + 2H_2O$

   In practice, a mixture of common salt and concentrated sulphuric acid is often used in place of HCl:

   $4NaCl + MnO_2 + 4H_2SO_4 \rightarrow MnCl_2 + 4NaHSO_4 + 2H_2O + Cl_2$

2. Action of hydrochloric acid on potassium permanganate:

   $2KMnO_4 + 16HCl \rightarrow 2KCl + 2MnCl_2 + 8H_2O + 5Cl_2$

Manufacture

On an industrial scale, two methods dominate:

1. Deacon's process — atmospheric oxygen oxidises hydrogen chloride gas, using copper(II) chloride as catalyst at 723 K:

   $4HCl + O_2 \xrightarrow{\ CuCl_2,\ 723\,K\ } 2Cl_2 + 2H_2O$

2. Electrolytic process — chlorine is liberated at the anode during electrolysis of brine (concentrated sodium chloride solution). Large quantities of chlorine are also obtained as a by-product of various chemical industries.

Properties

Chlorine is a greenish-yellow gas with a sharp, suffocating smell, noticeably heavier than air. It liquefies readily to a greenish-yellow liquid boiling at 239 K, and it is soluble in water.

Reaction with metals and non-metals — chlorine combines directly with a range of elements to form chlorides:

Affinity for hydrogen — chlorine reacts readily with hydrogen-bearing compounds, pulling hydrogen away to form HCl:

Reaction with alkalies — the products depend sharply on temperature and concentration. Cold, dilute alkali gives a mixture of chloride and hypochlorite, while hot, concentrated alkali gives chloride and chlorate:

Reaction with slaked lime gives bleaching powder:

Bleaching powder's composition is written as $Ca(OCl)_2 \cdot CaCl_2 \cdot Ca(OH)_2 \cdot 2H_2O$.

Reaction with hydrocarbons — chlorine gives substitution products with saturated hydrocarbons and addition products with unsaturated ones:

Bleaching action — chlorine water loses its yellow colour on standing as it forms $HCl$ and $HOCl$. The hypochlorous acid generated releases nascent oxygen, which is the true agent behind chlorine's oxidising and bleaching behaviour:

This same oxidising ability lets chlorine convert ferrous to ferric ion, sulphite to sulphate, sulphur dioxide to sulphur trioxide, and iodine to iodate — the last two, in the presence of water, going on to form sulphuric acid and iodic acid respectively:

$Na_2SO_3 + Cl_2 + H_2O \rightarrow Na_2SO_4 + 2HCl$ …