Modern inorganic chemistry

318 GROUP VII: THE HALOGENS

chloride containing one or more chlorides of rare-earth metals on a

silica support at a temperature of 600-670 K; the reaction is

exothermic:

4HC1 + O 2 ==* 2H 2 O 4- C1 2

The equilibrium constant for this reaction decreases with increase

in temperature but the higher temperature is required to achieve a

reasonable rate of conversion. Hydrogen chloride is now being

produced in increasing quantities as a by-product in organic

chlorination reactions and it is economic to re-convert this to

chlorine.

Chlorine has a boiling point of 238 K and is a greenish-yellow

diatomic gas at room temperature. It can be liquefied by cooling or

by a pressure of a few atmospheres at room temperature.

BROMINE

Bromides of sodium, potassium, magnesium and calcium occur in

sea water (about 0.07 % bromine) but the Dead Sea contains much

more (5% bromine). Salt deposits (e.g. at Stassfurt) also contain

these bromides. Silver bromide, AgBr, is found in South America.

In the laboratory, bromine is prepared by oxidation of bromide

ion; the oxidation is carried out by mixing solid potassium bromide

with manganese(IV) oxide and distilling with concentrated sulphuric

acid:

2KBr + MnO 2 + 3H 2 SO 4 -> Br 2 + 2KHSO 4 + MnSO 4 + 2H 2 O

The bromine is condensed and collected in a water-cooled

receiver as a dark-red liquid.

On the industrial scale, bromine is obtained from sea water by

using the displacement reaction with chlorine (the reaction by which

bromine was discovered):

2Br" + C1 2 -> 2CP + Br 2

The sea water is first treated with chlorine in acid solution (sul-
phuric acid is added) and very dilute bromine is obtained by blowing
air through the solution. This is mixed with sulphur dioxide and the
gases passed up a tower down which water trickles:

SO 2 + Br 2 + 2H 2 O -> 2HBr + H 2 SO 4

The mixture of the two acids (now much richer in bromine than
the sea water) is then treated with chlorine again, and bromine