Modern inorganic chemistry

320 GROUP VII: THE HALOGENS

Alternatively, the iodide is precipitated as copper(I) iodide by

addition of copper(II) sulphate, in presence of sulphite, thus:

21" + 2Cu2+ + SOi- -h H 2 O -> 2CuI 4- SOj" + 2H +

The iodine is then liberated by heating the copper(I) iodide with

sulphuric acid and iron(III) oxide:

2CuI + 6H 2 SO 4 -f 2Fe 2 O 3 -> 2CuSO 4 + 4FeSO 4 4- 6H 2 O 4- I 2

The copper(II) sulphate is recovered and used to precipitate more

copper(I) iodide.

Iodine and its compounds are. relative to the other halogens,

costly substances.

Iodine is a dark-coloured solid which has a glittering crystalline

appearance. It is easily sublimed to form a bluish vapour in vacno.

but in air, the vapour is brownish-violet. Since it has a small vapour

pressure at ordinary temperatures, iodine slowly sublimes if left in

an open vessel; for the same reason, iodine is best weighed in a

stoppered bottle containing some potassium iodide solution, in

which the iodine dissolves to form potassium tri-iodide. The vapour

of iodine is composed of I 2 molecules up to about 1000 K; above

this temperature, dissociation into iodine atoms becomes appreci-

able.

Like bromine, iodine is soluble in organic solvents, for example

chloroform, which can be used to extract it from an aqueous

solution. The iodine imparts a characteristic purple colour to the

organic layer; this is used as a test for iodine (p. 349). NB Brown

solutions are formed when iodine dissolves in ether, alcohol, and

acetone. In chloroform and benzene a purple solution is formed,

whilst a violet solution is produced in carbon disulphide and some

hydrocarbons. These colours arise due to charge transfer (p. 60) to

and from the iodine and the solvent organic molecules.

CHARACTERISTIC REACTIONS OF THE HALOGENS

WITH HYDROGEN

All the halogens combine directly with hydrogen, the reaction
generally occurring with less vigour in the series F 2 , C1 2 , Br 2 ,1 2 -
The rate of reaction between fluorine and hydrogen varies a
great deal with conditions. Solid fluorine and liquid hydrogen
explode even at 21 K but mixing of the gases at room temperature
in the dark may preclude any reaction; however a reaction can