Modern inorganic chemistry

GROUP VII: THE HALOGENS 333

acid. The latter may be prepared directly by slow hydrolysis of a

covalent bromide: a convenient one is disulphur dibromide, S 2 Br 2 ,

made by dissolving sulphur in excess liquid bromine. The mixture

is then hydrolysed, and hydrobromic acid distilled off:

S 2 Br 2 + 2Br 2 + 4H 2 O -> 6HBr -f H 2 SO 4 + Si

The acid which conies over is a constant boiling mixture con-

taining about 47% hydrogen bromide (density = 1.46gem"^3 ).

Hydrobromic acid is rather easily oxidised when exposed to light

and becomes brown due to the bromine liberated. Otherwise, its

properties are those of a strong acid, similar to hydrochloric acid.

HYDROGEN IODIDE, HI

Hydrogen iodide is prepared in a similar way to hydrogen bromide,

by the action of water on a mixture of iodine and violet phosphorus.

The hydrogen iodide evolved may be collected by downward

delivery or may be condensed (b.p. 238 K); it reacts with mercury

and so cannot be collected over the latter.

An aqueous solution of hydrogen iodide, up to 50% strength, may

be prepared by passing hydrogen sulphide (or sulphur dioxide) into

a suspension of iodine in water:

H 2 S + I 2 -*2H+ + 21- + Si

SO|" + I 2 + H 2 O -» 2H+ + 2I~ + SO|~

These reactions illustrate the oxidising action of iodine.
In the first reaction, sulphur may be filtered off, leaving only
hydriodic acid.

Properties—Hydrogen iodide is a colourless gas. It is very soluble
in water and fumes in moist air (cf. hydrogen chloride), to give
hydriodic acid. Its solution forms a constant boiling mixture (cf.
hydrochloric and hydrobromic acids). Because it attacks mercury
so readily, hydrogen iodide is difficult to study as a gas, but the
dissociation equilibrium has been investigated.

Hydriodic acid is a strong acid, reacting with bases to give iodides,
containing the ion I~. It is also a strong reducing agent (so also is
hydrogen iodide, particularly at high temperatures, when dissocia-
tion into hydrogen and iodine is considerable). Thus, it reduces
sulphuric acid to a mixture of sulphite, sulphur and hydrogen
sulphide, the last reaction predominating:

H,SO 4 -f 8HI -> H 2 S -f 41, + 4H 2 O