332 GROUP VII: THE HALOGENS
Uses of hydrogen chloride—Hydrogen chloride is sometimes used
in the preparation of an ester, for example ethyl benzoate, where it
acts as both an acid catalyst and a dehydrating agent. Hydrochloric
acid is used primarily to produce chlorides, for example ammonium
chloride. It is extensively used in the manufacture oi aniline dyes, and
for cleaning iron before galvanising and tin-plating.
HYDROGEN BROMIDE, HBrHydrogen bromide cannot be prepared readily by the action of
sulphuric acid on a bromide, because the latter is too easily oxidised
by the sulphuric acid to form bromine. It is therefore obtained by
BromineMoist violet phosphorus
on glass beads. = >. —3
Hydrogen
bromideMoist violet phosphorus
Figure 113, Preparation of hydrogen bromidethe hydrolysis of a covalent bromide; a convenient one is phos-
phorus tribromide. By dropping bromine on to a paste of violet
phosphorus and water, phosphorus tribromide is formed and
immediately hydrolysed thus:
PBr 3 + 3H 2 O -» H 3 PO 3 + BHBrtAny free bromine can be removed by passing the evolved gas through
a U tube packed with glass beads covered with moist violet phos-
phorus (Figure 113).
Hydrogen bromide may also be prepared by dropping bromine
into benzene containing aluminium powder, which acts as a catalyst
to the reaction:
C 6 H 6 + Br 2 -^-C 6 H 5 Br + HBrt
Hydrogen bromide is a colourless gas similar in properties to
hydrogen chloride. It is very soluble in water, giving hydrobromic