Modern inorganic chemistry

282 GROUP VI

porous concrete and foam rubber where the evolved oxygen leavens'

the product. Hydrogen peroxide is a useful antiseptic (for example

toothpaste). It is increasingly used to prepare organic peroxo-

compounds, which are used as catalysts in, for example, polymerisa-

tion reactions, and to prepare epoxy-compounds (where an oxygen

atom adds on across a carbon-carbon double bond); these are used

as plasticisers.

Hydrogen sulphide H 2 S

Sulphur can be reduced directly to hydrogen sulphide by passing

hydrogen through molten sulphur; the reversible reaction H 2 +

S ^ H 2 S occurs. In the laboratory the gas is most conveniently

prepared by the action of an acid on a metal sulphide, iron(II) and

dilute hydrochloric acid commonly being used:

FeS + 2HC1 -» FeCl 2 + H 2 St

The gas is washed with water to remove any hydrogen chloride.
Since iron(II) sulphide is a non-stoichiometric compound and
always contains some free iron, the hydrogen sulphide always
contains some hydrogen, liberated by the action of the iron on the
acid. A sample of hydrogen sulphide of better purity can be obtained
if antimony(HI) sulphide, (stibnite) Sb 2 S 3 , is warmed with concen-
trated hydrochloric acid:

Sb 2 S 3 4- 6HC1 -» 2SbCl 3 4- 3H 2 St

Alternatively pure hydrogen sulphide is obtained by the hydrolysis

of aluminium(III) sulphide:

A1 2 S 3 + 6H 2 O -> 2A1(OH) 3 4- 3H 2 St

PROPERTIES

Hydrogen sulphide is a colourless gas, b.p. 213 K, with a most
unpleasant odour; the gas is very toxic, but the intense odour
fortunately permits very minute concentrations of the gas to be
detected.
Hydrogen sulphide burns in air with a blue flame yielding
sulphur dioxide, but if the air supply is limited, preferential com-
bustion to form sulphur occurs:

2H 2 S + 3O 2 -* 2SO 2 + 2H 2 O

2H 2 S + O 2 -> 2Si + 2H 2 O