Modern inorganic chemistry

330 GROUP VII: THE HALOGENS

Hydrogen fluoride also effects replacement reactions in organic

compounds. For example, carbon tetrachloride yields a mixture of

chlorofluoromethanes CC1 3 F, CC1 2 F 2 and so on. Like all the other

hydrogen halides, hydrogen fluoride adds on to olefms, for example:

CH 2 =CH 2 + HF -> CH 3 CH 2 F

Aqueous hydrogen fluoride is a weak acid (see above) and dissolves

silica and silicates to form hexafluorosilicic acid; hence glass is

etched by the acid, which must be kept in polythene bottles.

In addition to the abnormal properties already discussed,

aqueous hydrofluoric acid has the properties of a typical acid,

attacking metals with the evolution of hydrogen and dissolving

most metallic hydroxides and carbonates.

Uses of hydrogen fluoride

By far the largest use of hydrogen fluoride is in the manufacture of

fluorocarbons which find a wide variety of uses including refriger-

ants, aerosol propellants and anaesthetics. Hydrogen fluoride is also

used in the manufacture of synthetic cryolite, Na 3 AIF 6 , and the

production of enriched uranium.

HYDROGEN CHLORIDE

Hydrogen chloride is formed:

1. By the direct union of hydrogen and chlorine. Very pure
   hydrogen chloride is made by direct union of pure hydrogen and
   chlorine in a quartz vessel.


2. As the product of the hydrolysis of many substances in which
   chlorine is covalently bound, for example:

SOC1 2 + 2H 2 O -> H 2 SO 3 + 2HC1

PC1 3 4- 3H 2 O -> H 3 PO 3 + 3HC1

It is prepared in the laboratory by warming sodium chloride with

concentrated sulphuric acid:

NaCl + H 2 SO 4 -> NaHSO 4 + HClt

The gas is dried by passage through concentrated sulphuric acid and
collected over mercury.
On the large scale, hydrogen chloride can be produced by the