Modern inorganic chemistry

GROUP VII: THE HALOGENS 329

The liquid, like water, has a high dielectric constant (permittivity)

and is weakly conducting. It is a good solvent for many inorganic

and organic substances, to give conducting solutions. Substances

which move the equilibria to the right when dissolved in hydrogen

fluoride, by taking up the fluoride ions, are^4 acids'. For example,

boron trifluoride forms the tetrafluoroborate anion in a solution of

hydrogen fluoride:

2HF + BF 3 ^H 2 F + + BFJ

tetrafluoroborate ion

However, many substances, notably alcohols, have a greater proton

affinity than the hydrogen fluoride molecule, and so behave as

bases, for example ethanol:

C 2 H 5 OH + HF ^ C 2 l _

Even nitric acid will do this, i.e.:

HNO 3 + HF ^ H 2 NOa 4- F"

Thus nitric acid behaves as a base in hydrogen fluoride. Hence

increases of conductivity when substances dissolve in hydrogen

fluoride may be due to "acidic' or 'basic' behaviour.

The preparation and reactions of hydrogen halides

HYDROGEN FLUORIDE, ANHYDROUS HYDROFLUORIC ACID, HF

Hydrogen fluoride is the most important compound of fluorine. It is
prepared in the laboratory, and on the large scale, by the reaction
of calcium fluoride with concentrated sulphuric acid:

CaF 2 + H 2 SO 4 -» CaSO 4 + 2HF?

The reaction is carried out in a lead retort; one suitable for the
laboratory can be made from a piece of lead piping, bent like a
retort and closed at the shorter end. This is charged with fluorspar
and the acid and heated, and the hydrogen fluoride is distilled into
a polythene vessel.

Anhydrous hydrogen fluoride (as distinct from an aqueous
solution of hydrofluoric acid) does not attack silica or glass. It
reacts with metals to give fluorides, for example with heated iron
the anhydrous iron(II) fluoride is formed; the same product is
obtained by displacement of chlorine from iron(II) chloride:

Fed, + 2HF -» FeF? + 2HC1T