Modern inorganic chemistry

328 GROUP VII: THE HALOGENS

correctly indicates the high thermal stability of hydrogen chloride,

with hydrogen bromide being rather less stable. Unlike the hydrogen

halides so far discussed, hydrogen iodide is an endothermic com-

pound, and reference has been made to the equilibrium

H 2 + I 2 ^ 2HI

This equilibrium is established when hydrogen iodide is heated,

hydrogen-iodine bonds being broken.

ACIDITY OF HYDROGEN HALIDES

All the hydrogen halides are freely soluble in water and react

according to the general equation

The steps involved are : (a) the breaking of the hydrogen-halogen
bond, (b) the hydration of the proton and (c) the hydration of the
halide ion. When HX is HC1, HBr or HI, the energy liberated by the
combined hydration energies of the proton and halide ion exceeds
the bond dissociation energy, step (a), and all three are strong acids
in water with acid strength increasing from HC1 to HI (p. 88).
The bond dissociation energy of the hydrogen-fluorine bond in
HF is so great that the above equilibrium lies to the left and hydrogen
fluoride is a weak acid in dilute aqueous solution. In more concen-
trated solution, however, a second equilibrium reaction becomes
important with the fluoride ion forming the complex ion HFJ. The
relevant equilibria are :

HF-h H 2 O^ H 3 O+ 4- F~

(HF) 2 + H 2 O^HF 2 - + H 3 O +

or more generally

The second equilibrium is displaced to the right as the concentration
of hydrogen fluoride is increased and it is found that at a concentra-
tion of approximately 5-1 5 M, hydrogen fluoride is effectively a
strong acid. In this way hydrogen fluoride differs from all the other
hydrogen halides. Anhydrous hydrogen fluoride ionises to a small
extent and the following equilibria are established:

HF ^ HF 2 ~ (H 2 F 3 ~, H 3 F4 etc.)