Modern inorganic chemistry

GROUP VII: THE HALOGENS 337

Iodine oxides

fhere appears to be only one true oxide of iodine, diiodine pentoxide,

I 2 O 5. It is a white solid prepared by heating iodic acid(V) to 450 K:

2HIO 3 -* H 2 O + I 2 O 5

As the equation indicates, it is the anhydride of iodic-acid(V), which

is re-formed when water is added to the pentoxide. Mixed with

concentrated sulphuric acid and silica, it is a quantitative oxidising

agent for carbon monoxide at room temperature:

SCO + I 2 O 5 -> 5CO 2 T + I 2

OXO-ACIDS AND THEIR SALTS

For many years it was thought that fluorine did not form any oxo-

acids or oxo-acid anions. Recent work, however, indicates the

existence of fluoric(I) acid (hypofluorous acid), HFO, formed by the

reaction of fluorine with water at 273 K. The acid forms colourless

crystals, m.p. 156K, is very unstable and has, as expected, very

strong oxidising properties.

The acids of chlorine(I), bromme(T) and iodine(I) are weak acids,

the pKa values being 7.4, 8.7 and 12.3 respectively. They are good

oxidising agents, especially in acid solutions. The acids decrease in

stability from chloric(I) to iodic(I).

Only chlorine forms a +3 acid, HC1O 2. This is also a weak acid

and is unstable. The +5 acids, HXO 3 , are formed by chlorine,

bromine and iodine; they are strong acids. They are stable com-

pounds and all are weaker oxidising agents than the corresponding

+1 acids.

The existence of chloric(VII) (perchloric) and iodic(VII) (periodic)

acids has long been known but bromic(VII) acid has only recently

been prepared.

Halic(I) acids of chlorine, bromine and iodine

The amount of halic(I) acid formed when the halogen reacts
reversibly with water decreases from chlorine to iodine and the
concentration of iodic(I) acid in a saturated solution of iodine is
negligible. However the equilibrium

2H 2 O + X 2 ^ HXO + H 3 O+ 4- X~