Modern inorganic chemistry

GROUP VII: THE HALOGENS 341

Haifc(VII) acids

The existence of chloric(VII), (perchloric), HC1O 4 , and several

periodic(VII) acids has long been established. Bromie(VII) acid and

the bromate(VII) ion have only recently been discovered.

These acids differ so greatly in their properties that they will be

considered separately.

CHLORIC(VIl) ACID AND CHLORATES(VIl)

Chloric(VII) acid is prepared by carefully distilling potassium

chlorate(VII) with concentrated sulphuric acid under reduced

pressure:

KC1O 4 + H 2 SO 4 -> KHSO 4 + HC1O 4

It is a liquid, b.p. 363 K, but if heated it decomposes and hence must
be distilled under reduced pressure; decomposition may occur with
explosive violence and this can occur even at room temperature if
impurities are present. Combustible material, for example paper
and wood, ignite spontaneously with explosive violence on contact
with the acid, and it can produce painful blisters on the skin.
Chloric(VII) acid fumes in moist air and is very soluble in water,
dissolving with the evolution of much heat. Several hydrates are
known; the hydrate HC1O 4. H 2 O is a solid at room temperature
and has an ionic lattice [H 3 O+] [C1OJ].
The oxidising properties of the aqueous solutions of chloric(VII)
acid change dramatically with temperature and the concentration
of the acid. Cold dilute solutions have very weak oxidising properties
and these solutions will react, for example, with metals, producing
hydrogen without reduction of the chlorate(VII) ion occurring:

Zn + 2HC1O 4 -» Zn(ClO 4 ) 2 + H 2 T

Hot concentrated solutions of chloric(VII) acid and chlorates(VII),
however, react vigorously and occasionally violently with reducing
agents.
Chloric(VII) acid is one of the strongest acids known, and it
behaves as such even when dissolved in solvents with poor proton
affinity; thus it can be used as an acid in pure ethanoic acid as a
solvent:

CH 3 COOH + HC1O 4 ^ CH 3 COOH^ + CIO 4