Modern inorganic chemistry

324 GROUP VII: THE HALOGENS

aqueous hydrogen peroxide). Bromine water undergoes a similar

decomposition in sunlight and oxygen is evolved but in general it is

more stable than chlorine water and the equilibrium

Br 2 + H 2 O ^ HBr + HBrO

lies further to the left.

If 'chlorine water' is boiled the chloric(I) acid decomposes as

above, but a little may break down into steam and the acid anhyd-

ride, dichlorine monoxide:

2HC1O ^ C1 2 O + H 2 O

The smell of chlorine water, somewhat different from that of

gaseous chlorine, may be due to minute amounts of evolved

dichlorine monoxide:

The reactions with alkalis

Oxygen difluoride, OF 2 , is obtained when gaseous fluorine is

passed through very dilute (2%) caustic soda solution:

2F 2 + 2NaOH -» 2NaF + F 2 O + H 2 O

but with more concentrated alkali, oxygen is formed:

2F 2 + 4NaOH -> 4NaF + 2H 2 O + O 2

The reactions of the other halogens can be summarised in the two

equations:

X 2 + 2OH~ -» X" + XO~ + H 2 O (11.3)

3X 2 + 6OH~ -> 5X~ 4- XOJ + 3H 2 O (11.4)

(Reaction (11.4) is really a disproportionation reaction of the halate(I)
anion: 3XO~ -> 2X~ + XO~.) Reaction (11.3) is favoured by the use
of dilute alkali and low temperature, since the halate(I) anions, XO~
are thermally unstable and readily disproportionate (i.e. reaction
(11.4)). The stability of the halate(I) anion, XO~, decreases from
chlorine to iodine and the iodate(I) ion disproportionates very
rapidly even at room temperature.
The formation of halate(V) and halide ions by reaction (11.4) is
favoured by the use of hot concentrated solutions of alkali and an
excess of the halogen.
When chlorine is passed over molten sodium or potassium
hydroxide, oxygen is evolved, the high temperature causing the
chlorate(V) ion to decompose:

2CKK -+2CP +30 2