Modern inorganic chemistry

GROUP VII: THE HALOGENS 335

boiling at 275 K. It is prepared by treating freshly prepared yellow

mercury(II) oxide with either chlorine gas, or with a solution of

chlorine in tetrachloromethane (carbon tetrachloride):

2HgO + 2C1 2 -> HgO. HgCU + C1 2 O

On heating (and sometimes at ordinary temperatures) it explodes,

yielding chlorine and oxygen — this decomposition also being

catalysed by light. It dissolves in water to give an orange-yellow

liquid containing some chloric(I) acid of which dichlorine monoxide

is the formal anhydride. It is a strong oxidising agent converting

many metals to a mixture of their oxides and chlorides.

Liquid chlorine dioxide, C1O 2 , boils at 284 K to give an orange-

yellow gas. A very reactive compound, it decomposes readily and

violently into its constituents. It is a powerful oxidising agent which

has recently found favour as a commercial oxidising agent and as a

bleach for wood pulp and flour. In addition, it is used in water

sterilisation where, unlike chlorine, it does not produce an un-

pleasant taste. It is produced when potassium chlorate(V) is treated

with concentrated sulphuric acid, the reaction being essentially a

disproportionation of chloric(V) acid :

3KC1O 3 + 3H 2 SO 4 -» 3KHSO 4 + 3HC1O 3

3HC1O 3 -» 2C1O 2 -h HC1O 4 + H 2 O

chloriefV) acid chloric(VII) acid

The reaction usually proceeds with explosive violence and a better
method of preparation is to heat, gently, moist crystals of ethane-
dioic acid (oxalic acid) and potassium chlorate(V) :

2KC1O 3 4- 2H 2 C 2 O 4 -» K 2 C 2 O 4 + 2H 2 O + 2CO 2 + 2C1O 2

Industrially an aqueous solution of chlorine dioxide can be prepared
by passing nitrogen dioxide up a packed tower down which sodium
chlorate(V) flows :

C1OJ + NO 2 -> NOa + C1O 2

The aqueous solution is safe to handle, the dissolution being
essentially physical. On standing in sunlight the solution slowly
decomposes to a mixture of acids. In alkaline solution a mixture of
chlorate(III), C1OJ, and chlorate(V), CIO J, ions is rapidly produced.
Chlorine dioxide is paramagnetic, the molecule containing an odd
electron and having a structure very like that of NO 2 (p. 231).

Dichlorine hexoxide. C1 7 O*. is formed when chlorine dioxide is