Modern inorganic chemistry

252 GROUP V

The reaction of ethanoic acid and phosphorus pentachloride may be

written:

CH 3 COOH + PC1 5 -> O=PC1 3 + HClT + CH 3 COC1

acetyl (cthanoyl) chloride

The trichloride oxide is also obtained by distillation of a mixture

of the pentachloride and anhydrous ethanedioic acid:

(COOH) 2 + PC1 5 -> O=PC1 3 + CO 2 T + COT 4- 2HC1!

This is a convenient laboratory method.

These reactions (and those ol the trichloride) indicate the great

tendency of (pentavalent) phosphorus to unite with oxygen (cf,

silicon).

Arsenic halides

ARSENIC TRIHALIDES

Arsenic forms a volatile trifluoride, AsF 3 , and a fairly volatile

trichloride, AsCl 3 , which fumes in air. The latter is prepared by

passing dry hydrogen chloride over arsenic(III) oxide at 500 K:

As 4 O 6 + 12HC1 -> 4AsCl 3 + 6H 2 O

Arsenic trichloride is not completely hydrolysed by water, and in

solution the following equilibrium is set up:

AsCl 3 + 3H 2 O ^ H 3 AsO 3 + 3HC1

arsenic(lll) acid

Hence addition of concentrated hydrochloric acid to a solution of

arsenic(III) acid produces arsenic(III) chloride in solution. The above

equilibrium may be written:

[As3+] + 3H 2 O ^ H 3 AsO 3 -h 3H +

where i[As^3 + ]^1 represents the complex mixture of cationic arsenic

species present. This behaviour of arsenic(III) chloride is in contrast

to that of phosphorus trichloride where hydrolysis by water is

complete.

ARSENIC PENTAHALIDES

Arsenic forms only the pentafluoride AsF 5 , a colourless liquid, b.p.
326 K. This resembles phosphorus pentafluoride.