Modern inorganic chemistry

GROUPV 235

Phosphorus(III) oxide reacts slowly with oxygen at ordinary

temperatures to give the pentoxide, P 4 Oi 0. The reaction is rapid if

the oxide is heated in air. It is oxidised vigorously by chlorine and

bromine which form the oxidehalides, POX 3.

Phosphorus(III) oxide dissolves slowly in cold water to yield

phosphoric(III) acid, H 3 PO 3 (phosphorousacid):

P 4 O 6 + 6H 2 O -* 4H 3 PO 3

With hot water a vigorous but complex reaction occurs, the products

including phosphine and phosphoric(V) acid. This disproportiona-

tion reaction can be approximately represented as:

P 4 O 6 + 6H 2 O -* PH 3 + 3H 3 PO 4

PHOSPHORUS(V) OXIDE, P 4 O 10

This oxide was originally given the formula P 2 O 5 and called
"phosphorus pentoxide'; but the vapour density and structure
indicate the formula P 4 O 10. It is prepared by burning phosphorus
in a plentiful supply of air or oxygen:

P 4 + 5O 2 -> P 4 O 10

It is a white, deliquescent solid, very powdery, which exhibits
polymorphism; on heating, several different crystalline forms appear
over definite ranges of temperature—iiltimately, the P4.O 10 unit in
the crystal disappears and a polymerised glass is obtained, which
melts to a clear liquid.
The most important property of phosphorus(V) oxide is its great
tendency to react with water, either free or combined. It reacts with
ordinary water with great vigour, and much heat is evolved; trioxo-
phosphoric(V) acid is formed, but the local heating may convert
some of this to tetraoxophosphoric(V) acid:

P 4 O 10 + 2H 2 O -» 4HPO 3

HPO 3 + H 2 O -> H 3 PO 4

Phosphorus(V) oxide will remove water from acids to give the acid
anhydride. For example, if nitric acid is distilled with it, dinitrogen
pentoxide is formed:

P 4 O 10 + 4HNO 3 -> 2N 2 O 5 + 4HPO 3

Phosphorus(V) oxide is an extremely effective desiccating agent,
reducing the vapour pressure of water over it to a negligibly small