Oxoacids of Phosphorus

⇒ Phosphorus forms a large number of oxoacids.

⇒ All these acids are based on tetrahedral four coordinated phosphorus atom containing at least one P = O unit and one P - OH group. Condensed systems are formed by P - O - P linkage or P - P linkage.

⇒ The basicity depends on the H atoms bonded to O in the form of - OH.

Some of the common oxoacids of phosphorus are as follows:

Name	Formula	Oxidation state of P	basicity	structure	preparation
Hypophosphorus acid	H3PO2	+1	1		white P4 + alkali
Phosphorus acid	H3PO3	+3	2		P2O3 + 3H2O → 2H3PO3
Hypophosphoric acid	H4P2O6	+4	4		red P4 + alkali
Orthophosphoric acid	H3PO4	+5	3		P(red) + 5HNO3 → H2O + H3PO4 + 5NO2 or P4O10 + 6H2O → 4H3PO4
Diphosphoric acid (Pyrophosphoric acid)	H4P2O7	+5	4		heating phosphoric acid 2H3PO4 → H4P2O7 + H2O
Metaphosphoric acid	HPO3	+5	1		heating orthophosphoric acid to about 850 K H3PO4 →HPO3 + H2O
Peroxophosphoric acid	H3PO5	+7	3		P2O5 + 2H2O2 + H2O → 2H3PO5

Oxides of phosphorous:

1. Phosphorus pentoxide:

⇒ It is a chemical compound with molecular formula P₄O₁₀ (with its common name derived from its empirical formula, P₂O₅).

⇒ This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

⇒ P₄O₁₀ is prepared by burning elemental phosphorus with sufficient supply of air:

P₄ + 5 O₂ → P₄O₁₀

2. Phosphorus trioxide:

⇒ It is the chemical compound with the molecular formula P₄O₆. Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today.

⇒ It is a colorless solid. It is formally the anhydride of phosphorous acid, H₃PO₃, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid.

⇒ It is obtained by the combustion of phosphorus in a limited supply of air at low temperature.

P₄(s) + 3 O₂ (g) → P₄O₆(s)