Oxyacids of phosphorus

Phosphorus forms two series of oxyacids namely; phosphorus acid and phosphoric acid.

(a) Phosphorus acid series
The series consists of p atom exhibiting oxidation state of +3. They act as reducing agents.

The p Block Elements Chemistry Notes

Special Note
(i) Meta is used for acid obtained by loss of H2O molecule.
(ii) Pyro is used for the acid obtained from two molecules with a loss of H2O.
(iii) Hypo is used for the acid having lower oxygen content than the parent acid.

Phosphorus acid (H3PO3 )

Preparation and Properties

The p Block Elements Chemistry Notes

(b) Phosphoric acid series
This series consist of acids having P in +5 oxidation state.

The p Block Elements Chemistry Notes

Meta phosphoric does not exist in monomeric form, but it exists as cyclometa phosphoric acid (HPO3)3 or polymeta phosphoric acid (HPO3)n.

Orthophosphoric acid (H3PO4 )

This acid is commonly called as phosphoric acid.
Preparation
(i) P4O10 + 6H2O → 4H3PO4
(ii) PCl + 4H2O → H3PO4 + 5HCl
(iii) Laboratory preparation
P + 5HNO3 → H3PO4 + H2O + 5NO2
This reaction is carried out in the presence of a crystal of iodine. Iodine acts as a catalyst.

(iv) Industrial preparation
It is prepared from bone ash.
⇒Ca3(PO4)2 + 3 H2SO4 → 3CaSO4 + 2 H3PO4
When bone ash is dissolved in HNO3 and lead acetate it forms Pb3(PO4)2
Pb3(PO4)2 + 3H2S → 3PbS + 2H3PO4

Properties
(a) It is a transparent deliquescent crystalline solid. It melts at 42.3°C. It absorbs water and forms colourless syrupy mass. It is highly soluble in water.

The p Block Elements Chemistry Notes

Illustration 1. Name the three series of salts formed when orthophosphoric acid reacts with sodium hydroxide.
Solution: NaH2PO4, Na2HPO4 and Na3PO4.

Illustration 2. Which of the following is a cyclic oxo acid?
(A)H4P2O7 (B) H4P2O6
(C) H3P3O9 (D) H5P5O15

Solution:
(C)

Illustration 3. Which of the following species is basic and reducing?
(A) SO32- (B) SO42- (C) S2O42- (D) HSO42-
Solution: (A)

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