CBSE Class 12 Chemistry Notes: The p Block Elements – Nitrogen Family (Oxyacids of phosphorus)

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.

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

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

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.

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) SO₃^2- (B) SO₄^2- (C) S₂O₄^2- (D) HSO₄^2-
Solution: (A)

Class 12 Chemistry “The p Block Elements” All Topic Notes Class 12 Chemistry All Chapters Notes