Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

Oxy-Acids of Phosphorus: Preparation, properties, Structure

1. Phosphorus acid : It is prepared by the action of cold water on phosphorus (III) oxide or phosphorus (III) chloride. II. Ortho phosphoric Acid, Preparation - It is prepared by dissolving phosphorus pentoxide in water and boiling the solution. III. B. Pyrophosphoric acid : Preparation: Pyrophosphoric acid is prepared by heating orthophosphoric

Oxy-Acids of Phosphorus

I. Phosphorus acid - H3PO3

It is prepared by the action of cold water on phosphorus (III) oxide or phosphorus (III) chloride.

P2O3 + 3H2O � 2H3PO3

PCl3 + 3H2O � H3PO3 + 3HCl

Physical properties

It is a white crystalline solid with garlic taste.

Chemical Properties

1. Acidic nature: It is a dibasic acid and gives salts of two types.

H3PO3 + NaOH � NaH2PO3 + H2O

Sodium dihydrogen Phosphite

H3PO3 + 2NaOH � Na2HPO3 + 2H2O

Disodium hydrogen Phosphite

2. When it is heated it undergoes auto-oxidation and reduction to form

phosphoric acid and phosphine.

4H3 PO3 � 3H3PO4 + PH3

3. It is a powerful reducing agent because it has P-H bond. It reduces silver

nitrate solution into silver.

2AgNO3 + H3PO3 + H2O � 2Ag +H3PO4+2HNO3

Electronic structure

Use: It is used as a reducing agent

II. Ortho phosphoric Acid, H3PO4

Preparation

1. It is prepared by dissolving phosphorus pentoxide in water and boiling the solution.

P2O5 + 3H2O � 2H3PO4

2. Laboratory preparation: In the laboratory orthophosphoric acid can be

prepared by boiling a mixture of red phosphorus with 50% nitric acid in a flask fitted with a reflux condenser on a water bath till no more oxides of nitrogen are liberated.

Iodine acts as a catalyst. The product is evaporated below 453 K and then

cooled in a vaccum desiccator surrounded by freezing mixture when crystals of orthophosphoric acid are deposited.

P+5HNO3 � H3PO4 +5NO2 +H2O

Physical properties

1. It is a deliquescent crystalline solid.

2. It is soluble in water.

Chemical properties

1. It is a tribasic acid. It combines with alkalies like NaOH to form three series

of salts.

H3PO4 +NaOH � NaH2PO4 + H2O

Sodium Di hydrogen Phosphate

H3PO4 +2NaOH � Na2HPO4 + 2H2O

Disodium hydrogen Phosphate

H3PO4 + 3NaOH � Na3PO4 + 3H2O

Sodium Phosphate

2. On heating it gives pyrophosphoric acid at 523 K and at 589 K gives

metaphosphoric acid

523K 589K

H3PO4 H4 P2O7 2HPO3 + H2O

3. On reaction with silver nitrate, it gives yellow precipitate of silver phosphate.

H3PO4 + 3AgNO3 � Ag3PO4+3HNO3

Uses

1. It is used in the preparation of HBr and HI as a substitute for sulphuric acid.

2. It is used as souring agent in the preparation of soft drinks.

3. It is used in the preparation of phosphate salts of sodium, potassium and ammonium.

4. It is used in the manufacture of phosphatic fertilisers.

Structure

Being a tribasic acid, the structure of phosphoric acid is represented as

III. B. Pyrophosphoric acid, H4 P2 O7

Preparation: Pyrophosphoric acid is prepared by heating orthophosphoric

acid to 523 K - 533 K.

2H3PO4 � H4P2O7 + H2O

Physical Properties

It is a colourless crystalline solid.

Chemical Properties

1. It is reconverted to orthophosphoric acid on boiling with water

H4P2O7 + H2O � 2H3PO4

2. When heated strongly, it yields metaphosphoric acid

H4P2O7 2HPO3 + H2O

Structure: The Structure of pyrophosphoric acid is represented as:

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11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes : Oxy-Acids of Phosphorus: Preparation, properties, Structure |