Oxides of phosphorus

(a) Phosphorus trioxide (P2O3) :

It is dimeric and has formula P4 O6

Preparation :

It is prepared by burning phosphorus in a limited supply of oxygen.

P4 + 3O(limited supply of oxygen)  P4O6

Properties :

(i) It is colourless crystalline solid having melting point 23.8oC and boiling point 178oC.

(ii) It dissolves in cold water to form phosphorus acid and in hot water liberating PH3.

 P4O6 + 6H2O (cold) ® 4H3PO3 

P4O6 + 6H2O (hot) ® 3H3PO4 + PH3

(iii) It burns in Cl2 gas forming phosphorus oxytrichloride (POCl3) and phosphoryl chloride (PO2Cl)

P4O6 + 4Cl2 ® 2POCl3 + 2PO2Cl

(b) Phosphorus pentaoxide (P2O5) :

It is dimeric and has the formula P4O10.

Preparation :

It is obtained by burning phosphorus in excess air.

P+ 5O2 ® P4O10

Properties:

(i) It is a white powder ,acidic in nature and is the anhydride of orthophosphoric acid.

(ii)  It sublimes on heating at 250oC.

(iii)  It dissolves in water with hissing sound forming metaphosphoric acid and finally orthophosphoric acid.

P4O10 + 2H2® 4HPO3

4HPO3 + 2H2® 2H4P2O7

2H4P2O7 + 2H2® 4H3PO4

(iv) It dehydrates concentrated H2SO4 and concentrated HNO3 to SO3 and N2O5 respectively.

4HNO3 + P4O10 https://www.edumple.com/media/Images/CkEditor/%20Tarun%20Jaiswal%20__2379/532295.png4HPO3 + 2N2O5

2H2SO4 + P4O10 https://www.edumple.com/media/Images/CkEditor/%20Tarun%20Jaiswal%20__2379/559504.png 4HPO3 + 2SO3

Uses:

1. For drying acidic gases.  

2. As a dehydrating agent

3. For the preparation of SO3 and N2O5.

4. For the preparation of phosphoric acid.

Oxoacids of Phosphorus :

Phosphorus forms a number of a oxoacids as given in following Table :

https://www.edumple.com/media/Images/CkEditor/%20Tarun%20Jaiswal%20__2379/832106.png

The structures of some of oxo-acids are as given below :

 

https://www.edumple.com/media/Images/CkEditor/%20Tarun%20Jaiswal%20__2379/153808.png

In oxoacids phosphorus is tetrahedrally surrounded by other atoms. All these acids contain one P=O and at least one P–OH bond. The oxoacids in which phosphorus has lower oxidation state (less than +5) contain in addition to P=O and P–OH bonds ,either P–P (e.g., in H4P2O6) or P–H (e.g., in H3PO2) bonds but not both. These acids in + 3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. For example, orthophosphorus acid (or phosphorus acid) on heating disproportionates to give orthophosphoric acid (or phosphoric acid) and phosphine.

 4 H3PO3  ® 3 H3PO4  + PH3

The acids which contanin P – H bond have strong reducing properties. Thus , hypophorous acid is a good reducing agent as it contains two P – H bonds and reduces , for example, AgNO3 to metallic silver.

4 AgNO3 + 2 H2O + H3PO2 ®4 Ag + 4 HNO3 + H3PO4 

These P–H bonds are not ionisable to give H+ and do not play any role in basicity. Only those H atoms which are attached with oxygen in P–OH form are ionisable and cause the basicity. Thus , H3PO3 and H3PO4 are dibasic and tribasic, respectively as the structure of H3PO3has two P – OH bonds and H3PO4 three.