Nitrogen Family : Compounds of phosphorus

Compounds of phosphorus

(1)     Oxides and oxyacids of phosphorus : Phosphorus is quite reactive and forms number of compounds in oxidation states of –3 , +3 and +5.

(i)      Oxides : Phosphorus forms two common oxides namely, (a) phosphorus trioxide (P4O6) and (b) phosphorus penta oxide (P4O10).

(a)     Phosphorus (III) oxide (P4O6) : It is formed when P is burnt in a limited supply of air, { P }_{ 4 }\quad +\quad \underset { limited }{ 3{ O }_{ 3 } } \rightarrow \quad { P }_{ 4 }{ O }_{ 6 }

It is a crystalline solid with garlic odour.  It dissolves in cold water to give phosphorous acid,

\quad { P }_{ 4 }{ O }_{ 6 }\quad +\quad \underset { cold }{ 6{ H }_{ 2 }O } \quad \rightarrow \quad \underset { Phosphoric\quad acid }{ 4{ H }_{ 3 }PO_{ 3 } }

It is therefore, considered as anhydride of phosphorus acid.

With hot water, it gives phosphoric acid and inflammable phosphine,

\quad { P }_{ 4 }{ O }_{ 6 }\quad +\quad \underset { }{ 6{ H }_{ 2 }O } \quad (hot)\quad \rightarrow \quad \underset { Phosphoric\quad acid }{ 3{ H }_{ 3 }PO_{ 4 }\quad +\quad P{ H }_{ 3 } }

It reacts vigorously with to form a mixture of phosphoryl chloride and meta phosphoryl chloride.

\quad { P }_{ 4 }{ O }_{ 6 }\quad +\quad \underset { cold }{ { 4Cl }_{ 2 } } \quad \rightarrow \quad \underset { Phosphoryl\quad chloride }{ 2POCl_{ 3 } } +\quad \underset { Metaphosphoryl\quad chloride }{ 2POCl_{ 3 } }

(b)     Phosphorus (V) oxide (P4O10): It is prepared by heating white phosphorus in excess of air, P4 + 5O2 (excess) → P4O10. It is snowy white solid. It readily dissolves in cold water forming metaphosphoric acid.

\quad { P }_{ 4 }{ O }_{ 10 }\quad +\quad \underset { cold }{ { 2H }_{ 2 }O } \quad \rightarrow \quad \underset { Metaphosphoric\quad acid }{ 4HPO_{ 3 } }  

With hot water, it gives phosphoric acid,  \quad { P }_{ 4 }{ O }_{ 10 }\quad +\quad \underset { Heat }{ { 6H }_{ 2 }O } \quad \rightarrow \quad \underset { Phosphoric\quad acid }{ 4H_{ 3 }PO_{ 4 } } .

P4O10 is a very strong dehydrating agent. It extracts water from many compounds including H2SO4 and HNO3,

H_{ 2 }SO_{ 4 }\quad \xrightarrow [ -H_{ 2 }O ]{ \quad P_{ 4 }O_{ 10 }\quad } \quad SO_{ 3 }

2HNO_{ 3 }\quad \xrightarrow [ -H_{ 2 }O\quad ]{ P_{ 4 }O_{ 10 } } \quad N_{ 2 }O_{ 5 }

\underset { Acetamide }{ CH_{ 3 }CONH_{ 2 } } \quad \xrightarrow [ -H_{ 2 }O\quad ]{ P_{ 4 }O_{ 10 } } \quad \underset { Methylcyanide }{ CH_{ 3 }CN }

(ii)     Oxyacids of phosphorus : Phosphorus forms a number of oxyacids which differs in their structure and oxidation state of phosphorus.

 

Oxyacids of phosphorus

Name

 Formula Oxidation state of P Basicity Structure
Hypophosphorous acid H3PO2 +1 Monobasic
Phosphorous acid H3PO3 +3 Dibasic  
Hypophosphoric acid H4P2O6 +4 Tetrabasic
Orthophosphoric acid H3PO4 +5 Tribasic  
Metaphosphoric acid (HPO3)n +5 Monobasic  
Pyrophospric acid

(Diphosphoric acid).

 H4P2O7 +5 Tetrabasic