Phosphorous Pentoxide

What is Phosphorous Pentoxide?

Phosphorous pentoxide is an inorganic chemical compound, composed of four phosphorus atoms and ten oxygen atoms. It is generally found as a dimer of P2O5 that’s why it is known as phosphorous pentoxide. Its chemical name is tetraphosphorous  decaoxide. It has several other names such as phosphoric anhydride, phosphorous (V) oxide, diphosphorus pentoxide etc. 

It is a white crystalline solid which is used as a powerful dehydrating agent. It is a strong desiccant. It is an anhydride of phosphoric acid. 


Its molecular formula is P4O10 but it is found as a dimer of P2O5

Formula of Phosphorous Pentoxide 

S. No.

Formula of Phosphorous Pentoxide

1.

Empirical Formula 

P2O5

2.

Molecular Formula 

P4O10

3.

Structural Formula 

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Structure of Phosphorous Pentoxide 

It shows polymorphism. It means it is found in more than one crystalline form. Its metastable form is the most common form. Its molecules are bonded together by weak van der Waals forces. Its P-O sigma bond length is 160 pm while its P=O bond length is 143 pm. O-P-O bond angle is 102° and P-O-P bond angle is 123°. Its structure is illustrated below –

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Preparation of Phosphorous Pentoxide 

It is prepared by oxidation of tetraphosphorous. P4 is burned in presence of oxygen and it gives P4O10. Reaction is given below –

P4 + 5O2 → P4O10

It is used as a source to provide pure concentrated phosphoric acid. 

Properties of Phosphorous Pentoxide 

Properties of phosphorous pentoxide are listed below –

  • It is a white crystalline solid.

  • Its molar mass is 283.9 g/mol.

  • It can absorb moisture easily. It is a very deliquescent compound. 

  • It is an odorless compound.

  • Its density is 2.39 g.cm-3.

  • Its melting point is 340 .

  • Its boiling point is 360 .

  • It may undergo explosive decomposition when subjected to heat.

  • It is not flammable. 

  • It is an acidic compound as it reacts with water and forms phosphoric acid. 

  • Its short or long exposure may cause residual injuries. 

  • When it undergoes hydrolysis, a large amount of heat is released. Thus, it shows exothermic hydrolysis. 

P4O10 + 6H2O → 4H3PO4 (–177 kJ)

  • It reacts with primary amide and converts it into nitrile. General reaction is given below – P4O10 + RC(O)NH2 → P4O9(OH)2 + RCN

  • It reacts with carboxylic acid and forms corresponding anhydride. General equation of its reaction with carboxylic acid is given below –

P4O10 + RCOOH → P4O9(OH)2 + [RC(O)]2O

  • It can easily corrode metals, so must be kept away from them. 

Applications of Phosphorous Pentoxide 

Due to its unique properties, it is used in many fields. few of its applications are listed below –

  • It is used as desiccant. It is an excellent absorber of moisture. In its granular form, it is used in desiccators. 

  • It is used as a potent dehydrating agent. but due to its tendency to form a viscous coating during the reaction, makes its use limited. 

  • It is used in synthesis of organic compounds. 

  • Its solution in dimethyl sulfoxide is used for oxidation of alcohols. 

  • Its desiccating powder can convert many mineral acids into their anhydrides. 

  • It is used in production of phosphorous compounds. 

  • It is used in purification of sugar.

  • It is used in manufacturing of optical glass and heat insulating glass. 

  • It acts as a drying agent for many substances especially for those with which it does not react. 

  • It is used in medicines. 

  • It is used in pesticides as well. 

Hazards From Phosphorous Pentoxide 

It is dangerous due to its exothermic hydrolysis. It vigorously reacts with water and releases a large amount of heat in a short time span. It reacts with water containing substances such as wood, leaves and cotton etc. in the same way. It is harmful due to corrosive nature against metals. It may cause itching and burning sensation when exposed to eyes and skin. Its exposure may harm mucous membrane and respiratory tract as well even at very low concentration such as 1.5 – 2 mg.m-3.

Phosphorous pentoxide: Summary in Tabular Form 

Phosphorous Pentoxide 

Chemical formula 

P4O10

IUPAC Name 

Tetraphosphorous decaoxide 

Molar mass 

283.9 g/mol

Melting point 

340

Boiling point 

360

Density 

2.39 g.cm-3

Appearance 

White crystalline powder 

Crystal Structure 

Hexagonal 

State at STP

Solid 

Solubility in water 

Soluble. Shows exothermic hydrolysis. 

Solubility in organic solvents 

Insoluble in acetic acid, acetone, benzene and chloroform. 

Solubility in acids and alkalis 

Soluble in sulfuric acid and reacts with some alkalis

Preparation 

By burning tetraphosphorous in air or presence of oxygen –

P4 + 5O2 → P4O10

Main properties 

Explosive decomposition, exothermic hydrolysis 

Main use 

As a desiccant and dehydrating agent. 

Disadvantage 

Exothermic hydrolysis of it, at large scale, may cause many hazards.

It is corrosive to metals. 


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