Phosphorous Pentoxide

Chemistry

Phosphorous Pentoxide

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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	Image will be uploaded soon

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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