What Is Phosphorus Pentoxide Definition Formula Reactions and Uses
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
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
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FAQs on Phosphorus Pentoxide Structure Properties and Chemical Behavior
1. What is phosphorus pentoxide?
Phosphorus pentoxide is a white, highly hygroscopic solid with the molecular formula P4O10 that acts as a powerful dehydrating agent. It is the oxide of phosphorus formed in excess oxygen and is commonly referred to by its empirical formula P2O5.
- It is a covalent compound.
- It reacts vigorously with water.
- It is classified as an acidic oxide.
2. What is the chemical formula of phosphorus pentoxide?
The correct molecular formula of phosphorus pentoxide is P4O10, although it is often written as P2O5 (empirical formula).
- Molecular formula: P4O10
- Empirical formula: P2O5
- The name “pentoxide” refers to five oxygen atoms per two phosphorus atoms in the empirical ratio.
3. How is phosphorus pentoxide prepared?
Phosphorus pentoxide is prepared by burning white phosphorus in excess oxygen. The balanced chemical equation is:
P4(s) + 5O2(g) → P4O10(s)
- This is a combination reaction.
- It is highly exothermic and produces dense white fumes of P4O10.
4. Why is phosphorus pentoxide called a dehydrating agent?
Phosphorus pentoxide is called a strong dehydrating agent because it readily removes water molecules from compounds. It reacts vigorously with water to form phosphoric acid:
P4O10(s) + 6H2O(l) → 4H3PO4(aq)
- It absorbs moisture from air (hygroscopic).
- It is used to dry gases and organic solvents.
5. What happens when phosphorus pentoxide reacts with water?
When phosphorus pentoxide reacts with water, it forms phosphoric acid or metaphosphoric acid depending on the amount of water.
- With excess water:
P4O10(s) + 6H2O(l) → 4H3PO4(aq) - With limited water:
P4O10(s) + 2H2O(l) → 4HPO3(s)
6. What is the molar mass of phosphorus pentoxide?
The molar mass of phosphorus pentoxide (P4O10) is 283.88 g/mol.
- Phosphorus: 4 × 30.97 = 123.88 g/mol
- Oxygen: 10 × 16.00 = 160.00 g/mol
- Total = 283.88 g/mol
7. Is phosphorus pentoxide acidic or basic?
Phosphorus pentoxide is an acidic oxide because it forms acids when it reacts with water.
- It reacts with water to form phosphoric acid (H3PO4).
- It reacts with bases to form phosphate salts.
- It is a non-metal oxide, which typically shows acidic character.
8. What is the structure of phosphorus pentoxide?
Phosphorus pentoxide (P4O10) has a cage-like molecular structure with four phosphorus atoms each bonded tetrahedrally to oxygen atoms.
- Each phosphorus atom forms three P–O–P bridges.
- Each phosphorus also has one terminal P=O bond.
- The structure is covalent and discrete in the solid state.
9. What are the uses of phosphorus pentoxide?
Phosphorus pentoxide is mainly used as a powerful drying and dehydrating agent in laboratories and industry.
- Drying gases and organic solvents.
- Preparation of phosphoric acid.
- Dehydration reactions in organic chemistry.
- Manufacture of phosphate compounds.
10. What is the difference between P2O5 and P4O10?
The difference is that P2O5 is the empirical formula, while P4O10 is the actual molecular formula of phosphorus pentoxide.
- P2O5 shows the simplest whole-number ratio (2:5).
- P4O10 represents the true number of atoms in one molecule.
- Both names refer to the same compound in practice.
