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
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 Phosphorous Pentoxide
1. What exactly is phosphorus pentoxide?
Phosphorus pentoxide is a chemical compound and an oxide of phosphorus. While its simplest or empirical formula is P₂O₅, it actually exists as a larger molecule with the formula P₄O₁₀. It is a white, crystalline solid that is best known as the chemical anhydride of phosphoric acid.
2. Why is the molecular formula of phosphorus pentoxide written as P₄O₁₀ and not just P₂O₅?
This is a common point of confusion. P₂O₅ is the compound's empirical formula, which shows the simplest whole-number ratio of atoms. However, experimental analysis shows that the actual, stable molecule is a dimer of this unit. The molecular formula, P₄O₁₀, accurately represents this structure, which consists of four phosphorus atoms and ten oxygen atoms bonded together in a cage-like shape.
3. What are the most common uses of phosphorus pentoxide?
Due to its strong affinity for water, phosphorus pentoxide has several key applications. Its main uses include:
- As a powerful dehydrating and drying agent for gases and liquids in laboratory and industrial settings.
- In the production of pure phosphoric acid through a controlled reaction with water.
- As a catalyst or reagent in various organic synthesis reactions, particularly in condensation reactions.
4. Can you describe the structure of the phosphorus pentoxide (P₄O₁₀) molecule?
The P₄O₁₀ molecule has a complex, cage-like structure based on the adamantane framework. It is formed by four phosphorus atoms located at the corners of a tetrahedron. Six oxygen atoms are inserted into the P-P bonds, forming P-O-P bridges, and the remaining four oxygen atoms are bonded to each phosphorus atom as terminal P=O double bonds.
5. What happens when you add phosphorus pentoxide to water?
When phosphorus pentoxide is added to water, it undergoes a vigorous and highly exothermic reaction. It readily dissolves and reacts with the water to form phosphoric acid (H₃PO₄). The reaction is so strong that it produces a hissing sound and releases a significant amount of heat. The balanced chemical equation for this reaction is: P₄O₁₀ + 6H₂O → 4H₃PO₄.
6. Why is phosphorus pentoxide considered such a powerful dehydrating agent?
Phosphorus pentoxide is a powerful dehydrating agent because its reaction with water (hydration) is extremely favourable from a thermodynamic standpoint. The formation of the P-O-H bonds in phosphoric acid is a very stable arrangement, releasing a large amount of energy. This strong drive to form phosphoric acid allows P₄O₁₀ to aggressively remove water molecules from other substances, even from strong acids like nitric acid and sulfuric acid.
7. What is the typical method for preparing phosphorus pentoxide?
The most common way to prepare phosphorus pentoxide is by burning elemental phosphorus, usually white phosphorus (P₄), in a plentiful supply of dry air or pure oxygen. The reaction is a combustion process that needs excess oxygen to ensure complete oxidation to the pentoxide form. The chemical equation for this preparation is: P₄ (s) + 5O₂ (g) → P₄O₁₀ (s).
8. What is the key difference between phosphorus pentoxide (P₄O₁₀) and phosphorus trioxide (P₄O₆)?
The main difference lies in the oxidation state of the phosphorus atom and their reactivity with water.
- In phosphorus pentoxide (P₄O₁₀), phosphorus is in its highest oxidation state of +5. It reacts with water to form phosphoric acid (H₃PO₄).
- In phosphorus trioxide (P₄O₆), phosphorus is in the +3 oxidation state. It reacts with cold water to form phosphorous acid (H₃PO₃).
9. What is the oxidation state of phosphorus in P₄O₁₀?
The oxidation state of phosphorus in P₄O₁₀ is +5. We can calculate this easily: oxygen almost always has an oxidation state of -2. If we let 'x' be the oxidation state of phosphorus, the sum of oxidation states in the neutral molecule must be zero. So, (4 * x) + (10 * -2) = 0. This simplifies to 4x - 20 = 0, which means 4x = 20, and therefore x = +5.
