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Phosphorus pentoxide is a white crystalline solid with the formula P4O10. This inorganic chemical compound is the anhydride of phosphoric acid and has several other names such as phosphoric anhydride, phosphorous (V) oxide, diphosphorus pentoxide,tetraphosphorus decaoxide, and tetraphosphorus decoxide. The compound is highly hygroscopic in nature and hence is used as a powerful desiccant and dehydrating agent.
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Keywords: Phosphorus pentoxide, P4O10, Atoms, polymorph, desiccant, exothermic, hydrolysis, dehydrating.
What is Phosphorus Pentoxide?
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Phosphorus pentoxide is a chemical compound that comprises four phosphorus atoms and ten oxygen atoms. As it is a dimer of P4O5 it is known as phosphorus pentoxide. Its chemical name is tetraphosphorus decaoxide. It is found as a dimer of P2O5 and has a molecular formula P4O5.
Structure of Phosphorus Pentoxide
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Phosphorus pentoxide shows polymorphism and crystallizes in at least four polymorphs. The most familiar one is a metastable form that comprises molecules of P4O10. These molecules are held together by Van Der Waals force. It has a P=O bond length of 143 pm and a P-O sigma bond length of 160 pm. The O-P-O bond angle is 102° and the P-O-P bond angle is 123°. The structure of the P4O10 cage is reminiscent of adamantane with a Td symmetry group.it is closely related to the corresponding anhydride of phosphorous acid P4O6
A tetrahedron of oxygen atoms binds the phosphorus atoms, one of which forms a terminal P=O bond in which the terminal oxygen p-orbital electrons are donated to the antibonding phosphorus-oxygen single bonds.
Phosphorus pentoxide structure
Formula of Phosphorus Pentoxide
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- The molecular formula is P4O10
- The empirical formula is P2O5
Phosphorus pentoxide formula
Preparation of Phosphorus Pentoxide
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Tetra phosphorus p4 is burned in the presence of a sufficient amount of oxygen which produces phosphorus pentoxide. The chemical reaction is as follows ;
P4 + 5O2 = P4O10
This substance is a pure phosphoric acid source.
Properties of Phosphorus Pentoxide
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Stated below are the physical and chemical properties of phosphorus pentoxide:
Physical Properties of Phosphorus Pentoxide
- Phosphorus pentoxide is an odorless white powder.
- Highly deliquescent, it absorbs moisture easily.
- The chemical formula is P4O10. It is, nevertheless, named from its empirical formula, P2O5.
- Nonflammable, but when subjected to heat, it undergoes explosive decomposition.
- The molar mass of phosphorus pentoxide is 283.9 g/mol.
- The density is 2.39 g/cm3.
- This P2O5 has a melting point of 613K.
- Its boiling point is 633K.
Chemical Properties of Phosphorus Pentoxide
- It is an acidic compound and reacts with water to form phosphoric acid.
- It crystallizes in four different polymorphs or forms.
- Phosphorus pentoxide undergoes Hydrolysis, a reaction with water, releasing a high amount of heat. The reaction is exothermic.
P4O10 + 6H2O → 4H3PO4 (–177 kJ)
- In reaction with amides, it converts it to nitrile. The reaction being
P4O10 + RC(O)NH2 → P4O9(OH)2 + RCN
- Phosphorus pentoxide is highly corrosive and hence must be kept away from metals.
Application of Phosphorus Pentoxide
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- It is used as a dehydrating agent as it has the potent property of absorbing moisture.
P4O10 + 6 H2O → 4H3PO4 (–177 kJ). The reaction is highly exothermic.
- Desiccators use a granular version of P4O10
- It is also used in some organic synthesis reactions for the preparation of nitriles from primary amides.
P4O10 + RC( O)NH2 → P4O9(OH)2 + RCN
- It is used in the production of phosphorous compounds.
- The "Onodera reagent", a solution of P4O10, is employed for the oxidation of Alcohol.
- It is used in manufacturing optical glass and heat-insulating glass.
- Many compounds, especially those with which it does not react, profit from its use as a drying agent.
- Used in medicines.
- In the agricultural field, it is used as a crop fertilizer.
Things to remember
- This white crystalline powder has the formula P4O10.
- The melting point is 340 C and the boiling point is 360 C.
- It is highly soluble in water and shows an exothermic reaction.
- The compound is insoluble in acetone, acetic acid, benzene, and chloroform (Organic Solvent ).
- Major properties include explosive decomposition and exothermic hydrolysis.
- Mainly used as a desiccating and dehydrating agent.
Sample questions
Ques. How many moles are there in P2O5? (2 Marks)
Ans. P2O5 is the empirical formula for the polymorphism of P4O10.
1 Mole is equal to 1 mole P2O5, or 141.944522 grams.
1 gram of Phosphorus Pentoxide is equal to 0.007045005935488 moles.
The conversion is based on the molar mass ratio for the two compounds.
Hence, 10 moles of phosphorus pentoxide is equal to 20 moles of P atoms and 50 moles of O atoms.
Ques. Mention a few physical and chemical properties of tetraphosphorus decoxide. (3 Marks)
Ans. Tetraphosphorus dioxide is another name for phosphorus pentoxide.
The physical properties are:
- Phosphorus pentoxide is an odourless white powder that is highly deliquescent, it absorbs moisture easily.
- The chemical formula is P4O10 but is named after its empirical formula, which is P2O5.
- The molar mass of phosphorus pentoxide is 283.9 g/mol and has a density of 2.39 g/cm3.
- The melting point of this P2O5 is 613K and the boiling point is 633K.
The chemical properties are
- Phosphorus pentoxide is an acidic compound that reacts with water to form phosphoric acid.
- It crystallizes in four different polymorphs or forms.
- Phosphorus pentoxide undergoes hydrolysis in reaction with water, releasing a high amount of heat. The reaction is exothermic.
Ques. What is a chemical reaction? (2 Marks)
Ans. The process in which one or more substances are converted to one or more different substances is a chemical reaction. It is represented in the form of a chemical equation and the substances on the left side are the reactants and on the right side are the products.
Ques. Write down an equation where phosphorus pentoxide is formed. (2 Marks)
Ans. Phosphorus pentoxide is formed when tetra phosphorus p4 is burned in the presence of a sufficient amount of oxygen which produces phosphorus pentoxide. The chemical reaction is as follows ;
P4 + 5O2 = P4O10
This substance is a pure phosphoric acid source.
Ques. Is phosphorus pentoxide an acid? Is it also a phosphate? (2 Marks)
Ans. Phosphorus pentoxide is an acidic oxide because it is an anhydride of phosphoric acid. Phosphorus pentoxide is a chemical compound with the molecular formula P4O10.The anhydride of phosphoric acid is this white crystalline substance. It's used to dehydrate and desiccate things.
Ques. What are the other names of P2O5? (2 Marks)
Ans. The IUPAC name of P2O5 is Phosphorus pentoxide.The following are other names for it:
- Diphosphorus pentoxide
- Phosphorus(V) oxide
- Phosphoric anhydride
- Tetraphosphorus decaoxide
- Tetraphosphorus decoxide
Ques. Mention 4 uses of phosphorus pentoxide.(4 Marks)
Ans. The following are the uses of phosphorus pentoxide:
- The most important use would be as a strong desiccating and dehydrating agent since the compound has the property of moisture absorption.
- It is also used in some organic synthesis reactions for the preparation of nitriles from primary amide.
P4O10 + RC( O)NH2 → P4O9(OH)2 + RCN
- In the agricultural field, it is used as a fertilizer.
- Also used in medicines.
Ques. Are there any hazards associated with the use of phosphorus pentoxide? (3 Marks)
Ans. Due to its exothermic hydrolysis, it is fatal as it vigorously reacts with water and releases a large amount of heat in a short period. It reacts easily with water-containing substances such as wood, leaves, and cotton. It has Corrosion properties against metals. The compound causes itching and burning sensation when exposed to the eyes and skin. Exposure may harm mucous membrane and respiratory tract as well even at very low concentrations such as 1.5 – 2 mg/m3.
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