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Additional recommended knowledge
P2O5 crystallizes in at least four forms or polymorphs. The most familiar one shown in the figure, comprises molecules with the formulae P4O10. Weak van der Waals forces hold these molecules together in a hexagonal lattice (However, in spite of the high symmetry of the molecules, the crystal packing is not a close packing). The structure of the P4O10 cage is reminiscent of adamantane with Td symmetry point group. It is closely related to the corresponding anhydride of phosphorous acid, P4O6. The latter lacks terminal oxo groups. Its density of 2.30 g/cm³. It can be boiled at 423 °C, if the sample is heated more rapidly than it can sublime.
The other polymorphs are polymeric, but in each case the phosphorus atoms are bound by a tetrahedron of oxygen atoms, one of which forms a terminal P=O bond. The O-form (density 3.05 g/cm³, m.p. 580 °C), adopts a layered structure consisting of interconnected P6O6 rings, not unlike the structure adopted by certain polysilicates. A lower density phase, the so-called O' form, consists of a 3-dimensional framework is also known, density 2.72 g/cm³.
P2O5 is prepared by burning elemental phosphorus with sufficient supply of air :
For most of the 20th century, phosphorus pentoxide was used to provide a supply of concentrated pure phosphoric acid. In the thermal process, the phosphorus pentoxide obtained by burning white phosphorus was dissolved in dilute phosphoric acid to produce concentrated acid. Improvements in filter technology is leading to the "wet phosphoric acid process" taking over from the thermal process, obviating the need to produce white phosphorus as a starting material.
Phosphorus pentoxide is a potent dehydrating agent as indicated by the exothermic nature of its hydrolysis:
However, its utility for drying is limited somewhat by its tendency to form a protective viscous coating that inhibits further dehydration by unspent material. A granular form of P4O10 used in desiccators.
The indicated coproduct P4O9(OH)2 is an idealized formula for undefined products resulting from the hydration of P4O10.
Apparently, when combined with a carboxylic acid, the result is the corresponding anhydride (however, what is noticeable is that no references have been provided to substantiate this apparent transformation - although it might happen at around 400C when used in conjunction with Aluminium Oxide):
Related phosphorus oxides
Between the commercially important P4O6 and P4O10, phosphorus oxides are known with intermediate structures.
In Anthony Burgess' The Wanting Seed, phosphorus pentoxide is a highly prized compound.
In Detective Comics #825, Batman notices that phosphorus pentoxide was at the scene of a fire, indicating that the villain Dr. Phosphorus was involved.
In Aldous Huxley's Point Counter Point, Lord Edward bemoans societal loss of phosphorous pentoxide to his assistant Illidge.
In Aldous Huxley's Brave New World, Henry Foster tells Lenina about the recovery of phosphorus pentoxide.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Phosphorus_pentoxide". A list of authors is available in Wikipedia.|