Phosphorous pentoxide is the common name for the compound with a chemical formula P4O10. Phosphorus pentoxide is a covalent compound that is composed of 4 phosphorus (P) atoms and 10 oxygen (O) atoms. It sometimes referred to as diphosphorus pentoxide, phosphoric anhydride, and tetra phosphorus decoxide.
“Without phosphorous, there would be no thoughts.” — Ludwig Buchner
Phosphorous pentoxide is a solid, white, waxy substance at room temperature and comes in 4 distinct crystalline structures. It is an anhydride of phosphoric acid and is very hygroscopic, meaning that it readily absorbs water from the surrounding atmosphere. As such, phosphorous pentoxide is often used as a desiccant to keep places dry and free from airborne moisture.
While being stored, phosphorous pentoxide will react with the atmosphere to form a skin of phosphoric acid around the compound. This layer of acid can prevent phosphorous pentoxide from drawing water out of the air, which can make it less effective as a desiccant. To prevent this, phosphorous pentoxide is usually processed into a granular form for desiccant use. Historically, phosphorus pentoxide was created via a reaction with white phosphorus and oxygen, but other more efficient methods of production have taken precedence.
Molecular Vs. Empirical Formula
An astute reader may notice something strange. What is the compound with the chemical formula P4O10 named phosphorous pentoxide? After all, pent- is the prefix of chemical nomenclature meaning “five”, and the formula P4O10 clearly indicates that there are 10 oxygens. What gives?
In chemistry, there are 2 kinds of chemical formulas, the molecular formula and empirical formula. Both represent the atomic constituents of a given compound but in different ways. The molecular formula tells you the kind and number of atoms in a free-standing single molecule of the compound. The empirical formula tells you the simplest integer ratio of elements in a compound. 2 compounds could have same different molecular formulas and the same empirical formula, such as acetylene (C2H2) and benzene (C6H6), both of which share the empirical formula of CH. Likewise, the empirical formula for both ethylene (C2H4) and butene (C4H8) is CH2. A compound’s molecular formula is either equal to or is a whole number multiple of its empirical formula.
Sometimes, the common name for a compound is derived from its empirical formula instead of its molecular formula. Such is the case with phosphorus dioxide. Phosphorus pentoxide has a molecular formula of P4O10 and so has an empirical formula of P2O5. The “pent-” in “pentoxide” comes from the empirical formula P2O5.
In the case of phosphorus pentoxide, molecules with a formula of P2O5 will associate with each other to form larger molecules of P4O10. So even though the molecular formula of phosphorus pentoxide is P4O10, it is still called phosphorus pentoxide on account of its empirical formula P2O5.
Phosphorus Pentoxide: Physical Properties
“Who knows whether it is not true that phosphorous and mind are not the same thing?” — Stendhal
Phosphorous pentoxide is unique in that it can exist in up to 4 distinct polymorphs. The most common form is a single molecule of P4O10, which forms from the cohesion of two smaller P2O5 molecules. P2O5 has a very unstable molecular configuration, so two molecules will join up to for a larger single molecule of P4O10, and arrange themselves according to the following diagram:
This particular configuration is structured as 4 tetrahedra, each sharing a leg with another. Each tetrahedron is composed of a central phosphorus atom surrounded by 4 oxygen atoms, where the three base oxygen atoms of each tetrahedron are shared. A single molecule of phosphorus pentoxide looks a little like a small hexagonal cell with terminal oxygen atoms protruding from the sides. The particular configuration of the molecules makes phosphorus pentoxide less dense than most crystalline solids, with a density of only 2.3 g/cm3. The geometric structure of phosphorus pentoxide is similar to the hydrocarbon crystal adamantane and it has a relatively high melting point for covalently bonded compounds at 340 °C. The boiling point of phosphorus pentoxide is only 20 °C higher than its melting point, so it will often skip melting and sublime directly into a gas.
The hexagonal cell of a molecule of phosphorus pentoxide is held together by weak van der Waals forces—the electrostatic attraction between molecules. Phosphorus pentoxide contains 6 P–O–P bonds and 4 P=O bonds. The dipole-dipole interactions of the P–O–P bonds are what keep the molecule together. P–O–P bonds are polar with a negative valence on the oxygen atom.
All the polymorphs of phosphorus pentoxide are based around tetrahedral arrangements of phosphorus and oxygen atoms. They generally are formed with P=O double bonds such as the o’-(P2O5) form shown below.
Many of the polymorphs have a slightly different molecular arrangement than regular phosphorus pentoxide. For instance, the stable “O” form consists of cyclical arrangements of P6O6 rings, similar to the structure of various silicate minerals. One of phosphorus pentoxide’s polymorphs is an amorphous glass created by fusing together any two different polymorphs.
Phosphorus Pentoxide: Chemical Properties
Phosphorus pentoxide is a polar compound. The electronegativity difference between oxygen and phosphorus is 1.4, making P-O bonds rather polar in nature. Even though phosphorus pentoxide is polar, it will not be dissolved by water because it instead undergoes exothermic hydrolysis. Phosphorus pentoxide is an anhydride, meaning that it is formed by removing water (H2O) from a compound. Phosphorus pentoxide is the corresponding anhydride of phosphoric acid (H3PO4) and will react violently with water to form phosphoric, according to the equation:
P4O10 + 6H2O → 4H3PO4
The enthalpy of change of this reaction is -177 kJ/mol, meaning that for every 1 mole of P4O10, 177 kJ of energy are released in the form of heat. This reaction with water is one of the main methods for producing industrial amounts of phosphoric acid, an extremely important ingredient in fertilizers.
Phosphorus pentoxide is non-combustible and will not react with oxygen to produce a flame. However, the hydrolysis reaction of phosphorus pentoxide with water and water-containing substances like wood is very exothermic and can release enough energy to catalyze a combustion reaction between the water-containing material and the atmosphere. Phosphorus pentoxide is very corrosive to metal and will form various metal oxides and phosphate metals when brought into contact with metals. It is also very corrosive to human tissue and can cause chemical burns and respiratory inflammation, even at low concentrations.
Phosphorus Pentoxide: Production and Use
Historically the primary method of forming phosphorus pentoxide is through the combustion of elemental phosphorus and oxygen. White phosphorus, one of the allotropes of elemental phosphorus, is made of molecules composed of 4 phosphorus atoms arranged in a tetrahedral structure. Elemental tetra phosphorus burns in oxygen to form phosphorus pentoxide according to the following reaction:
P4 + 5O2 → P4O10
Most phosphorus pentoxide produced in this manner is for the purpose of making phosphoric acid, though recent methods have removed the need to start with white phosphorus to make phosphoric acid.
The main use of phosphorus pentoxide is as a desiccant. Because it reacts readily with water, phosphorus pentoxide can draw traces of water out of the atmosphere to keep a space dry and moisture-free. The hydrolysis of water with phosphorus pentoxide creates a gummy layer of phosphoric acid that can inhibit its water-removing properties. This is why most phosphorus pentoxide used for industrial purposes is made in a granular form. It is not possible to form phosphorus pentoxide via the dehydration of phosphoric acid, as the heat required to catalyze the reaction is enough to boil off any excess water.
The desiccant properties of phosphorus pentoxide are often used to convert a number of acids into their corresponding anhydrides. For instance, phosphorus pentoxide will convert nitric acid (HNO3) into its anhydride dinitrogen pentoxide (N2O5). It will also convert sulfuric acid (H2SO4) into sulfur trioxide (SO3) by removing one oxygen and two hydrogens; a single molecule of water, and it will convert hydrogen perchlorate (HClO4) into dichlorine heptoxide (Cl2O7).
“We define organic chemistry as the chemistry of carbon compounds.” — August Kekule
The majority of phosphorus pentoxide that is not used as a desiccant is used as an intermediary reactant to create other compounds. In organic chemistry, phosphorus pentoxide is used to dehydrate organic compounds, like converting amides into nitriles, an important class of organic molecules used in rubber manufacturing and lab procedures.
In summary, phosphorus pentoxide is an anhydride covalent compound that is formed via the combustion of elemental phosphorus and oxygen. Phosphorus pentoxide is highly hygroscopic, so it will draw water out of the nearby environment and react to form phosphoric acid. Phosphorus pentoxide is normally used as an industrial desiccant and plays a role as an intermediate reactant to turn acid into their anhydride counterparts. Even though a single molecule of the compound has a molecular formula of P4O10, it is still called phosphorus pentoxide on account of its empirical formula P2O5. Phosphorus pentoxide is unique in that it exists in several different polymorphs that have different molecular geometries. The most common form is a hexagonal cell composed of 4 distinct phosphorus tetrahedra. Phosphorus pentoxide is corrosive to metals and can damage human tissue even in small concentrations.
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