Phosphorus Oxygen Compounds

Phosphorus forms two common oxides, phosphorus(III) oxide (or tetraphosphorus hexaoxide), P₄O₆, and phosphorus(V) oxide (or tetraphosphorus decaoxide), P₄O₁₀, both shown in the figure below. Phosphorus(III) oxide is a white crystalline solid with a garlic-like odor. Its vapor is very poisonous. It oxidizes slowly in air and inflames when heated to 70 °C, forming P₄O₁₀. Phosphorus(III) oxide dissolves slowly in cold water to form phosphorous acid, H₃PO₃.

Two ball-and-stick models are shown. In the left model, three orange atoms labeled, “P,” are single bonded to red atoms labeled, “O,” in an alternating, six-sided ring structure. Each of the orange atoms are also single bonded to another red atom, which are in turn single bonded to a single orange atom. The right model shows three orange atoms labeled, “P,” single bonded to red atoms labeled, “O,” in an alternating, six-sided ring structure. Each of the orange atoms are also single bonded to two more red atoms, one in an upward position and one facing the outside of the molecule. The upward red atoms are single bonded to a single orange atom which is single bonded to a final red atom.

This image shows the molecular structures of P₄O₆ (left) and P₄O₁₀ (right).

Phosphorus(V) oxide, P₄O₁₀, is a white powder that is prepared by burning phosphorus in excess oxygen. Its enthalpy of formation is very high (−2984 kJ), and it is quite stable and a very poor oxidizing agent. Dropping P₄O₁₀ into water produces a hissing sound, heat, and orthophosphoric acid:

\({\text{P}}_{4}{\text{O}}_{10}(s)+{\text{6H}}_{2}\text{O}(l)\;⟶\;{\text{4H}}_{3}{\text{PO}}_{4}(aq)\)

Because of its great affinity for water, phosphorus(V) oxide is an excellent drying agent for gases and solvents, and for removing water from many compounds.

This lesson is part of:

Metals, Metalloids, and Nonmetals

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