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Chemistry of Phosphorus

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Phosphorus and Group 5A Elements

Nitrogen and phosphorus of Group 5A (or group 15) are essential elements of life on the planet earth. Among the group 5A elements are also non metals arsenic (As) and metals antimony (Sb) and bismuth (Bi).

Phosphorus and Life

We get most elements from nature in the form of minerals. In nature, phosphorus exists in the form of phosphates. Rocks containing phosphate are fluoroapatite (3Ca3(PO4)2.CaF2), chloroapaptite, (3Ca3(PO4)2.CaCl2), and hydroxyapatite (3Ca3(PO4)2.Ca(OH)2). These minerals are very similar to the bones and teeth. The arrangements of atoms and ions of bones and teeth are similar to those of the phosphate containing rocks. In fact, when the OH- ions of the teeth are replaced by F-, the teeth resist decay. This discovery led to a series of social and economical issues.

Nitrogen, phosphorus and potassium are key ingredients for plants, and their contents are key in all forms of fertilizers.

From an industrial and economical view point, phosphorus containing compounds are important commodities. Thus, chemistry of phosphorus has academic, commercial and industrial interests.

Sources and Properties of Elementary Phosphorus

Any phosphorus rock can be used for the production of elemental phosphorus. Crushed phosphate rocks and sand (SiO2) react at 1700 K to give phosphorus oxide, P4O10. 2 Ca3(PO4)2 + 6 SiO2 ® P4O10 + 6 CaSiO3 The, P4O10 can be reduced by carbon, P4O10 + 10 C ® P4 + 10 CO. Waxy solids of white phosphorus are molecular crystals consisting of P4 molecules. They have an interesting property in that they undergo spontaneous combustion, P4 + 5 O2 ® P4O10 The structure of P4 can be understood by thinking of electronic configuration (s2 p3) of P in bond formation. Sharing three electrons with other P atoms gives rise to the 6 P-P bonds, leaving a lone pair occupying the 4th position in a distorted tetrahedron.

When burned with insufficient oxygen, P4O6 is formed,

P4 + 3 O2 ® P4O6 To each of the P-P bonds, an O atom is inserted.

Burning phosphorus with sufficient oxygen results in the formation of P4O10. An additional O atom is attached to the P directly.

P4 + 5 O2 ® P4O10

Thus, the oxides P4O6 and P4O10 share interesting features.

Phosphoric Acid

As mentioned earlier, phosphates are important minerals. Oxides of phosphorus, P4O10, dissolve in water to give phosphoric acid, P4O10 + 6 H2O ® 4 H3PO4 Phosphoric acid is a polyprotic acid, and it ionizes at three stages: H3PO4 ® H+ + H2PO4-
H2PO4- ® H+ + HPO42-
HPO42- ® H+ + PO43-

Elemental

Phosphorus has two forms, white and red phosphorus. White phosphorus consists of P4 molecules, whereas the crystal structure of red phosphorus has a complicated net work of bonding. White phosphorus has to be stored in water to prevent natural combustion, but red phosphorus is stable in air.

When burned, red phosphorus als forms the same oxides as those obtained in the burning of white hosphosrus, P4O6 when air supply is limited, and P4O10 when sufficient air is present.

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