Chemistry of Phosphorus
Skills to develop
- Compare properties of Group 5A elements.
- Explain the major application of phosphate
- Describe the equilibria of the ionization of phosphoric acid
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
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
+ 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
P4 + 5 O2 ®
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
When burned with insufficient oxygen, P4O6 is formed,
P4 + 3 O2 ®
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
P4 + 5 O2 ®
Thus, the oxides P4O6 and P4O10
share interesting features.
As mentioned earlier, phosphates are important minerals.
Oxides of phosphorus, P4O10, dissolve in water to give
P4O10 + 6 H2O ®
Phosphoric acid is a
polyprotic acid, and it ionizes at three stages:
H+ + H2PO4-
H+ + HPO42-
H+ + PO43-
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.
Confidence Building Questions
About 85% of the total industrial output of phosphoric acid is used
a. in the detergent industry
b. to produce buffer solutions
c. in the paint industry
d. to produce superphosphate fertilizers
e. in the manufacture of plastics
The middle number, (for example, 6-5-8) specifies a the percentage
of phosphorus compound in a fertilizer. Phosphorus is an important element
for plant life.
What is the product when phosphorus pentoxide P4O10
reacts with water? Give the formula of the product.
P4O10 + 6 H2O ®
What is the phosphorus-containing product when PCl3 reacts with
water? Give the formula.
PCl3 + 3 H2O ®
H3PO3 + 3 HCl
This is a weaker acid than H3PO4.