Compounds of Phosphorus
a) Halides of Phosphorus
Phosphorus combines with allthehalogens forming phosphorus halides which are all covalent compounds. Phosphorus chlorides are more important. Tri and pentachlorides of phosphorus are most common.
I. Phosphorus Trichloride, PCl3
Preparation: PCl3 is prepared by heating white phosphorus in a current of dry chlorine.
P4 + 6Cl2 ® 4PCl3
Dry white phosphorus is placed in the retort and gently heated on a water bath. A current of pure, dry chlorine is led over the phosphorus. The phosphorus trichloride formed being volatile distils over and is collected in a water cooled receiver.
The phosphorus trichloride obtained as above contains some PCl5 as impurity. This is removed by distilling the PCl3 over white phosphorus.
1. Colourless low boiling liquid
2. It fumes in moist air
3. It has pungent odour.
1. It is violently hydrolysed by water giving phosphorus acid and hydrochloric
PCl3 + 3 H2O ® 3HCl + H3PO3
In a similar manner it reacts with organic compounds containing hydroxyl (OH)
group, such as acids and alcohols.
PCl3 + 3CH3COOH ® 3CH3COCl + H3PO3
Acetic Acid Acetyl Chloride
PCl3 + 3C2H5OH ® 3 C2H5Cl + H3PO3
Ethyl alcohol Ethyl Chloride
2.. It reacts with chlorine or sulphuryl chloride forming phosphorus pentachloride.
PCl3 + Cl2 ® PCl5
PCl3 + SO2Cl2 ® PCl5 + SO2
3. It readily combines with oxygen forming phosphorus oxychloride
2PCl3 + O2 ®2POCl3
4. It reacts with SO3 to form phosphorus oxychloride and SO2
SO3 + PCl3 ® POCl3 + SO2
Structure: PCl3 molecule has a pyramidal shape, which arises from sp3
hybridisation of phosphorus atom. One of the tetrahedral positions is occupied by a lone pair of electrons.
II. Phosphorus pentachloride, PCl5
Preparation: Phosphorus pentachloride is usually prepared by the action of an
excess of chlorine on phosphorus trichloride.
PCl3 + Cl2 ® PCl 5
1. Phosphorus pentachloride is a yellowish white crystalline solid.
2. It sublimes on heating at 473 K and melts at 318 K under pressure.
Phosphorus pentachloride dissociates on heating into phosphorus trichloride and chlorine.
PCl5 -- > < --- PCl3 + Cl2
It is violently hydrolysed by water giving phosphorus oxychloride or
phosphoric acid depending upon the quantity of water.
PCl5 + H2O -- insufficient water --- > POCl3 + 2HCl
PCl5 + 4H2O -- Excess of water -- > H3PO4 + 5HCl
PCl5 molecule has trigonal bipyramidal shape in vapour state which arises from sp3d hybridisation of phosphorus atom.
b) Oxides of phosphorus
I Phosphorus trioxide P2O3 or P4O6
It is obtained by the combustion of phosphorus in a limited supply of air.
4P + 3O2 ® 2P2O3
1. It is a white waxy substance
2. It has a garlic odour.
1. It reacts with cold water, gives phosphorus acid.
P2O3 +3H2O ® 2H3PO3
2. It reacts with hot water vigorously to form inflammable phosphine.
2P2O3 + 6H2O ® PH3 + 3H3 PO4
II Phosphorus pentoxide P2O5 or P4O10
Phosphorus pentoxide can be prepared by burning phosphorus with sufficient
supply of air.
P4 + 5O2 ®P4 O10
It is a white solid and an acidic oxide.
1. It reacts with moisture to form metaphosphoric acid.
P4O10 + 2H2O ® 4HPO3
When the solution is boiled, the metaphosphoric acid is changed to
HPO3 + H2O ® H3PO4
P4O10 + 6H2O ® 4H3 PO4
2. Phosphorus pentoxide extracts water from many inorganic compound
including sulphuric acid, nitric acid and several organic compounds. It is therefore, used as a powerful dehydrating agent.
Use: It is used as a dehydrating agent.
c) Oxy-Acids of Phosphorus
I. Phosphorus acid - H3PO3
It is prepared by the action of cold water on phosphorus (III) oxide or phosphorus (III) chloride.
P2O3 + 3H2O ® 2H3PO3
PCl3 + 3H2O ® H3PO3 + 3HCl
It is a white crystalline solid with garlic taste.
1. Acidic nature: It is a dibasic acid and gives salts of two types.
H3PO3 + NaOH ® NaH2PO3 + H2O
Sodium dihydrogen Phosphite
H3PO3 + 2NaOH ® Na2HPO3 + 2H2O
Disodium hydrogen Phosphite
2. When it is heated it undergoes auto-oxidation and reduction to form
phosphoric acid and phosphine.
4H3 PO3 ® 3H3PO4 + PH3
3. It is a powerful reducing agent because it has P-H bond. It reduces silver
nitrate solution into silver.
2AgNO3 + H3PO3 + H2O ® 2Ag +H3PO4+2HNO3
Use: It is used as a reducing agent
II. Ortho phosphoric Acid, H3PO4
1. It is prepared by dissolving phosphorus pentoxide in water and boiling the
P2O5 + 3H2O ® 2H3PO4
2. Laboratory preparation: In the laboratory orthophosphoric acid can be
prepared by boiling a mixture of red phosphorus with 50% nitric acid in a flask fitted with a reflux condenser on a water bath till no more oxides of nitrogen are liberated.
Iodine acts as a catalyst. The product is evaporated below 453 K and then
cooled in a vaccum desiccator surrounded by freezing mixture when crystals of orthophosphoric acid are deposited.
P+5HNO3 ® H3PO4 +5NO2 +H2O
1. It is a deliquescent crystalline solid.
2. It is soluble in water.
1. It is a tribasic acid. It combines with alkalies like NaOH to form three series
H3PO4 +NaOH ® NaH2PO4 + H2O
Sodium Di hydrogen Phosphate
H3PO4 +2NaOH ® Na2HPO4 + 2H2O
Disodium hydrogen Phosphate
H3PO4 + 3NaOH ® Na3PO4 + 3H2O
2. On heating it gives pyrophosphoric acid at 523 K and at 589 K gives
H3PO4 H4 P2O7 2HPO3 + H2O
3. On reaction with silver nitrate, it gives yellow precipitate of silver phosphate.
H3PO4 + 3AgNO3 ® Ag3PO4+3HNO3
1. It is used in the preparation of HBr and HI as a substitute for sulphuric acid.
2. It is used as souring agent in the preparation of soft drinks.
3. It is used in the preparation of phosphate salts of sodium, potassium and ammonium.
4. It is used in the manufacture of phosphatic fertilisers.
Being a tribasic acid, the structure of phosphoric acid is represented as
III. B. Pyrophosphoric acid, H4 P2 O7
Preparation: Pyrophosphoric acid is prepared by heating orthophosphoric
acid to 523 K - 533 K.
2H3PO4 ® H4P2O7 + H2O
It is a colourless crystalline solid.
1. It is reconverted to orthophosphoric acid on boiling with water
H4P2O7 + H2O ® 2H3PO4
2. When heated strongly, it yields metaphosphoric acid
H4P2O7 2HPO3 + H2O
Structure: The Structure of pyrophosphoric acid is represented as:
d) Phosphine - PH3
Phosphine is the best known hydride of phosphorus.
Laboratory preparation: It is usually obtained by boiling white phosphorus with 30-40% solution of caustic soda in an inert atmosphere of CO2.
4P + 3NaOH + 3H2O ® PH3 + 3NaH2PO2
Phosphine so obtained is impure. It is passed into an aqueous solution of hydrogen iodide, PH4I is formed. PH4I is heated with KOH or NaOH, pure
phosphine is obtained.
PH3 + HI ® PH4I
PH4I + NaOH ® PH3 + NaI + H2O
Phosphine is colourless gas with rotten fish odour.
1. Dissociation: Phosphine dissociates at about 723 K and gives red
2. Action of air: It burns with oxygen and produces phosphorus pentoxide.
3. Action of chlorine: Phosphine burns in chlorine spontaneously forming
PCl3 and PCl5.
PH3 + 3Cl2 ® PCl3 + 3HCl PH3 + 4Cl2 ® PCl5 + 3HCl
4. Reducing properties: PH3 is a powerful reducing agent. When it is passed
through the salt solutions, corresponding metal is formed.
PH3 + 6AgNO3+ 3H2O ® 6Ag + 6HNO3 + H3PO3
1. Smoke screens
When PH3 burns it produces smoke which is dense enough to serve as
2. Holme's signal : Containers which have a perforated bottom and a hole at
the top are filled with calcium phosphide and calcium carbide. These are thrown into the sea. Water enters the container through the bottom and reacts with calcium carbide and calcium phosphide to give acetylene and phosphine. Phosphine gets ignited spontaneously as it comes in contact with air and also ignites acetylene. Thus a bright red flame is produced which is accompanied by huge smoke due to the burning of phosphine. This serves as a signal to the approaching ships.
Ca3P2 + 6H2O ® 2 PH3 + 3Ca(OH)2
CaC2 + 2H2O ® C2H2 + Ca(OH)2
An element 'A' occupies group number 15 and period number 3 reacts
with chlorine to give B which further reacts with chlorine to give C at 273 K. Both B and C are chlorinating agent for organic compounds. C is a better
chlorinating agent because it chlorinates metals also. B reacts with SO3 and
reduces it to SO2. B has a pyramidal shape. C has trigonal bipyramidal shape by
sp3d hybridisation. Identify the element A and the compounds B and C. Write
1. The element which occupies group number 15 and period number 3 is
phosphorus. Therefore A is phosphorus. Phosphorus reacts with chlorine
to give PCl3. Therefore compound B is phosphorus trichloride and it has a
P4 + 6Cl2 ® 4PCl3
2. PCl3 further reacts with Cl2 to give PCl5. Therefore, the compound C is
phosphorus pentachloride and it has a trigonal bipyramidal shape.
PCl3 + Cl2 ® PCl5
3. PCl3 and PCl5 are chlorinating agents for organic compounds. So, both
reacts with C2H5OH gives C2H5Cl.
PCl3 + 3C2H5OH ® 3C2H5Cl+ H3PO3
PCl5 + C2H5OH ® C2H5Cl + POCl3 + HCl
4. PCl5 is a better chlorinating agent. So it chlorinates copper.
PCl5 + 2Cu ® 2CuCl + PCl3
5. PCl3 reacts with SO3 and reduces it to SO2.
PCl3 + SO3 ® POCl3 + SO2