Compounds of Phosphorus

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.

Physical properties

1.         Colourless low boiling liquid

2.         It fumes in moist air

3.         It has pungent odour.

Chemical Properties

1. It is violently hydrolysed by water giving phosphorus acid and hydrochloric

acid gas.

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

Physical properties

1.           Phosphorus pentachloride is a yellowish white crystalline solid.

2.     It sublimes on heating at 473 K and melts at 318 K under pressure.

Chemical properties

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

Structure

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

Physical properties

1.     It is a white waxy substance

2.     It has a garlic odour.

Chemical properties

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

Physical properties

It is a white solid and an acidic oxide.

Chemical properties

1.    It reacts with moisture to form metaphosphoric acid.

P4O10 + 2H2O ® 4HPO3

When the solution is boiled, the metaphosphoric acid is changed to

orthophosphoric acid.

HPO3 + H2O ® H3PO4

or

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

Physical properties

It is a white crystalline solid with garlic taste.

Chemical Properties

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.

D

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

Electronic structure

Use: It is used as a reducing agent

II. Ortho phosphoric Acid, H3PO4

Preparation

1. It is prepared by dissolving phosphorus pentoxide in water and boiling the

solution.

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

Physical properties

1. It is a deliquescent crystalline solid.

2. It is soluble in water.

Chemical properties

1. It is a tribasic acid. It combines with alkalies like NaOH to form three series

of salts.

H3PO4 +NaOH      ®   NaH2PO4 + H2O

Sodium Di hydrogen Phosphate

H3PO4 +2NaOH     ®              Na2HPO4 + 2H2O

Disodium hydrogen Phosphate

H3PO4 + 3NaOH   ®           Na3PO4         + 3H2O

Sodium Phosphate

2. On heating it gives pyrophosphoric acid at 523 K and at 589 K gives

metaphosphoric acid

523K                589K

H3PO4             H4 P2O7          2HPO3 + H2O

3.    On reaction with silver nitrate, it gives yellow precipitate of silver phosphate.

H3PO4 + 3AgNO3 ® Ag3PO4+3HNO3

Uses

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.

Structure

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

Physical Properties

It is a colourless crystalline solid.

Chemical Properties

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

Sodium hypophosphite

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

Physical properties

Phosphine is colourless gas with rotten fish odour.

Chemical properties

1. Dissociation: Phosphine dissociates at about 723 K and gives red

phosphorus.

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

Uses

1.    Smoke screens

When PH3 burns it produces smoke which is dense enough to serve as

smoke screens.

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

Problem

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

the reactions.

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

pyramidal shape.

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