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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Phosphine - PH3: Preparation, properties, Structure

hosphine 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.

Phosphorus  -

Atomic Number :  15  Electronic Configuration  :  [Ne] 3s2 3p3  Group Number : 15   Periodic Number  : 3

 

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

 

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