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