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
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