Phosphine ($PH_3$): Preparation, Reactivity & Holme's Signal
Phosphine is a colorless, poisonous gas with a distinct rotten fish smell. It is widely known for its use in Holme's Signals and smoke screens due to its spontaneous combustion when mixed with impurities.
1. Preparation of Phosphine
Phosphine can be prepared by hydrolysis of metal phosphides or by heating white phosphorus with a concentrated alkali solution.
Safety Note
Pure $PH_3$ is non-flammable, but it often catches fire spontaneously due to the presence of $P_2H_4$ (Diphosphine) vapors. To purify it, it is absorbed in HI to form $PH_4I$.
Figure: Complete Reaction Chart of Phosphine ($PH_3$)
2. Chemical Properties & Reactions
Phosphine acts as a Lewis base (weaker than Ammonia) and reacts with acids to form phosphonium salts. It also reacts with metal salts to form phosphides.
Key Chemical Reactions
- Basic Nature: Reacts with $HBr$ to form Phosphonium Bromide.
$PH_3 + HBr \rightarrow PH_4Br$ - Reaction with Metal Salts:
$3CuSO_4 + 2PH_3 \rightarrow Cu_3P_2 + 3H_2SO_4$ (Black ppt)
$3HgCl_2 + 2PH_3 \rightarrow Hg_3P_2 + 6HCl$ - Combustion: Burns to form Phosphoric acid fumes.
$PH_3 + 2O_2 \rightarrow H_3PO_4$
Holme's Signal
Containers containing Calcium Carbide ($CaC_2$) and Calcium Phosphide ($Ca_3P_2$) are pierced and thrown into the sea. The gases evolved ($C_2H_2$ and $PH_3$) catch fire, serving as a signal.
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