Phosphine Gas: Preparation, formula, and Uses
Phosphides – When calcium and sodium Phosphides reacts with water, phosphine gas is obtained.
Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3
Ca3P + 3H2O → 3CaOH + PH3
2AlP + 3H2SO4 → Al2(SO4)3 + 2PH3
Phosphine gas is formed by the action of phosphonium iodide with sodium hydroxide.
PH4I + NaOH → PH3 + NaI + H2O
Phosphine gas can also be made by heating phosphorus acid.
4H3PO3 → 3H3PO4 + PH3
Phosphine gas is formed by heating white phosphorus with concentrated caustic soda.
4P + 3NaOH + 3H2O → 3NaH2PO2 + PH3
This method of making phosphine is used for manufacture in its laboratory. To make this gas in the laboratory, in a round bottomed flask, a few pieces of white phosphorus and a mixture of sodium hydroxide of about 40% concentrations are first heated slowly and then rapidly.
This device is air tight and the cork on the mouth of the flask has two tubes. One of these flows coal gas through the tube, displacing the air in the flask. Oxygen of air can lead to unnecessary charges from reactants and products.
Whereas coal gas does not do this. Shortly after the reaction begins, the flow to the coal gas is stopped. The second tube in the flask exits the phosphine gas. If the other end of this second tube is immersed in water, the gas coming out of the water makes white smoke rings.
This is due to the phosphorus hydrides(P2H4). Phosphorus hydrides gas (Diphosphane) rises as it comes in contact with air. White smoke is obtained due to the formation of Meta Phosphoric Acid(HPO3).
6P + 4NaOH + 4H2O → 4NaH2PO2 + P2H4
2P2H4 + 7O2 → 4HPO3 + 2H2O
In order to obtain pure phosphine gas, the gaseous mixture obtained from the exhaust pipe is flowed through the ice water. Where the impurities are condensed and separated and the pure phosphine gas is collected by downward displacement of water.
In the above method, pure phosphine gas is obtained by using an alkaline solution of KOH in place of aqueous NaOH solution.
Phosphine Properties
Physical Properties
It is a colorless, century fish-like odor and poisonous gas.
It changes into a liquid state at – 85°C and a solid state at – 135°C.
It is a small amount of fusion in water and heavier than air.
Heat Effect:
Pure phosphine gas is not flammable at ordinary temperature in the presence of air, it burns when heated to about 150°C.
2PH3 + 4O2 → P2O5 + 3H2O
In the absence of air, it decomposes on heating to a temperature above 400°C.
2PH3 → 2P + 3H2
Reaction with Chlorine:
At ordinary temperature, it reacts with chlorine as follows –
PH3 + 4Cl2 → 3HCl + PCl5
Reaction with halogen acids
It reacts with halogen acids under high pressure to form phosphonium compound.
PH3 + HCl → PH4Cl (Phosphonium Chloride)
PH3 + HBr → PH4Br (Phosphonium Bromide)
PH3 + HI → PH4I (Phosphonium Iodide)
Phosphonium compounds are unstable in comparison to ammonium compounds and decompose easily.
example :
PH4I → PH3 + H+ + I–
Reaction with nitric acid
It reacts with nitric acid to form phosphorus pentoxide (P2O5) and nitrogen dioxide (NO2).
[2HNO3 → 2NO2 + H2O + O] x 8
2PH3 + ???? → P2O5 + 3H2O
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2PH3 + 16HNO3 → P2O5 + 11H2O + 16NO2
It reacts with nitric acid to form phosphorus pentoxide (P2O5) and nitrogen dioxide (NO2).
Reaction with copper sulfate
A black precipitate of cupric phosphide (Cu3P2) is obtained when phosphine gas is passed through a solution of copper sulfate.
3CuSO4 + 2PH3 → Cu3P2 + 3H2SO4
Reaction with mercuric chloride:
On passing phosphine gas in a solution of murcuric chloride, a black precipitate of mercuric phosphide (Hg3P2) is obtained.
3HgCl2 + 2PH3 → Hg3P2 + 6HCl
Reaction with silver nitrate:
When passed in a solution of silver nitrate, it reduces silver nitrate to metallic silver.
6AgNO3 + 3H2O + PH3 → 6Ag + 5HNO3 + H3PO3
Reaction with metallic chlorides:
Phosphine is absorbed by anhydrous AlCl3 and anhydrous SnCl4.
AlCl3 + 2PH3 → AlCl3.2PH3
SnCl2 + 2PH3 → SnCl4.2PH3
It is also absorbed on passing through acidic solution of cuprous chloride and forms additive compound.
Cu2Cl2 + 4PH3 → Cu2Cl2.4PH3
On heating the solution, phosphine gas is again liberated.
Identification of Phosphine
Its smell is like rotten fish.
A black precipitate is obtained on passing through an aqueous solution of copper sulfate.
A black precipitate is obtained on passing through an aqueous solution of silver nitrate.
Uses of Phosphine
It is used in the preparation of phosphide compounds of various elements. Calcium phosphide (CP) is used to make the Holmes signal to signal ships in the seas. It is also used to make smoke screen in riots and wars.
For this purpose tin filled with calcium phosphide and calcium carbide is thrown into the sea. They react with water to form phosphine and acetylene gases.
Ca3P2 + 6H2O → 3Ca(OH)2 + 2PH3
Ca3C2 + 2H2O → Ca(OH)2 + C2H2
Due to the impurity of phosphorus di hydride (P2H4) in phosphine, it immediately burns up. The heat obtained in this process also burns acetylene, due to which it seems that the ocean is on fire.
The compounds formed by the burning of phosphine, acetylene and other gases, they take the form of smoke, make smoke screen or act as a signal.
Structure of Phosphine
Like ammonia, the structure of phosphine is also pyramidical. The hybridization of P is of sp3 type and phosphorus atom forms a single bond with each hydrogen atom.
The H – N – H bond angle in NH3 is 107° whereas in PH3 the H – P – H bond angle is about 93°.
Comparison of ammonia and phosphine
Similarities
Both the gases are colourless. The molecular formula and molecular structures of both the gases are the same.
Both the gases do not help in the combustion of other substances. Both the gases burn on their own in the presence of oxygen.
Both the gases decompose into their constituents by electric spark.
Both react rapidly with chlorine.
Compounds with both halogen acids to form ammonium and phosphonium compounds respectively.
Inequalities
ammonia has a characteristic and strong odor whereas phosphine has a rotten fish-like smell.
Sniffing ammonia in excess is harmful. It is not harmful in small amounts. A small amount of ammonia is present in the atmosphere. In contrast, phosphine is a very toxic gas.
ammonia is highly soluble in water whereas phosphine is sparingly soluble in water.
ammonia is lighter in air and phosphine is heavier than air.
In ammonia, intermolecular hydrogen is bonded while phosphine does not form hydrogen bond. Due to this the boiling point of ammonia is higher than that of phosphine.
Aqueous solution of ammonia is alkaline to litmas. Aqueous solution of phosphine is neutral to litmus.
Phosphine is a weak base as compared to ammonia.
Ammonium salts are permanent compounds whereas phosphinium compounds are relatively temporary.
On passing ammonia to aqueous solution of metal salts or adding ammonium hydroxide, their hydroxides precipitate while phosphides of metals are precipitated by phosphine.
Ammonia forms complex compounds with salts of Cu(II), Ag(I) etc. whereas phosphine does not.
ammonia does not show reducing property whereas phosphine also shows reducing property.
example :
ammonia forms complex with AgNO3 while phosphine reduces AgNO3 to Ag.
These gases react with sodium to form compounds of different nature (NaNH2 and Na3P).