Question

Give the properties of the following:

PCl₅

Answer 1

1. The impure sample is often greenish yellow in colour, but pure phosphorus pentachloride is a nearly colourless solid. It smells strong.

2. Action of heat: It sublimates at 373 K and melts at 421 K when heated. It dissociates as follows above 573 K.

\[\ce{PCl5 <=> PCl3 + Cl2}\]

3. Action of water: Water and phosphorus pentachloride react violently. Phosphorus oxychloride is created when there is not enough water present, while phosphoric acid is produced when there is too much water.

\[\ce{PCl5 + \underset{amount)}{\underset{(Insufficient}{H2O}} -> \underset{oxychloride}{\underset{Phosphorus}{POCl3}} + HCl}\]

\[\ce{PCl + \underset{(excess)}{4H2O} -> \underset{Phosphoric acid}{H3PO4} + 5HCl}\]

4. Action on metals: When heated, it reacts with finely split metals to produce the appropriate chlorides.

For example:

\[\ce{PCl5 + Zn -> PCl3 + ZnCl2}\]

\[\ce{PCl5 + 2Cu -> PCl3 + 2CuCl}\]

\[\ce{PCl5 + 2Ag -> PCl3 + 2AgCl}\]

\[\ce{PCl5 + Cd -> PCl3 + CdCl2}\]

5. Reaction with sulphuric acid: Chlorosulphonic acid is created when it reacts with concentrated sulphuric acid.

\[\ce{PCl5 + H2SO4 -> \underset{acid}{\underset{Chiaro sulphonic}{Cl * SO3H}} + POCl3 + HCl}\]

6. Reaction with sulphur dioxide: It reacts with sulphur dioxide to form thionyl chloride.

\[\ce{PCl5 + SO2 -> \underset{Thionyl chloride}{SOCl2} + POCl3}\]

7. Reaction with hydroxy compounds: It reacts with substances that have hydroxy (-OH) groups, especially organic ones, and swaps out the -OH groups for chlorine atoms.

For example:

\[\ce{\underset{Ethyl alcohol}{CH3CH2OH} + PCl5 -> \underset{Ethyl chloride}{CH3CH2Cl} + POCl3 + HCl}\]

\[\ce{\underset{Acetic acid}{CH3COOH} + PCl5 -> \underset{Acetyl chloride}{CH3COCl} + POCl3 + HCl}\]