Question

How do we account for the volatility of the hydrogen halide series?

Answer 1

Volatility is the inherent property of a chemical compound which describes the vaporizing behaviour of the compound. Volatility determines the ease of vaporization of the product. Volatility changes with change in temperature and pressure of the substance.

Complete answer:
Hydrogen halides are the covalent compounds which are formed by formation of hydrogen bond between hydrogen atom and electronegative halide metal. The halide used in these cases may be $Cl,Br,I,F$.
There are some factors which affects the volatility of the substance like: -
As the boiling point of the substance increases, the tendency of the substance to vaporize decreases gradually.
With increase in molecular mass of the compound, the tendency of the substance to vaporize decreases. The main reason behind it is that as the molecular mass of the compound increases, the boiling point of the substance increases therefore, volatility decreases.
The strength of different intermolecular forces of attraction present in compounds also affects the volatility of articular substances.
Presence of van der Waals forces between the molecules decreases the volatility of the compound.
Presence of intermolecular hydrogen bonding between the molecules also decreases the volatility of the compound.
In the case of hydrogen halide, the halide group of one molecule tends to form intermolecular hydrogen bonding with the hydrogen atom of another molecule. Therefore, as the strength of hydrogen bonding increases between the hydrogen halides, their volatility decreases.
As we move from top to bottom in the halogen family, size of the halogen atom increases. Therefore, due to the large difference between the size of hydrogen atom and halogen atom, its interaction to form hydrogen bonding decreases. Hence, the intermolecular attraction decreases as we move down.
Hence, volatility order of the hydrogen halide is: -
$HCl > HBr > HI > HF$
Due to the large size of the chlorine atom, it does not form hydrogen bonding and therefore, it has higher volatility than other hydrogen halides.

Note:
Volatility of the substance plays a major role during separation by distillation method. The basic method to compare volatility power of the compound by determining its rate of vaporization when placed to the atmosphere. Substances which are highly volatile in nature are like rubbing alcohol, camphor etc.