Question

Give Reason :

Hydrogen fluoride $\left(\mathrm{HF}\right)$ is the least volatile and hydrochloric acid $\left(\mathrm{HCl}\right)$ is the most volatile among hydrogen halides.

Answer 1

Properties of Halogens:

• Halogens are Group 17 elements in a periodic table.
• $\mathrm{HX}$ is hydrogen halide.
• Down the periodic table, atomic masses increases and so the mass of $\mathrm{HI}$ is highest and that of $\mathrm{HF}$ is lowest.
• Due to the increase in molecular mass, the boiling point increases due to an increase in the strength of Van der Waals force of intermolecular attraction.
• With the increase in the boiling point, volatility decreases.
• Hydrogen fluoride $\left(\mathrm{HF}\right)$ has Hydrogen bonding as an intermolecular force in addition to the dispersion force or Van der Waals force whereas Hydrogen chloride $\left(\mathrm{HCl}\right)$ does not have intermolecular H-bonding.
• So, $\left(\mathrm{HF}\right)$ has the highest boiling point and least volatility.
• The order of volatility of Hydrogen halides is as follows.

$\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}>\mathrm{HF}$

Therefore, among Hydrogen halides, hydrogen fluoride $\left(\mathrm{HF}\right)$ is the least volatile and Hydrochloric acid $\left(\mathrm{HCl}\right)$ is the most volatile because of the presence of hydrogen bonds in $\mathrm{HF}$ which causes an increase in boiling point and decrease in volatility.