Question

which of the following lists the substances f₂, hcl, and hf in order of increasing boiling point?
a hf < hcl < f₂
b hcl < f₂ < hf
c hcl < hf < f₂
d f₂ < hcl < hf

Explanation:

Step1: Analyze intermolecular forces

Boiling - point is related to the strength of intermolecular forces. $F_2$ is a non - polar molecule, and the only intermolecular force is London dispersion force. $HCl$ is a polar molecule, and it has dipole - dipole forces in addition to London dispersion forces. $HF$ has hydrogen bonding (a special and strong type of dipole - dipole force) along with London dispersion forces.

Step2: Compare strength of intermolecular forces

London dispersion forces are the weakest. Among the three substances, $F_2$ has only London dispersion forces, so it has the weakest intermolecular forces and the lowest boiling point. $HCl$ has dipole - dipole and London dispersion forces, and $HF$ has hydrogen bonding which is stronger than the dipole - dipole forces in $HCl$. So, $HCl$ has a lower boiling point than $HF$.

Answer:

D. $F_2 < HCl < HF$