Question

which of the following correctly ranks the substances below in order of increasing boiling point. lif, n₂, f₂, hf, hcl

Explanation:

• $N_2$ and $F_2$: These are non - polar covalent molecules. The boiling point of non - polar covalent molecules is determined by London dispersion forces. The strength of London dispersion forces increases with the size of the molecule. Since $F_2$ has a larger electron cloud (F has more electrons than N in $N_2$), $F_2$ has stronger London dispersion forces than $N_2$. So, $N_2
• $HCl$ and $HF$: These are polar covalent molecules. $HF$ has hydrogen bonding (because F is highly electronegative and H is bonded to it), while $HCl$ has dipole - dipole forces. Hydrogen bonding is stronger than dipole - dipole forces. So, $HCl < HF$.
• Ionic compound ($LiF$): Ionic compounds have very strong ionic bonds. The energy required to break ionic bonds (to convert the solid ionic compound to a gas) is much higher than the energy required to overcome intermolecular forces (London dispersion, dipole - dipole, or hydrogen bonding) in covalent compounds. So, the boiling point of $LiF$ is the highest among all the given compounds.

Answer:

$N_2 < F_2 < HCl < HF < LiF$