WAEC Chemistry 2023 Paper

Concept:Boiling point depends on the strength of intermolecular forces. Stronger forces require more energy to overcome, leading to higher boiling points.Explanation:Hydrogen bonding is the strongest intermolecular force among these compounds.$\mathrm{HF}$ has the strongest hydrogen bond because fluorine is the most electronegative element. Thus, it has the highest boiling point.$\mathrm{NH}_3$ also forms hydrogen bonds, but these are weaker than in $\mathrm{HF}$ because nitrogen is less electronegative than fluorine. So $\mathrm{NH}_3$ has the next highest boiling point.$\mathrm{SiH}_4$ and $\mathrm{CH}_4$ are nonpolar molecules, so only weak van der Waals forces act between them.$\mathrm{SiH}_4$ has a larger molar mass than $\mathrm{CH}_4$, so its van der Waals forces are stronger and its boiling point is higher.Therefore, the decreasing order of boiling points is: $\mathrm{HF} > \mathrm{NH}_3 > \mathrm{SiH}_4 > \mathrm{CH}_4$.Answer:Option A: $\mathrm{HF}, \mathrm{NH}_3, \mathrm{SiH}_4, \mathrm{CH}_4$.