Quick questions on Intermolecular Forces (Topic 8) - Edexcel A-Level Chemistry

All three are weak compared with covalent bonds (a few $\text{kJ mol}^{-1}$ for London forces, up to about $40\ \text{kJ mol}^{-1}$ for a hydrogen bond, versus several hundred $\text{kJ mol}^{-1}$ for a typical covalent bond). London forces are always present; polar molecules have London plus dipole-dipole; molecules that meet the N/O/F rule also have hydrogen bonding on top.

Boiling overcomes intermolecular forces; it does not break covalent bonds. Down group 4 the hydrides $\text{CH}_4, \text{SiH}_4, \text{GeH}_4, \text{SnH}_4$ show a steady rise in boiling point because each has more electrons and stronger London forces. The hydrides of groups 5, 6 and 7 follow the same trend except that $\text{NH}_3$, $\text{H}_2\text{O}$ and $\text{HF}$ are anomalously high because they hydrogen bond. Water is the most striking: its boiling point of $100\ ^\circ\text{C}$ is roughly $160\ ^\circ\text{C}$ higher than the size trend alone would predict.

Explain why the boiling point of $H_2O$ is much higher than that of $H_2S$. [2 marks]

Explain why ice is less dense than liquid water. [2 marks]