The boiling points of the hydrogen halides HClHClHCl, HBrHBrHBr, and HI HI\,HI increase down Group 17, from −85 ∘C-85\ ^\circ\text{C}−85 ∘C to −35 ∘C-35\ ^\circ\text{C}−35 ∘C. Which statement correctly explains this trend?
The H−XH-XH−X covalent bond enthalpy increases from HClHClHCl to HIHIHI, requiring more thermal energy to break.
The electronegativity difference between hydrogen and the halogen increases down the group, strengthening permanent dipole–dipole forces.
The total number of electrons increases, resulting in stronger London (induced dipole–dipole) forces that outweigh the weaker permanent dipole–dipole forces.
Molecules of HIHIHI can form intermolecular hydrogen bonds due to the high polarisability of iodine.
31 exam-style questions on OCR A A Level Chemistry Bonding and structure. Each one has a worked solution and a mark scheme showing where the marks go.