DP Chemistry (first assessment 2025)
Question 22M.2.SL.TZ2.c
| Date | May 2022 | Marks available | [Maximum mark: 1] | Reference code | 22M.2.SL.TZ2.c |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Suggest | Question number | c | Adapted from | N/A |
c.
[Maximum mark: 1]
22M.2.SL.TZ2.c
Suggest why hydrogen chloride, HCl, has a lower boiling point than hydrogen cyanide, HCN.
[1]
Markscheme
HCN has stronger dipole–dipole attraction ✔
Do not accept reference to H-bonds.
Examiners report
This proved to be the most challenging question (10%). It was a good question, where candidates had to explain a huge difference in boiling point of two covalent compounds, requiring solid understanding of change of state where breaking bonds cannot be involved). Yet most considered the triple bonds in HCN as the cause, suggesting covalent bonds break when substance boil, which is very worrying. Others considered H-bonds which at least is an intermolecular force, but shows they are not too familiar with the conditions necessary for H-bonding.
Syllabus sections
Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.9—Given comparable molar mass, the relative strengths of intermolecular forces are generally: London (dispersion) forces < dipole–dipole forces < hydrogen bonding. Explain the physical properties of covalent substances to include volatility, electrical conductivity and solubility in terms of their structure.