Question 19M.2.sl.TZ2.b(iii)
| Date | May 2019 | Marks available | [Maximum mark: 1] | Reference code | 19M.2.sl.TZ2.b(iii) |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Identify | Question number | b(iii) | Adapted from | N/A |
Ethyne, C2H2, reacts with oxygen in welding torches.
Identify the type of interaction that must be overcome when liquid ethyne vaporizes.
[1]
London/dispersion/instantaneous dipole-induced dipole forces [✔]
Note: Do not accept just “intermolecular forces” or “van der Waals’ forces”.
Some candidates could not relate evaporation of a liquid to the breaking of its intermolecular forces and gave irrelevant answers such as “evaporation”. Other candidates gave general answers such as “the intermolecular forces” or used the term “van der Waals’ forces” which did not gain credit as too vague. The current guide is clear that “London/dispersion forces” is the appropriate term to use for instantaneous dipole-induced dipole forces. Less than 40 % of the candidates scored the mark. It was disappointing to see some candidates state “covalent bonding” as the type of interaction that must be overcome when liquid ethyne vaporizes. Some teachers thought the wording of the question may have been vague and candidates may have been confused about what was meant by the “type of interaction”.
