Question 19M.2.sl.TZ2.1d(iii)
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.2.sl.TZ2.1d(iii) |
| Level | sl | Paper | 2 | Time zone | TZ2 |
| Command term | Explain | Question number | d(iii) | Adapted from | N/A |
Ethyne, C2H2, reacts with oxygen in welding torches.
Product B, CH3CHO, can also be synthesized from ethanol.
Explain why product B is water soluble.
[3]
Any three of:
has an oxygen/O atom with a lone pair [✔]
that can form hydrogen bonds/H-bonds «with water molecules» [✔]
hydrocarbon chain is short «so does not disrupt many H-bonds with water molecules» [✔]
«large permanent» dipole-dipole interactions with water [✔]
This was a challenging question with a demanding markscheme. Most students missed the fact that ethanal can form hydrogen bonds with water. And students who did state this often achieved only 1 out of the 3 marks because they did not offer a full explanation. Some candidates stating "hydrogen bonding" showed confusion by mentioning the hydrogen of the aldehyde group. Few identified the lone pairs on oxygen as the reason for the ability to hydrogen bond. Most candidates just stated that ethanal is polar and dissolves in polar water achieving no marks. However, one mark was awarded for “dipole-dipole interactions with water”.
