Question 22N.2.SL.TZ0.4
| Date | November 2022 | Marks available | [Maximum mark: 8] | Reference code | 22N.2.SL.TZ0.4 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Deduce, Describe, Explain | Question number | 4 | Adapted from | N/A |
An organic compound, A, reacts with ethanoic acid to produce B using concentrated sulfuric acid as a catalyst.
Deduce the structural and empirical formulas of B.
[3]
Structure:
ester functional group ✔
rest of structure ✔
Empirical Formula:
C3H5O ✔
Accept condensed/skeletal formula.
A question that discriminated well between high-scoring and low-scoring candidates. The average mark on this three-mark question was 1.3. The majority of candidates did not recognize it as an esterification reaction and the ester functional group was only seen in a small proportion of the scripts. Some candidates earned a mark for the remainder of the structure. Only about half of the candidates earned error carried forward for the mark allocated for the empirical formula. Some candidates had the molecular formula instead, and some candidates miscounted the numbers of atoms in the structure they drew.
Explain, with reference to Le Châtelier’s principle, the effect of using dilute rather than concentrated sulfuric acid as the catalyst on the yield of the reaction.
[2]
dilute adds «excess» water
OR
water is a product ✔
shift left AND decreases yield ✔
This was the most challenging question on the paper according to the difficulty index. Many candidates stated that catalysts do not affect the position of an equilibrium and hence the yield is not changed. Some candidates stated that the rate of reaction would be slower and the yield per unit time would be lower. Only a few candidates recognized that the dilute sulfuric acid catalyst would introduce more water, and since water is a product it would shift the equilibrium to the left and lower the yield of the ester. 23% of the candidates did not answer the question. Some teachers commented in their feedback that it was not fair to expect the students to know about the dehydrating property of H2SO4, but this was not intended. The students were expected to deduce the effect.
Explain, with reference to intermolecular forces, why B is more volatile than A.
[2]
A has hydrogen bonding/bonds «and dipole-dipole and London/dispersion forces» AND B has dipole-dipole «and London/dispersion forces»
OR
A has hydrogen bonding/bonds AND B does not ✔
intermolecular forces are weaker in B
OR
hydrogen bonding/bonds stronger «than dipole-dipole» ✔
This question about intermolecular forces discriminated well between high-achieving and low-achieving candidates. Stronger candidates showed excellent understanding of the types of intermolecular forces found between the molecules of each compound and how they compared in strength. They gave more detail than the markscheme required. The average mark on the question was 0.8 out of 2. 21% of the candidates left the question blank. Error carried forward was applied whenever it was possible based on the answer in (a)(i).
Compound A can also react with bromine. Describe the change observed if A is reacted with bromine.
[1]
brown/orange/red/yellow to colourless ✔
Do not accept clear for colourless.
Although it was a straightforward organic question, 22% of the candidates left it blank, indicating less confidence in answering the organic chemistry questions. 40% of the candidates gained the mark for the decolourization of bromine. One of the common mistakes was reversing the colour change and another was using the term "clear" instead of "colourless".
