Question 22N.2.sl.TZ0.2
| Date | November 2022 | Marks available | [Maximum mark: 9] | Reference code | 22N.2.sl.TZ0.2 |
| Level | sl | Paper | 2 | Time zone | TZ0 |
| Command term | Annotate, Compare, Deduce, Determine, Draw, Explain, Justify, State | Question number | 2 | Adapted from | N/A |
Chloroquine is a medication used to prevent and treat malaria.
Draw a circle around the secondary amino group in chloroquine.
[1]
✔
Half of the candidates circled the secondary amino group in the structure. Many candidates circled the N in the ring and some circled N1.
Determine the index of hydrogen deficiency, IHD, of chloroquine.
[1]
seven/7 ✔
A challenging question that 28% of the candidates did not attempt. Only 10% of the candidates were able to determine the index of hydrogen deficiency of chloroquine.
Compare, giving a reason, the length of the carbon-nitrogen bond in the ring to the length of the carbon-N1 bond.
[1]
«bond in ring is» shorter AND more electrons are shared
OR
«bond in ring is» shorter AND partial double/multiple bonding/bond order 1.5 ✔
30% of the candidates deduced that the C−N bond in the ring is shorter than the single C−N1 bond and were able to give a valid reason in terms of bond multiplicity or resonance.
State, giving a reason, whether carbon or nitrogen is the most electronegative element.
[1]
nitrogen AND larger number of protons/nuclear charge/Zeff
OR
nitrogen AND smaller «atomic» radius ✔
Accept nitrogen AND further along period.
This question was rather vague. 40% of the candidates gave a good reason for N being more electronegative than C.
Chloroquine can be synthesized by reacting 4,7-dichloroquinoline with another reactant, B.
4,7-dichloroquinoline chloroquine
Deduce the structure of B.
[2]
primary amine/–NH2 ✔
rest of structure ✔
Do not penalize using “N1”.
Teachers commented that the question included an organic reaction that was beyond the syllabus. They also commented that it would have been better to provide a balanced equation with HCl as the byproduct to help the candidates deduce the reactant B, which is a fair point. Most candidates achieved one mark for the general structure but lost the mark allocated for the NH2 functional group.
This reaction can be done with a copper catalyst. State the ground-state electron configuration for copper.
[1]
1s22s22p63s23p64s13d10 / 1s22s22p63s23p63d104s1
OR
[Ar]4s13d10 / [Ar]3d104s1 ✔
Half the candidates wrote the correct ground-state electron configuration for copper. Many candidates wrote 1s2 2s2 2p6 3s2 3p6 4s2 3d9.
Annotate the Maxwell–Boltzmann distribution curve showing the activation energies, Ea, for the catalysed and uncatalysed reactions.
[1]
both Ea values marked AND left one labelled catalysed ✔
This question discriminated well between high-scoring and low-scoring candidates. Half the candidates annotated the Maxwell-Boltzmann distribution curve to show the activation energies for the uncatalysed and catalysed reactions. Mistakes included drawing a second curve (confusing this effect with changing temperature), reversing the labels and labelling other parts of the graph.
Explain, referring to the Maxwell–Boltzmann distribution curve, the effect of a catalyst on a chemical reaction.
[1]
increases rate AND there is a greater area under the curve past activation energy
OR
increases rate AND greater proportion of/more molecules have «kinetic» E ≥ Ea «(cat) than Ea (uncat)» ✔
Do not award a mark for general statements about catalysts such as “provides alternative pathway” or “lowers Ea”.
Most candidates identified that catalysts provided an alternative reaction pathway with a lower activation energy but most of them did not relate this to the Maxwell-Boltzmann distribution and hence did not gain the mark. Only 20% of the candidates were awarded the mark. A few candidates were confused and referred to an increase in the frequency of collisions when a catalyst is used.