Some thoughts on the May 2022 IB Chemistry examinations
Monday 30 May 2022
Questions with more than one 'best' answer
About a year ago I wrote a blog Thinking through multiple choice questions on the difficulty of writing good multiple choice questions. This is partly due to the fact that because the numbers of words should be kept to a minimum it is a challenge to cover all the parameters so that only one of the four answers can be correct (or at least much better than any of the other three possible answers). This seems to be the case with one of the questions that has appeared on this year’s May multiple choice papers.
Question 28 (SL) and 38 (HL) asks how many signals would be observed in the 1H NMR spectrum of a compound for which no name is given but the structure is shown.
The molecular formula is C6H8O2 and all eight hydrogen atoms are in the same chemical environment so students should fairly easily be able to see that it will give one signal. The problem is that the question does not actually ask how many signals the compound will give – it asks how many will be observed. 1H NMR spectra need a reference so that the chemical shift can be measured in ppm. This reference is often tetramethylsilane Si(CH3)4 which gives a strong single signal at 0 ppm due to the 12 equivalent hydrogen atoms so actually two signals may well be observed, one from the compound and one from the reference. I hope the IB will consider giving both 1 and 2 as the correct answer. Of course, this could easily have been clarified in the wording of the question either by asking how many signals will the compound give in its 1H NMR spectrum or alternatively by adding “ignore the signal due to the reference”.
(Incidentally, I think it is bad practice not to name a compound in a question but I must admit that I am having trouble naming it and cannot actually find either the compound or its 1H NMR spectrum online – maybe someone can enlighten me? - Update someone has! I’m grateful to Adrian Dingle who has identified it using ChemDoodle as 3,7-dioxabicyclo[3.3.0]oct-1(5)-ene. In fact the first report of its synthesis by C. Mlynek et al. was in 2005 in the Journal of Molecular Structure.)
I think there are also issues with another question on the HL paper.
Question 21 asks which factor influences the pre-exponential factor, A in the Arrhenius equation and the students can choose from A. Nature of reactants, B. Temperature of reaction, C. Activation energy of reaction and D. Overall order of reaction.
I’ve shared my thoughts about this question with three very experienced IB teachers and like me they all feel that it seems way too hard for IB students to answer in a multiple choice question, where they cannot explain their single answer, as there are multiple answers. The question does not include “in a specified reaction” so anything that might alter the value of A in the general equation could be considered correct. Clearly different reactions will have different values of A so it depends upon the nature of the reactants. The temperature dependence of the reaction is usually accounted for in the Ea/RT exponential part of the equation but A refers to the orientation of the successful collisions and at higher temperatures there will be more collisions so it is generally accepted that A is also temperature dependent. The activation energy of a reaction can be lowered by adding a catalyst which provides a different pathway, this will also affect the mechanism and hence the orientation of how the reactants collide so affecting the value of A. Finally A has the same units as the rate constant, k. The overall order of the reaction determines the units of the rate constant so therefore changing the overall order of the reaction will also change the value of A, as its units change.
... and some pedantry!
There are also a few pedantic points which I’m sure will not have affected how the students performed on the exams but the proof reading does not appear to have been very thorough which mars the overall professionalism and quality of the papers.
There are inconsistencies in the use of the language of chemistry. Several of the questions contain constants in normal lettering and (correctly) in italics in different questions and even in the same question, e.g. A and A in Question 21 discussed above in HLP1 and Question 13 in SLP1 has ΔH in answers A, B and D but ΔH in answer C. Similarly Question 30 has the E in E⦵ not in italics whereas in Question 3(a)(i) in HL Paper 2 it is shown correctly with the E in E⦵ In italics. There is confusion between the use of “what” and “which” in the multiple choice questions (e.g. compare Questions 26 and 29 on the SL Paper 1). In the past the rule followed was In general, use "which" before a noun and "what" before a verb. Now the choice of “which” or “what” seems random. The IB has for the past few years asked for “calculate the amount (in mol)” and this year in Question 1 on Paper 2 has reverted back to “calculate the number of moles” - perhaps because this is how it was worded in Question 2 on the June 2014 Cambridge International AS examination from which the question appears to have been taken from?