Unnecessary equations?
Wednesday 28 May 2025
This blog questions whether IB chemistry is becoming too reliant on simply plugging data into given equations rather than encouraging students to think through problems using their understanding of the underlying principles and concepts of chemistry. This is particularly the case when it comes to the importance of using units correctly. The blog also contains some comments on the Zone B HL May 2025 exam papers.
Equations in exam questions
The new syllabus was designed to encourage students to understand the underlying concepts and to make connections between them. I wonder if in fact it isn't encouraging students to learn more by rote and rely on simply ‘plugging’ values into given equations rather than thinking through problems logically. In the recent May 2025 HL Paper 1A exam (Zone B Europe & Africa) Question 19 asked students to work out the standard enthalpy change for the combustion of a fuel. The data given listed the amount of fuel burned (0.110 mol), the mass of water heated (200 g) with its initial and final temperatures (21.0 oC and 25.0 oC respectively) and the specific heat capacity of water.
Similar questions have been asked in the past but the big difference in this question is that the equation Q = mcΔT was also given so that students simply needed to feed the values into the equation. This of course is the first year that students had access to the whole data booklet for multiple choice questions so in a sense this equation was actually given to them not once but twice, once in the question and once in Section 1 in the 2023 data booklet. The specific heat capacity of water is given in the question as 4.18 J g-1 K-1, i.e. it requires 4.18 J of heat energy to raise the temperature of 1 g of water by 1.0 K (or 1.0 oC ). Without the need for any equation, it is obvious that when the combustion of 0.110 mol of fuel raises the temperature of 200 g of water by 4.0 oC, the heat given out is 200 x 4.0 x 4.18 J = 3344 J. From this it follows that the amount given out in this exothermic reaction when 1 mol of the fuel is combusted = 3344/0.110 = 30400 J = 30.4 kJ, i.e. ΔHcombustion = − 30.4 kJ mol-1. In my personal view "spoon-feeding" students unnecessarily with the equation Q = mcΔT twice is effectively ‘dumbing down’ IB chemistry.
Equations in the data booklet
In fact, Section 1 of the 2023 data book contains many unnecessary equations. For example, the units of concentration are mol dm-3 so there is no need for the equation n = CV; the units of M are g mol-1 so there is no need for the equation n = m/M; from the general gas equation pV = nRT it is obvious that when the pressure, temperature and volume change for a fixed amount of an ideal gas there is no need for the equation P1V1/T1 = P2V2/T2 since both equal nR. You can find other examples too. There is no need for the two rather confusing equations ΔH⦵ = Σ(ΔHf⦵products) - Σ(ΔHf⦵reactants) and ΔH⦵ = Σ(ΔHc⦵reactants) - Σ(ΔHc⦵products) as students can simply use energy cycles or bond enthalpies etc. to determine enthalpies of formation and combustion directly. Some teachers have commented upon the fact that now that buffer calculations are on the main syllabus (previously they were only on two of the options) it seems odd that the Henderson-Hasslebalch equation which was in the data booklet for the previous syllabus has now been left out. Students know how to use the equation Kw = 1 x 10-14 = [H+][OH-] to find the pH of pure water at 298 K as [H+] = [OH-]. They also know how to use the equation Ka = [H+][A-]/[HA] to find the pH of a weak acid as [H+] = [A-]. It is only one step further to use the same equation to find the pH of an acidic buffer solution where [H+] is not equal to [A-], so the Henderson-Hasslebalch equation is not really necessary (see R3.1 (AHL) Buffer solutions).
Some comments on the Zone B HL M2025 Exams
I haven't really looked at the May 2025 papers in great detail but I notice that some chemistry errors are present in the Zone B HL papers taken in Europe & Africa. For example, the table in HL Paper 1A Question 27 gives the units for [NO] and [Cl2] but then makes two errors in the heading of the next column.
It should be Initial rate (not just Rate) and there are no units given even though a previous question (Question 24) specifically asks what units are used for rate of reaction.1
In HL Paper 2 Question 3 (a)(v) it asks students to calculate the standard entropy change, ΔS⦵ for the reaction between carbon monoxide and chlorine to form phosgene. It then gives the standard entropy changes ΔS⦵ of chlorine and phosgene.
This is wrong. What is given are the standard entropy values, S⦵ for chlorine and phosgene, not the standard entropy changes, ΔS⦵.
In the next part of the question, (a)(vi), it states, "if you did not get an answer for (a)(v) use = -150 mol-1 K-1".
This is missing J in the units. Note that the data booklet in Section 13, to which students are also referred to in the question, correctly gives the units of S⦵ as J K-1 mol-1 rather than J mol-1 K-1 so to be consistent it should have stated use “-150 J K-1 mol-1”.
There are also examples of the now common inconsistencies in exam papers with the use of italics. For example it incorrectly uses Ka (sic) when giving the value of Ka for HCN in HL Paper 2 Question 1 (b)(iii) but in part (c)(iv) of the same question it correctly gives K for the equilibrium constant for the formation of [Fe(CN)6]4-. HL Paper 1A Question 18 confuses energy with temperature. Endothermic reactions take heat from the surroundings but it cannot necessarily be concluded that the temperature of the surroundings falls. If an ice-water bath is being used to keep the temperature constant the heat is provided by some of the liquid water changing into ice but the temperature remains constant.
These may be all be considered rather pedantic points - none of these errors or inconsistencies is likely to seriously affect how students performed on the paper - but they should have been picked up by the proof readers and checkers before the final version of the examination paper was published. They also provide good material for you to discuss with your students to get them to think critically.
Footnote
1It might be argued that the units were omitted in the table so as not to give the answer away for Question 24 but this argument is not really valid. Firstly, because rates of reaction should always have units unless they are relative rates (which could have been used to get round this problem) and secondly, in Paper 2 Question 1 (c)(ii) it asks students to deduce the oxidation state of iron in [Fe(CN)6]4- then, just below in part (c)(iv), it effectively provides the answer (+2) to (c)(ii) by giving the concentration of Fe2+ in a 1.00 mol dm-3 solution of the same complex ion.
