IB Chemistry: Enthalpy Changes & Calorimetry FAQ

Enthalpy changes and calorimetry sit in Reactivity 1.1 of the 2025 DP Chemistry guide, covering how much heat a reaction releases or absorbs and how you measure it experimentally using q = mcΔT. It's examined in Paper 1 (data-based multiple-choice), Paper 2 (calculations worth 2–4 marks each), and often chosen as an IA topic.

Standard enthalpy of reaction (ΔH°r) is the heat change for a reaction exactly as written. Enthalpy of combustion (ΔH°c) is specifically for one mole of a substance burning completely in oxygen. Enthalpy of formation (ΔH°f) is for one mole of a compound made from its elements in their standard states — all measured at 298 K and 100 kPa.

You need q = mcΔT, where m is the mass of the solution (not the solid dissolved or burnt in it), c is the specific heat capacity — 4.18 J g⁻¹ K⁻¹ for water — and ΔT is the temperature change. Divide q by moles of the limiting reagent to get the molar enthalpy change in kJ mol⁻¹.

It's not conceptually difficult, but it's mark-heavy and unforgiving on arithmetic — unit conversions, sign errors and misreading which mass to use trip up more students than the actual chemistry does. HL adds entropy and Gibbs free energy calculations, which raise the difficulty considerably once temperature dependence gets involved.

At SL, you calculate enthalpy changes using calorimetry, Hess's law and average bond enthalpies. HL adds Born-Haber cycles for ionic lattice enthalpies, plus entropy changes and Gibbs free energy (ΔG = ΔH − TΔS) to predict whether a reaction is spontaneous at a given temperature — content examined in Reactivity 1.2 and 1.4 of the current guide.

The biggest one, every single cohort I teach, is forgetting heat loss to the surroundings and the calorimeter itself — which makes the experimental enthalpy value look smaller (less exothermic) than the data booklet's theoretical value. Students also confuse the mass of the solid reactant with the mass of the solution when substituting into q = mcΔT.

Full marks come down to showing every step: the correct formula, correct substitution with units, the correct sign, and a final answer with kJ mol⁻¹ stated to a sensible number of significant figures. Examiners award method marks even when your final number is wrong, so never skip lines of working just to save time.

Three mistakes cost the most marks: using the mass of the solid instead of the solution, forgetting the negative sign for exothermic reactions, and rounding too early in a multi-step Hess's law calculation. Each is easy to avoid once you know an examiner is specifically checking for it.

Hess's law lets you calculate an enthalpy change you can't measure directly by combining known enthalpy changes from other reactions, since ΔH depends only on the initial and final states, not the pathway taken. IB exams typically give you two or three equations to combine with correct multipliers and signs before subtracting.

The Born-Haber cycle is an HL-only application of Hess's law that breaks the formation of an ionic compound into steps — atomisation, ionisation energy, electron affinity and lattice enthalpy — so you can find a lattice enthalpy that can't be measured directly. It's part of Reactivity 1.2 and regularly appears as a 6–8 mark structured question.

It appears across all three papers: Paper 1 tests the concepts through data-based multiple-choice questions, Paper 2 carries the heaviest weighting with multi-step calculations worth several marks each, and Paper 3 (HL and SL) can bring in experimental data analysis linked to a calorimetry practical.

Yes — calorimetry is one of the most common IA choices because it's accessible at both SL and HL and gives you a genuine independent variable to manipulate, such as fuel type, alcohol chain length, or concentration. It suits the methodology and data-processing criteria well, but your heat-loss error analysis needs to be thorough to score highly.

The IB Chemistry data booklet gives you the specific heat capacity of water (4.18 J g⁻¹ K⁻¹), average bond enthalpy values, and standard enthalpies of formation and combustion for common substances. You're expected to know which table to pull data from rather than memorise the values yourself.

Struggling with one calculation topic early in Year 1 isn't a reason on its own to rule out HL — enthalpy and calorimetry are taught near the start of the course, and most students who find q = mcΔT shaky in the first term are comfortable with it by the mocks. What matters more is how they cope with the multi-step reasoning in Hess's law and entropy questions later on.

The combination that works best is targeted calculation practice alongside concise notes explaining sign conventions and formulas, rather than re-reading a textbook chapter. On RevisionPrep, the Chemistry question bank and Topical Worksheets are organised by subtopic — including Reactivity 1 — so your child can drill enthalpy calculations specifically rather than working through everything at once.