A 7 in IB Chemistry Reactivity 2 starts with stoichiometry
A senior-tutor walk through IB Chemistry Reactivity 2: how much, how fast, and how far. Mole ratios, rate expressions, and equilibrium constants mapped to Paper 1 and Paper 2 marks.
Volkan Demir2 min read
Note (July 2026): This article was written against the pre-2025 IB Chemistry syllabus. From the May 2025 examination session a new guide is in force: Paper 3 and the option units were removed and the content was reorganised into Structure 1-3 and Reactivity 1-3. The article is kept online for reference. For the current syllabus and exam structure see IB Chemistry tutoring.
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Frequently asked questions
How much of the IB Chemistry Paper 2 comes from Reactivity 2?
On a typical Paper 2, Reactivity 2 contributes between 15 and 22 of the 50 SL marks or 25 and 35 of the 90 HL marks, distributed across Section A structured questions and one or two Section B extended-response items. Stoichiometry, kinetics and equilibrium are the most common homes for the section B choice questions.
Do SL candidates need to learn the Arrhenius equation?
No. The Arrhenius equation in the form ln k = -E_a/RT + ln A, the interpretation of a ln k against 1/T plot, and the calculation of activation energy from the slope are all HL extension material. SL candidates are expected to describe the effect of temperature on rate qualitatively and to interpret a Maxwell-Boltzmann distribution diagram, but not to extract E_a numerically.
Should Kc and Kp be written with units?
By convention Kc and Kp are treated as dimensionless, because activities and fugacities are used in their rigorous definitions. In an IB exam, omitting the unit is fine. Writing a unit is not penalised, but writing an inconsistent unit (for example mol dm-3 on a second-order Kc) is a small mark loss. The safe choice is to leave the value as a number.
How do I tell if a Paper 1 question is testing 2.1, 2.2 or 2.3?
Read the units in the stem. A question with masses, volumes in cm3, and concentrations in mol dm-3 is a 2.1 stoichiometry question. A question with rates, time units, and a table of initial concentrations is a 2.2 kinetics question. A question with partial pressures in kPa, an equilibrium symbol, or the words 'at equilibrium' is a 2.3 equilibrium question. The unit cue is more reliable than the surface wording.
What is the single most efficient revision activity for Reactivity 2?
Working through the past Paper 2 Section A questions on stoichiometry, kinetics and equilibrium, marking them with the IB mark scheme, and rewriting the lost marks as a one-sentence lesson on the page. The lesson sentence is the unit of progress. Two or three cycles of this across the weeks before the exam produces a measurable band shift, whereas rereading the syllabus guide does not.