IB Chemistry Reactivity 2.3: why 'how far' is a number, not a feeling
IB Chemistry Reactivity 2.3 explained: Kc, Kp, Le Chatelier, ICE tables and the Paper 1/Paper 2 split that decides a 5 from a 7 in the IB Diploma.
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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
What is the difference between Kc and Kp in IB Chemistry Reactivity 2.3?
Kc is the equilibrium constant written using concentrations of aqueous species and gases, in mol dm⁻³. Kp is the gas-phase analogue written using partial pressures, in kPa or atm. At HL, candidates are expected to know the relationship Kp = Kc(RT)^Δn, where Δn is the change in the number of moles of gas between reactants and products.
Does changing the concentration of a reactant change the value of Kc?
No. Kc is constant at a given temperature. Changing the concentration of a reactant or product shifts the position of equilibrium, which changes the individual concentrations at the new equilibrium, but the ratio defined by the Kc expression returns to the same value. Only a change in temperature alters Kc.
Why are pure solids and pure liquids omitted from the equilibrium expression?
The activity of a pure solid or pure liquid is defined as 1, so including them in the numerator or denominator of the Kc expression would not change the numerical value. The IB mark scheme accepts the convention of omitting them, and a candidate who includes them in a heterogeneous equilibrium such as CaCO₃(s) ⇌ CaO(s) + CO₂(g) will be marked down.
How many marks is a Reactivity 2.3 question worth on IB Chemistry Paper 2?
On Paper 2, a typical Reactivity 2.3 structured question is worth 4 to 6 marks at SL and 8 to 12 marks at HL, often split between a Kc calculation and a Kp calculation. The marks are distributed across the equilibrium expression, the ICE table, the algebraic solution, the substitution, the units, and the final answer.
Is Kp on the SL syllabus for IB Chemistry?
No. Kp is an HL extension to Reactivity 2.3. SL candidates are expected to know Kc for homogeneous and heterogeneous equilibria, but not partial-pressure calculations. A Paper 1 SL item will not test Kp directly, although it may test the qualitative idea that partial pressure is proportional to mole fraction.