IB Chemistry: Chemical Equilibrium & Kc — Why Students Lose Marks

Most marks vanish because students write the Kc expression with the wrong powers, forget that solids and pure liquids are omitted, or state units without checking if the expression even needs them. Examiners also penalise vague Le Chatelier answers that don't reference the specific stress and direction of shift.

It's conceptually one of the more demanding SL/HL topics because it blends algebra (ICE tables), abstract reasoning (Le Chatelier), and thermodynamics (Kc and temperature). Students who've memorised definitions without practising numerical problems tend to struggle most on Paper 2 calculation questions.

Kc uses equilibrium concentrations (mol dm⁻³); Kp uses partial pressures (usually kPa or atm) and only applies to gaseous equilibria. They're related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas between products and reactants.

Write Initial, Change and Equilibrium rows under each species in the balanced equation, using the stoichiometric ratio for the Change row and a single unknown (usually x). Then substitute the Equilibrium row values into the Kc expression and solve.

Temperature is the only factor that actually changes the value of Kc. Increasing temperature shifts equilibrium toward the endothermic direction, increasing Kc if the forward reaction is endothermic, or decreasing it if the forward reaction is exothermic. Concentration and pressure changes shift position but never change Kc's value.

Expect 'deduce' or 'predict' for Le Chatelier shift questions, 'calculate' for Kc/Kp numerical problems, 'state and explain' for linking temperature to ΔH, and 'sketch' or 'construct' for equilibrium graphs showing concentration versus time.

At HL, Kc connects to thermodynamics through ΔG° = −RT ln K. A large positive Kc means a very negative ΔG°, meaning the forward reaction is spontaneous under standard conditions. This links equilibrium directly to the Chemistry: Reactions option content on energetics.

According to the IB Chemistry guide (first exams 2025), Reactivity 2 covers the equilibrium law, Kc expressions, Le Chatelier's principle, and — at HL only — the relationship between Kc, ΔG° and temperature dependence via the Reactivity 3 thermodynamics link.

Both. Paper 1 tests equilibrium as multiple-choice, often on Le Chatelier predictions or identifying correct Kc expressions. Paper 2 features full calculation questions with ICE tables, sometimes combined with acid-base or energetics content in longer structured questions.

Yes — SL students learn qualitative Le Chatelier reasoning and basic Kc expressions, while HL students must also connect Kc quantitatively to ΔG° and temperature, often solving quadratics from ICE tables where x can't be approximated away.

Acid-base equilibria (Ka, Kb, pH calculations) use exactly the same equilibrium law logic as Kc, just applied to weak acid/base dissociation. Students who understand ICE tables for general equilibrium usually find Ka/Kb calculations far more manageable.

Topical worksheets that isolate Kc calculations from Le Chatelier reasoning let students build each skill separately before combining them in mock-style questions. Past paper practice matters most here, since equilibrium questions repeat similar structures year to year with different numbers and contexts.

For most HL students, 3-4 focused sessions of 45-60 minutes each — covering Kc expressions, one full ICE table worked problem, Le Chatelier reasoning, and (for HL) the ΔG°-K link — is enough to move from shaky to confident before a mock exam.