IB Chemistry Entropy & Gibbs Free Energy (HL): Your Questions Answered

Entropy (S) measures how many ways energy and particles can be arranged in a system — loosely, how disordered it is. In IB Chemistry HL, you use entropy to judge whether a reaction is thermodynamically favourable, not whether it's fast. More gas particles or more moles of product usually means higher entropy.

Gibbs free energy (G) combines enthalpy and entropy into one value that tells you whether a reaction happens spontaneously at a given temperature. The formula is ΔG = ΔH − TΔS, with T in kelvin. A negative ΔG means the reaction is spontaneous as written; a positive ΔG means it isn't, at that specific temperature.

Enthalpy (ΔH) measures heat energy transferred in a reaction; entropy (ΔS) measures disorder; Gibbs free energy (ΔG) combines both to tell you whether a reaction is spontaneous at a given temperature. Enthalpy and entropy are separate physical quantities — Gibbs free energy is the calculated value that actually predicts feasibility.

Entropy and Gibbs free energy sit in Reactivity 1.4, an HL-only sub-topic under the current DP Chemistry guide, first assessed in 2025. SL students cover enthalpy, Hess's law and bond enthalpies but stop short of spontaneity calculations — the extra abstraction and the temperature-dependent maths are reserved for HL's additional teaching hours.

Use ΔS°reaction = ΣS°(products) − ΣS°(reactants), using standard entropy values from the data booklet or the question itself. Add up entropies of all products, subtract the sum for reactants, and keep units in J K⁻¹ mol⁻¹ — mixing these up with ΔH's kJ mol⁻¹ is one of the most common calculation errors I mark.

Calculate ΔG = ΔH − TΔS; a negative result means the reaction is spontaneous at that temperature. Because T multiplies ΔS, a reaction can flip from non-spontaneous to spontaneous — or vice versa — as temperature changes, which is exactly why some reactions only proceed above or below a specific threshold.

The three mistakes I see most in mock papers: mixing up units (J versus kJ) when substituting into ΔG = ΔH − TΔS; forgetting to convert Celsius to kelvin before calculating; and assuming a negative ΔG means a reaction is fast, when it only confirms the reaction is thermodynamically favourable, not that it's kinetically quick.

Yes — it's one of the more abstract HL-only topics, mainly because it asks you to reason about a system without a colour change or a diagram to anchor your intuition. Students who've memorised patterns for Le Chatelier's principle often struggle here, since spontaneity questions demand genuine calculation, not recall.

It appears in Paper 1 (data-based multiple choice), Paper 2 (calculation and extended-response questions), and occasionally Paper 3 (data analysis linking entropy to reaction feasibility). Expect to calculate ΔS° from standard entropy values, calculate ΔG from ΔH and ΔS, and explain spontaneity using command terms such as 'determine' and 'predict'.

Yes — the IB Chemistry data booklet lists ΔG° = ΔH° − TΔS° in section 1, so you don't need to memorise it. You do need to remember to convert temperature to kelvin and match units (kJ for ΔH and ΔG, J K⁻¹ for ΔS) before substituting any values.

Recent Paper 2 questions typically give you standard entropy or enthalpy values and ask you to calculate ΔS, ΔH or ΔG for a named reaction — combustion, decomposition, or an industrial process like the Haber process are common contexts. Examiners often follow with 'state whether this reaction is spontaneous at [temperature]', testing interpretation, not just arithmetic.

Entropy and Gibbs free energy are a genuine reason HL Chemistry feels like a bigger step up — it's a wholly HL-only sub-topic with no SL equivalent, layering extra thermodynamic reasoning on top of the enthalpy content both levels share. It's not the hardest HL topic, but it's one where the maths and the concept both bite at once.

IB HL Chemistry's entropy and Gibbs free energy content sits roughly level with A-Level Chemistry's thermodynamics coverage, though the IB folds it into core HL content rather than an optional module. That means your child can't choose to skip it if they're studying HL Chemistry — every HL student meets Reactivity 1.4.

Past paper questions are non-negotiable here — this topic rewards repeated calculation practice far more than re-reading notes. Look for topical worksheets that isolate Reactivity 1.4 questions by sub-skill, and revision notes that summarise the sign-based spontaneity rules in one clear table your child can memorise in ten minutes.

Sit with your child while they redo one calculation from scratch, out loud, rather than watching them re-read notes — entropy and Gibbs free energy is a 'do it, don't just read it' topic. Ask them to explain why ΔG can flip sign with temperature; if they can't, that's the gap worth targeting before the next mock.