IBDP · Chemistry SL
Reactivity: What Drives Chemical Reactions?
Reactivity 1.1–1.4
30 min readStandardCore Reactivity 1 content — calorimetry calculations and Hess's Law cycles appear in almost every Paper 1 and Paper 2 series. Entropy/Gibbs free energy (Reactivity 1.4) is HL-only and never appears on an SL paper.
Every reaction is an energy trade: bonds break (costs energy), new bonds form (pays energy back), and the balance is . This topic is four different ways of getting at that number — measure it directly with a thermometer, calculate it indirectly with a Hess cycle, judge fuels by it, and (HL only) work out whether it even matters compared to entropy.
Overview
only tracks heat exchanged at constant pressure — nothing else. Reactivity 1 builds outward from that one quantity: how to measure it in a school lab, how to calculate it without ever running the reaction, how it's used to compare fuels commercially, and (at HL) why it isn't the full story on whether a reaction actually happens.
- Calorimetry gives you a number from a thermometer — but every simple calorimetry experiment has known, nameable sources of error, and examiners want those named, not vaguely gestured at.
- Hess's Law exists because enthalpy is a state function: the total change only depends on start and end points, so you can build a legal detour through data you can actually measure.
- Fuels get judged by two different numbers that get mixed up constantly — enthalpy of combustion in versus energy density in .
- HL only: spontaneity is decided by , not by alone — which is exactly why some endothermic processes (like an instant cold pack) still happen on their own.
The shape of the chapter
Command terms that decide your marks here
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Obtain a numerical answer, showing all relevant working | AO2 | Each formula line (q = mcΔT, then n, then ΔH = −q/n) is its own mark — a bare final number with the wrong sign loses the last mark even if the arithmetic was right. |
| Determine | Obtain a numerical or algebraic answer using given data or a data booklet value | AO2/AO3 | Quote the actual data value you used (e.g. ΔHf° figure) — unsupported working loses a mark even with the correct answer. |
| Deduce | Reach a conclusion from the information given, not from memorised rules alone | AO3 | Credit only given if your reasoning explicitly references the data in the question (e.g. a given sign of ΔS), not a generic textbook statement. |
| Sketch | Draw an approximate diagram showing the important features and relative positions | AO2 | Axes must be labelled (enthalpy/reaction progress) — an unlabelled sketch loses the mark even if the shape is correct. |
| Evaluate | Weigh up strengths and limitations to reach a judgement | AO3 | Name specific limitations (heat loss, container heat capacity ignored) — 'there could be errors' earns nothing. |
| State | Give a specific answer with no supporting argument required | AO1 | One correct word, value or sign is enough — don't spend time justifying it. |
Key point
Every calorimetry answer is only as good as its assumptions — no heat loss, container has negligible heat capacity, reaction is instantaneous. When asked to evaluate, name the exact assumption you're breaking, not just 'there might be errors'.
Overview