Reactivity: What Drives Chemical Reactions?
Reactivity 1
Every reaction either dumps energy into its surroundings or drags energy out of them — this topic is about pinning that number down experimentally (calorimetry), then explaining it using bookkeeping tricks (Hess's Law, Born–Haber cycles) before HL adds the deeper question of why a reaction goes forward at all (entropy and ). Examiners are ruthless about sign conventions and state symbols here: get one arrow backwards in a cycle and a perfectly good calculation collapses at the final line.
Overview — The Shape of the Chapter
Why this chapter exists
Reactivity 1 asks one question with three layers: how much energy does a reaction release or absorb, how do we calculate that when we can't measure it directly, and (at HL) why does a reaction happen spontaneously at all. The first layer is calorimetry and sign conventions. The second is Hess's Law, bond enthalpies and Born–Haber cycles. The third is entropy and Gibbs free energy — the HL upgrade that finally explains why some endothermic reactions still happen on their own.
- is always defined as — every sign error in this topic traces back to forgetting this order.
- Standard conditions (100 kPa, a stated , 1 mol dm solutes) must be quoted whenever a value carries the ° symbol.
- HL adds nothing new to how you measure — it adds entropy as the missing piece that finally predicts spontaneity properly.
The shape of the chapter
Command terms this topic loves to test
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Define | Give the precise textbook meaning, including every condition (e.g. '1 mole', 'standard conditions', 'excess oxygen'). | AO1 | Dropping one qualifier (e.g. forgetting 'excess oxygen' for combustion) typically costs the mark even if the general idea is right. |
| Determine / Calculate | Produce a numerical answer with full working shown. | AO2 | Working must be shown step by step; a bare correct final answer without units or sign can lose the answer mark on ECF-based mark schemes. |
| Sketch | Draw a reaction profile or graph with correct relative shape and labelled features, not to precise scale. | AO2 | Axes must be labelled and relative heights of reactants/products/activated complex must be qualitatively correct. |
| Deduce | Use given data to reach a conclusion that isn't stated outright, showing the reasoning link. | AO3 | The reasoning step (e.g. 'more gas moles → higher entropy') must be written, not just the final sign. |
| Suggest | Propose a plausible chemical explanation where more than one reasonable answer exists. | AO3 | Any chemically sound explanation is accepted — vague answers like 'heat was lost' without naming a mechanism earn nothing. |
Key point
Overview