Reaction Rates & Collisions
Reactions happen one collision at a time — so everything that makes collisions harder, faster or more frequent makes chemistry quicker.
Builds on: 5.1 Exothermic & Endothermic3.2 Gases & the Ideal Gas Law
Collision theory in one sentence
Two particles react only if they collide, collide hard enough (≥ Ea), and collide pointing the right way. Reaction rate is just successful collisions per second — so every rate trick in chemistry is a way of rigging that count.
The four levers
- Temperature — hotter particles move faster: more collisions, and a far bigger share of them clear the energy bar. Rule of thumb: +10 °C roughly doubles many everyday reaction rates. This is why fridges exist — food spoilage is chemistry, and cold chemistry is slow chemistry.
- Concentration (or gas pressure) — more particles per litre, more meetings per second. Pure oxygen turns a glowing ember into a torch.
- Surface area — reactions happen where phases touch. A log burns for an hour; the same wood as flour dust can explode (a genuine hazard in mills and silos).
- Catalyst — last lesson’s tunnel: same collisions, lower bar, vastly more of them count.
The energetic tail of the speed distribution grows exponentially with temperature. A modest 10% rise in average speed can double or triple the tiny fraction of collisions that clear the bar — small cause, huge effect. Life exploits the same math in reverse: a fever of just +3 °C measurably accelerates your immune chemistry.
⚗️ Lab — The Collision Counter
Red A and cyan B particles react to green AB — but only fast collisions count.
- Raise the temperature and watch the rate meter respond disproportionately.
- Double the concentration at fixed temperature — roughly what happens to the rate?
- Add the catalyst: same speeds, lower bar, green everywhere.