Chapter summary
Reaction Kinetics, at a glance
Extract a rate law from data, test a mechanism, and explain how conditions and catalysts change reaction speed.
A-Level 9476 (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Rate of reaction | The change in concentration of a reactant or product per unit time. |
| Order of reaction with respect to a reactant | The power to which the concentration of that reactant is raised in the experimentally determined rate equation. |
| Overall order | The sum of the orders with respect to each reactant in the rate equation. |
| Rate constant, k | The proportionality constant in the rate equation. It is constant at a given temperature (and for a given catalyst). |
| Half-life, t1/2 | The time taken for the concentration of a reactant to fall to half of its initial value. |
| Rate-determining step | The slowest step in a reaction mechanism. |
| Activation energy, Ea | The minimum energy that colliding particles must have for a reaction to occur. |
| Catalyst | A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy, and is chemically unchanged at the end of the reaction. |
Find orders from rate data or from a given elementary mechanism, never from the overall coefficients. A zero-order concentration-time graph is a straight line. A first-order graph has a constant half-life. Work out the units of k from the rate units and the overall order.
A higher concentration makes collisions more frequent. A higher temperature also makes collisions more frequent, and, more importantly, raises the fraction with energy at or above Ea, so k increases. A catalyst gives a new pathway with a lower barrier. The energy distribution and the equilibrium constant stay the same at that temperature. Enzymes also show active-site specificity and are sensitive to temperature and pH.
Quick recall
Define order of reaction with respect to a reactant.