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wise otter.Chemistry
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Reaction Kinetics

Topic 4 of 5

Collisions, activation energy and temperature

Separate collision frequency from the fraction energetic enough to react.

A-Level 9476 (2026-2027)

Reaction needs both encounters and sufficient energy

Heating changes the energy distribution; concentration mainly changes collision frequency.

The activation energy Ea is the minimum energy barrier that must be overcome along the reaction pathway. Colliding particles need sufficient energy and an appropriate orientation to react. At higher reactant concentration, more particles occupy a given volume, usually producing more frequent relevant collisions; the measured rate law tells how strongly rate responds.

Heating changes the Boltzmann energy distribution

At higher temperature the energy distribution has a lower, broader peak and a larger fraction of particles beyond the same activation-energy threshold. Both distributions are normalised; temperature does not move the threshold.

Schematic energy scale for the same number of particles. The area to the right of Ea is the energetic fraction. At higher temperature this fraction increases substantially; collision frequency also increases because particles move faster.

The distribution begins at zero and has a long tail: particles do not all have the same energy, and no sharp maximum energy exists. Raising temperature shifts the population towards higher energies and greatly increases the fraction able to cross the barrier. Together with a moderate increase in collision frequency, this raises k and hence the rate at fixed concentrations. Do not say that heating lowers activation energy.

Adding more particles at the same temperature does not change their normalised energy distribution or the fraction above Ea. It increases the number of potentially successful collisions. A temperature change and a concentration change therefore operate differently even when both make a reaction faster.

A catalyst supplies another mechanism

More particles can react at the same temperature because the barrier is lower.

A catalyst changes the threshold, not the distribution

One energy distribution at a fixed temperature is crossed by an uncatalysed activation threshold and a lower catalysed threshold. The catalysed threshold admits a larger fraction of particles without changing their energies.

At fixed temperature the distribution stays the same. Lowering the relevant activation barrier increases the rate constant. Shading marks the fraction above the original higher barrier; catalysis also admits particles between the two thresholds.

A catalyst participates in intermediate steps and is regenerated by the end of its cycle. It accelerates the approach to equilibrium in both directions and does not change ΔH, the equilibrium constant or the equilibrium composition at the same temperature. It can be deactivated or poisoned in practice even though it is not consumed in the ideal overall equation.