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2.2.5—Catalysts

Syllabus
First assessment 2025
Objective
2.2.5
Level
SL

Catalysts and Activation Energy

A catalyst provides an alternative reaction pathway with lower activation energy. It does not change the energy levels of the reactants or products.

Because the catalysed Ea is lower, a larger fraction of the same distribution lies beyond the threshold. The reaction therefore has more effective collisions at the same temperature.

At fixed temperature a catalyst does not change the Maxwell–Boltzmann distribution; it moves the threshold to a lower Ea, increasing the area beyond it. On an energy profile it changes the pathway and peak, not ΔH, reactant/product energies or the equilibrium constant.

Showing a Catalyst on Energy Diagrams

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

Sketch the Maxwell-Boltzmann energy distribution curve for this reaction. Label the activation energy with and without a catalyst on the diagram.

Rate and Mechanisms Summary

Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.

Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.