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2.2.3—Factors affecting rate

Syllabus
First assessment 2025
Objective
2.2.3
Level
HL

Factors Affecting Rate

Change Main collision consequence
concentration or pressure up more frequent collisions
surface area up more collisions at a solid surface
temperature up more frequent and more energetic collisions
catalyst alternative lower-Ea pathway

Explain a predicted rate change through collision frequency or the fraction of effective collisions, not just by saying particles move faster.

Powdered CaCO₃ reacts faster than equal-mass chips because more surface sites are exposed, not because its particles have higher kinetic energy. For each changed condition, identify exactly what changes—collision frequency, energy distribution or pathway—and hold other variables constant in a fair comparison.

Predicting Rate Changes

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

The student then carried out the experiment at other acid concentrations with all other conditions remaining unchanged.

[H+]/ mol dm−3\left[\mathbf{H}^{+}\right] / \mathbf{~ m o l ~ d m}^{-\mathbf{3}}Relative rate of reaction
0.050.0025
0.100.0051
0.200.0100

State and explain the relationship between the rate of reaction and the concentration of acid.

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.