Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Collision Theory
What is Collision Theory?
The model that explains reaction rates by requiring reacting particles to collide, and to collide with combined energy at least equal to the activation energy. Rate is therefore governed by the frequency of successful collisions rather than by the total number of collisions.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Collision Theory in context
Chemical reactions are changes in which new substances form as bonds break and re-form between particles, and this chapter answers three questions about them. Rate asks how fast a reaction happens, and is governed by collision theory: particles must collide with combined energy at least equal to the activation energy, \(E_\mathrm{a}\), and factors such as concentration, pressure, surface area, temperature and catalysts all work by changing how often successful collisions occur. Equilibrium asks how far a reversible reaction goes before the forward and reverse rates become equal. Redox asks which species lost electrons and which gained them, tracked through oxidation numbers.
Examiner tips on Collision Theory
- The word that carries the explanation. A rate explanation is only complete when it contains the word successful, or an equivalent such as “collisions with energy greater than or equal to the activation energy”. “More collisions, so faster” leaves out the entire energy condition, which is the half of collision theory that does the explaining.