Cambridge IGCSE Chemistry · Syllabus 0620 · 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 IGCSE Chemistry.
Examiner tips on Collision Theory
- Describe against explain is the tier line in miniature. Core statement 6.2.1 says describe the effect of each rate factor; Supplement statement 6.2.6 says describe and explain the same five effects using collision theory. A Core candidate who writes a collision-theory explanation has not earned extra marks, and has spent time they needed elsewhere. An Extended candidate who writes only the effect has answered half the question.
- 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.