Cambridge O Level Chemistry · Syllabus 5070 · Chemical Reactions
Successful Collision
What is Successful Collision?
A collision between reacting particles in which the particles have a combined energy at least equal to the activation energy, so that bonds break and products form. The rate of a reaction is set by how many successful collisions occur per unit time, not by the total number of collisions.
This definition is part of the Chemical Reactions chapter in Cambridge O Level Chemistry.
Successful Collision 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 Successful Collision
- The two-sentence rule for “explain” in this topic. Sentence one names the particle-level change. Sentence two names the consequence for successful collisions or for the position of equilibrium. An answer with only the first sentence has described rather than explained, and an “explain” question is asking for the second one.
- Marking yourself honestly. Award a mark only where your answer contains the idea in the marking point, not merely a related word. If you wrote “more collisions” where the point says “more successful collisions”, that is not a mark. These marking points are original to this chapter and are a guide to completeness, not a Cambridge mark scheme, so treat your score as a map of what to revise rather than as a prediction. Whatever the total, the useful step is the same: list the sections that produced the losses and return to those, rather than repeating the paper.
Questions students ask about Successful Collision
Why does increasing concentration speed up a reaction?
A more concentrated solution packs more particles into the same volume, so collision frequency rises and more successful collisions — ones with combined energy at least equal to the activation energy — happen per second. Concentration does not change how fast individual particles move; only temperature does that.
What is the examiner-safe way to answer an "explain the rate" question?
Use two sentences: the first names the particle-level change, such as higher concentration or smaller particle size; the second states the effect on successful collisions, using the word "successful" or "energy at least equal to the activation energy". Writing only "more collisions, so faster" leaves out the energy condition and only describes rather than explains.