Collision Theory
Reactions occur when particles collide with enough energy and the right orientation.
About this topic
Collision theory says that for a reaction to occur, particles must collide with enough energy (at least the activation energy) and with the correct orientation. Rate depends on the frequency of effective collisions. Increasing concentration, pressure (for gases), surface area, temperature or adding a catalyst all increase the rate.
Most collisions do not result in reaction, either because they are too gentle or because the molecules strike the wrong way. Raising concentration puts more particles in the same space, so collisions are more frequent. Raising temperature makes particles move faster, so collisions are more frequent and, importantly, a much larger fraction have energy above Ea. Crushing a solid exposes more surface for collisions. A catalyst lowers Ea so more collisions succeed.
Example: powdered zinc reacts with acid much faster than a single granule; hot water dissolves sugar more quickly than cold; and flour dust can explode in a mill because of enormous surface area, although a heap of flour barely burns. Ketchup and kitchen refrigeration illustrate the temperature effect.
A misconception is that raising temperature only increases collision frequency; the main effect is on the energy of the collisions. Another is that every collision causes a reaction, when in reality only a tiny fraction does.
Key ideas
- Higher concentration means more collisions
- Higher temperature means harder collisions
- Surface area matters
Quick check
Use cases: where Collision Theory helps
- Homework: review the key idea behind Collision Theory before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Collision Theory without notes.
- Visual learning: play with the interactive lab to see how Collision Theory changes when you adjust its inputs.
- Teaching a friend: explain Collision Theory in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Collision Theory?
Reactions occur when particles collide with enough energy and the right orientation.
What will I learn in Collision Theory?
Higher concentration means more collisions. Higher temperature means harder collisions. Surface area matters.
Is Collision Theory part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Collision Theory lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Collision Theory?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.