Collision theory
For two molecules to react, they must collide with enough energy and with the right orientation. Only a small fraction of collisions do both. More concentration, higher temperature and a bigger surface raise the number of effective collisions.
Last updated
Analogy
Two keys must hit a lock correctly: hard enough and the right way round, or nothing happens.
Example
Higher concentration: more collisionsHigher temperature: more energetic collisionsBigger surface area: more contactCatalyst: easier path
In practice
Powdered sugar burns more fiercely than a sugar cube because more surface is exposed to collisions with oxygen.
Practice
Flashcards
- What two things must a collision have to react?Enough energy and the right orientation.
Interview questions
- Explain with collision theory why raising temperature and adding a catalyst both increase rate.
- Temperature raises the fraction of molecules above Ea
- Catalyst gives a lower-Ea path
- Both raise the number of effective collisions
- Catalyst does not change ΔH
Questions people ask
Why does raising the temperature speed up reactions so much?
A small temperature rise increases the fraction of molecules that have at least the activation energy, and that fraction grows exponentially. Faster molecules also collide a bit more often, but the energy effect is much bigger. A 10 °C rise often doubles the rate.
What is the steric factor?
The steric factor accounts for orientation. Even when molecules collide with enough energy, they react only if their reactive parts meet. Many collisions are in the wrong orientation and just bounce apart. This explains why measured rates are lower than predicted from collision energy alone.