Collision Theory
Collision theory explains why a reaction needs more than particles simply touching: they need enough energy and the right orientation to react.
The two conditions for a reaction
Particles are colliding constantly in any gas or liquid, yet most collisions do nothing at all. Collision theory says a collision only leads to a reaction when both of these are true:
- Enough energy — the particles' combined kinetic energy must be at least the activation energy (Ea), the minimum energy needed to start breaking old bonds.
- Correct orientation — the particles must hit each other with the right atoms facing, so new bonds can form where old ones broke.
Anything that raises the frequency of effective collisions — more particles with enough energy, hitting more often, in the right orientation — speeds the reaction up: higher temperature, higher concentration, greater surface area, and catalysts (which lower Ea).
Elastic collision calculator
The maths of a head-on collision is the same whether it is reacting molecules or billiard balls. Try two masses and velocities below:
| Object 2's velocity after | 5.333 m/s |
|---|
Assumes a head-on elastic collision in 1D: both momentum and kinetic energy are conserved.
Exemple résolu
Doubling the concentration of a reactant roughly doubles the collision frequency, but raising the temperature by just 10 °C can double or triple the reaction rate — because it both speeds particles up (more frequent collisions) and shifts far more of them above the activation energy at once.
See momentum and collisions acted out in 3D in Momentum et Chocs below, or balance the resulting equation in the Equation Balancer.
Foire aux questions
What is collision theory?
The idea that a chemical reaction happens only when reactant particles collide with enough energy (at least the activation energy) and in the correct orientation to break and form bonds.
What two conditions does collision theory require for a reaction?
Sufficient energy — at least the activation energy, Ea — and correct orientation, so the reactive parts of the colliding particles actually meet.
How does temperature affect collision theory?
A higher temperature gives particles more kinetic energy, so they move faster, collide more often, and a larger fraction of collisions clear the activation energy — both effects speed up the reaction.
How does concentration affect collision rate?
A higher concentration packs more particles into the same volume, so collisions happen more often, which increases the reaction rate.
