Collision frequency

Advanced

Number of collisions per second.

Each molecule makes about v/λ collisions per second: roughly 5×10⁹ per second in air at STP.

Worked example

Ensures rapid thermal equilibrium.

Common mistake

Increases with density and temperature.

In more detail

The collision frequency is the average number of collisions per second for a molecule. It equals the mean speed divided by the mean free path.

Chemists estimate reaction rates in gases from collision frequency.
Vacuum engineers estimate how often molecules hit surfaces.
Chemists estimate how often reactant molecules collide in a gas, which sets the rate of gas-phase reactions.

Why is it so high?

Molecules move quickly and are close together, so collisions happen billions of times per second.

Does temperature change it?

Yes, higher temperature increases speed and collision frequency.

Does every collision cause a reaction?

No. Only collisions with enough energy and the right orientation lead to a reaction.