The most obvious factor that affects k is temperature; k increases as T increases. All reactions go faster at higher temperatures, because the molecules move faster, and collide more often and more effectively.

We could calculate k from basic principles if we knew the answers to two questions:
1. How frequently do two molecules in a reaction collide?
2. What is the probability that when they do collide, they will react instead of rebounding and going harmlessly on their way?

The collision frequency of gas molecules depends on the number of molecules in a given volume, how large they are, and how fast they are moving. The measurable variables that control these are concentration, molecular weight, and temperature.

Molecules move faster as the temperature increases, and for a given temperature (or kinetic energy), lighter molecules move faster than heavier ones. The kinetic theory of gases can give us an exact expression for the collision frequency.

What is the probability that any given collision will lead to chemical reaction rather than to recoil?

The simplest possible model of chemical reactivity proposes a threshold energy, Ea, such that if the kinetic energy of two colliding molecules along their line of approach is greater than Ea they will react, but if the collision energy is less than Ea the molecules will rebound without reacting (see opposite).