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, E_{a},
such that if the kinetic energy of two colliding molecules along
their line of approach is greater than E_{a}
they will react, but if the collision energy is less than E_{a}
the molecules will rebound without reacting (see opposite).
