This threshold energy, Ea
is called the energy of activation.
It can be thought of as a barrier that the reaction must surmount
before it can go to completion (see fig 10 opposite).
From the usual random distribution of
energy among molecules, the fraction of molecules that have enough
kinetic energy to collide with energy Ea
or greater is
This fraction increases with temperature.
At absolute zero, it has the value:
f = e-Ea/0
This is reasonable, since if the molecules
are motionless at absolute zero, none of them have enough energy
As the temperature approaches infinity,
the fraction of molecules capable of reacting approaches unity,
no matter how large the activation energy, Ea,
At any finite temperature, the larger
the activation energy, the smaller the fraction of molecules that
have enough energy to surmount this barrier and react. The simple
collision theory of chemical
reaction states that the rate
constant can be represented by
in which A is a constant derived
from the Collision
frequency. It depends on the molecular weight, the molecular
diameter, and the square root of the temperature.