Consider a container with two gases. The gas particles will move freely, colliding with each other and with the walls of the container. Since there is a lot of empty space between the particles they will also miss each other.

Imagine we could measure the kinetic energy of each particle. What we would find is that in any population of these particles there will be a distribution of kinetic energies. Only some particles will have enough energy to react. These are the particles that are represented by the shaded area on the graph below.

INS [AE_CI_01_IMA]

If we heat the container energy is transfered to the particles. Some of the particles move faster and their kinetic energy increases. Now the distribution of energies is different at this new temperature. You can see that the distributions is shifted to the right.

INS [AE_CI_02_IMA]Now a greater proportion of the particles have energy above the threshold energy for a reaction to take place. This means there can be more successful collisions per second. So the rate of reaction increases.

This threshold energy is called activation energy and is represented using the symbol E_A.