Temperature and equilibrium

The average kinetic energy of any molecules depends on their temperature.   However, all molecules do not travel at the same speed, so even at one temperature, some molecules will be travelling slowly, while others will move more quickly.  As the temperature is increased, the molecules will have a more dispersed range of speeds, with more molecules having higher velocites, and larger kinetic energies.

The different kinetic energies in a collection of molecules is called its Maxwell-Boltzmann distribution.  Here, the energy of molecules at a low temperature is shown by the blue line.  The red line represents the energy distribution of molecules that have a higher temperature.

Let's show this in a bar graph form, with the more intense color representing a greater number of molecules.  At a high temperature, the bar graph is longer and more spread out than it is at a cold temperature.  Now, if we rotate the graph around, we can show energy increasing in the vertical direction.  Near the bottom of the bar graph, there are very few molecules, and they are travelling slowly.  Near the top of the graph there are also few molecules, but they are travelling much more rapidly.  It is obvious that for a high temperature, there are more molecules with higher energy.

A chemical reaction can only occur if molecules have enough kinetic energy to pass over the activation energy barrier.  Because an equilibrium reaction takes place in both the forward and reverse directions, there are two activation energies we need to be aware of.  One is for the forward reaction, from reactants to products.  The other is for the reverse reaction, from products back to reactants.

In the forward direction, when the temperature is low, only those molecules shown outlined in green will have enough energy to react.  If the temperature was higher, then many more molecules would do so.  Similarily, in the reverse direction, only some of the molecules can react, though, because its activation energy is lower, many more molecules do.

If we had a way to count the molecules capable of making it over the activation energy barrier in the two directions, we could compare the rates when the temperature changes.   Notice that an increase in temperature causes an increase in rate in both the forward, and reverse directions.  However, there is a bigger change in rate when the activation energy is high.

Therefore we can conclude that changing temperature will change the position of equilibrium.  The reaction with the higher activation energy is affected more by a temperature change.