Equilibrium and Changes to Pressure

According to Le Châtelier’s Principle (LCP), if the pressure of an equilibrium reaction involving gases is increased, then the system will shift in the direction of the least moles of gas to partially counteract this change.

For example, if the volume of a closed system with equilibrium reaction...

...is decreased at a constant temperature (thus increasing the pressure), then the equilibrium will shift to the right (the side with the least moles of gas) and particlly counteract this change in pressure.

Similarly, if the volume of the same system is doubled at a constant temperture, then the equilibrium will shift to the left (the side with the greater number of moles of gas) and partially counteract this decrease in pressure.

However, LCP does not provide an explanation for these predictions - for that, we must look to collision theory.

When there is a change in the volume of an equilibrium system, the change in concentration for the side with more gaseous particles will be larger than the change in concentration for the side with less gaseous particles. This means that the change in the rate of reaction for the side with more gaseous particles will be larger than the change in the rate of reaction for the side with less gaseous particles.

For example, if we were to re-examine the doubling of volume for the system...

The decrease in the concentration of A is larger than the decrease in the concentration of B.

This means that the rate of forward reaction will decrease more than the rate of reverse reaction. Hence, the equilibrium position shifts to the left (the side with more gaseous particles).