Le Chatelier's Principle
Volume change of a gaseous mixture.
Increasing the volume of a gaseous equilibrium mixture causes a decrease in the partial pressure of each species present . The reaction therefore moves to the side with most particles in an attempt to increase pressure. Decreasing the volume drives the reaction to the side with least particles. For example consider the reaction H2(g) + O2(g) <=> H2O(g) at equilibrium in a reaction chamber. Decreasing the volume of the equilibrium mixture cause the pressure to increase. The reaction therefore drives forwards and decreases the amount of particles in an attempt to lower the pressure.
ONLY a change in pressure due to a volume change will effect the equilibrium position. Increasing the pressure of a gaseous mixture by adding an inert gas will have no effect on the reaction as the concentration of the reactants and products are not changed.
We can try and visualise what happens to the equilibrium position with an animation as shown on the left.
Consider the reaction on the left. It shows two particles reacting to form one particle.
As the volume of the reaction vessel decreases the partial pressure of reactants and products suddenly increases. The system responds by trying to reduce the pressure. This involves the reaction moving in the direction which creates less particles, that is the reaction moves to the right.
Initially the reaction has 8 particles at equilibrium .
After the pressure is increased the reaction moves such that only 6 particles are present and the pressure is decreased.
Note that the pressure never returns to its original value.
Some equilibrium mixtures will not be effected by a change in volume. Take the reaction below, the same number of particles appear on the left and the right so there is no way the reaction can react to a change in pressure due to a volume change.
