Now let's take a look at a reversible reaction when we vary the pressure of the system. One method that can be used to produce hydrogen is in the "Steam Methane Reforming" reaction, according to the equation below:


The way in which the principle works where pressure variation is concerned is to try to reduce the volume to compensate for the increased pressure, if we increase the volume of the system we decrease the pressure and so the process attempts to do this.


From the equation above we can see that there are more gas molecules on the left-hand side than there are on the right. More correctly we should be talking in terms of moles by saying that on the left-hand side of the equilibrium we have two moles of gas, but on the right-hand side of equilibrium we have four moles of gas. Stop at this point and study the equation if you're not sure where this comes from.


If we increase the pressure in the system, Le Chatelier's principle states that the equilibrium will be shifted in favour of that side of the equation which has the lowest moles of gas, trying to keep the gas pressure down by moving to the side with fewer gas particles. In other words a pressure increase in this system would favour the reverse reaction as the system attempted to compensate by reducing the overall volume.

You should be able to work out for yourself that if we decrease the pressure of the system, the principle will attempt to compensate for the reduction in pressure by the production of more molecules/moles. Doing this will be the system's way of trying to bring the pressure of the system back up.