Why does increasing pressure sometimes favour the spontaneity of reactions?

Increasing pressure can favour the spontaneity of reactions by shifting the equilibrium towards the side with fewer gas molecules.

In more detail, the principle that explains this behaviour is Le Chatelier's Principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In the context of pressure, if the pressure is increased, the equilibrium will shift to reduce the pressure. This is achieved by favouring the side of the reaction that has fewer gas molecules, as fewer gas molecules will exert less pressure.

For example, consider the reaction of nitrogen and hydrogen to form ammonia:

N2(g) + 3H2(g) ⇌ 2NH3(g)

In this reaction, there are four moles of gas on the reactant side (one mole of nitrogen and three moles of hydrogen) and two moles of gas on the product side (two moles of ammonia). If the pressure is increased, the equilibrium will shift to the right, towards the side with fewer gas molecules, to decrease the pressure. This means that the formation of ammonia is favoured, making the reaction more spontaneous.

However, it's important to note that this only applies to reactions where there is a change in the number of moles of gas. If the number of moles of gas is the same on both sides of the reaction, an increase in pressure will not affect the position of equilibrium.

Furthermore, the effect of pressure on the spontaneity of reactions is more significant for reactions that involve gases, as gases are much more compressible than solids or liquids. Therefore, a change in pressure can have a large effect on the volume of a gas, and hence on the position of equilibrium.

In conclusion, increasing pressure can favour the spontaneity of reactions by shifting the equilibrium towards the side with fewer gas molecules, according to Le Chatelier's Principle. However, this effect is most significant for reactions that involve gases and where there is a change in the number of moles of gas.