How does pressure influence equilibrium in gases?

Pressure influences equilibrium in gases by shifting the equilibrium position 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 change in conditions is applied to a system at equilibrium, the system will adjust itself to counteract that change. In the context of gases, if the pressure is increased, the system will try to reduce it. It does this by favouring the reaction that produces fewer gas molecules, as fewer gas molecules will exert less pressure.

For example, consider the equilibrium between nitrogen gas, hydrogen gas and ammonia:

N2(g) + 3H2(g) ⇌ 2NH3(g)

In this reaction, four molecules of gas (one nitrogen and three hydrogen) react to form two molecules of ammonia. If the pressure is increased, the equilibrium will shift to the right, towards the side with fewer gas molecules (the ammonia side), in an attempt to reduce the pressure. Conversely, if the pressure is decreased, the equilibrium will shift to the left, towards the side with more gas molecules (the nitrogen and hydrogen side), to increase the pressure.

It's important to note that this only applies to reactions where there is a change in the number of gas molecules. If the number of gas molecules is the same on both sides of the equation, a change in pressure will have no effect on the equilibrium position.

Remember, this is a simplification and in reality, the situation can be more complex. Other factors such as temperature and concentration can also affect the position of equilibrium. But for your GCSE Chemistry, understanding how pressure affects equilibrium in gases through Le Chatelier's Principle is a good starting point.