6.2.7 The Effect of Pressure Changes on Equilibrium (HT only)

In gaseous reactions, changes in pressure can affect the position of equilibrium. The effect of pressure changes depends on the total number of molecules on each side of the balanced equation.

Increase in Pressure

When the pressure is increased, the equilibrium position shifts towards the side of the reaction with the smaller number of gas molecules. This shift is based on the principles of Le Chatelier's Principle.

The increase in pressure causes the system to respond by minimising the overall number of gas molecules. Consequently, the equilibrium position shifts to the side with fewer gas molecules, as indicated by the balanced equation.

Decrease in Pressure

Conversely, a decrease in pressure causes the equilibrium position to shift towards the side with a larger number of gas molecules. The system responds to the decrease in pressure by maximising the overall number of gas molecules.

As a result, the equilibrium position shifts to the side with more gas molecules, as indicated by the balanced equation.

Conclusion

By considering the total number of gas molecules involved in the reaction, we can determine which side of the reaction will be favoured under increased or decreased pressure. Pressure changes primarily affect gaseous reactions - reactions involving only solids or liquids are not significantly influenced by pressure changes.