Revise: EquilibriaPressure and catalysts

Pressure

Changing the pressure of the equilibrium mixture can affect the position when the equilibrium involves chemicals in the gaseous state.

An increase in pressure favours the side with the lower gas volume.

In the above reaction, sulfur dioxide and oxygen react together to form sulfur trioxide.

There are three moles of gaseous reactants on the left hand side of the equation and two moles on the right hand side.

This means that an increase in pressure would move the equilibrium to the right and result in more sulfur trioxide being formed.

Pressure can only affect the position of equilibrium if there is a change in the total gas volume. The reaction between hydrogen and iodine to form hydrogen iodide is an example of a reaction that pressure does not affect.

Both sides of the reaction have two moles of gases, so changing the pressure does not favour either side of the equilibrium.

Catalysts

Adding a catalyst to a reaction at equilibrium has no effect on the position of equilibrium.

It does however allow equilibrium to be reached more quickly, or established at a lower temperature, which makes reactions more profitable.