Question

Effect of partial pressure on equilibrium

Answer 1

Changing pressure

This only applies to systems involving at least one gas.

The facts

Equilibrium constants aren't changed if you change the pressure of the system. The only thing that changes an equilibrium constant is a change of temperature.

The position of equilibrium may be changed if you change the pressure. According to Le Chatelier's Principle, the position of equilibrium moves in such a way as to tend to undo the change that you have made.

That means that if you increase the pressure, the position of equilibrium will move in such a way as to decrease the pressure again - if that is possible. It can do this by favouring the reaction which produces the fewer molecules. If there are the same number of molecules on each side of the equation, then a change of pressure makes no difference to the position of equilibrium.

Explanation

Where there are different numbers of molecules on each side of the equation

Let's look at the same equilibrium we've used before. This one would be affected by pressure because there are 3 molecules on the left but only 2 on the right. An increase in pressure would move the position of equilibrium to the right.


Because this is an all-gas equilibriium, it is much easier to use Kp:

Once again, it is easy to suppose that, because the position of equilibrium will move to the right if you increase the pressure, Kpwill increase as well. Not so!

To understand why, you need to modify the Kp expression.

Remember the relationship between partial pressure, mole fraction and total pressure?
Hope it helps you buddy..