[ptryon]

Textbooks claim that the equilibrium constant remains constant at a fixed temperature.

For the reaction...

A + B  C

Kc = C / AB

If you increase the pressure by decreasing the volume of the reacting vessel and keeping temperature and total amount of substance fixed, Le Chataliers principle predicts that the equilibrium will shift to the right (where there are less gas molecules). If the equilibrium shifts (without adding more reactants or products) surely the ratio of C : AB has to change, meaning the equilibrium constant also changes.

[Jorriss]

The biggest cover up in general chemistry is that the equilibrium constant does have a pressure dependency to it, but, at 'normal' pressures and for liquids and solids, it is not very high.

The equilibrium constant also has a temperature dependency to it.

[If you really want to get messed up, Le Chateliers Principle doesn't even always hold, sometimes adding more reactant, produces more reactant!]

[ptryon]

So you're saying that if you change the pressure of a system in chemical equilibrium by changing the volume, the equilibrium constant will change but only very slightly?

P.S. When does Le Chatalier's principle not hold? I am curious!!

[Jorriss]

Take the Ammonia example:

3H2 + N2 <-> 2NH3

If you add a very tiny amount of H2 and then calculate Q, it'll shift left further. It has to do with how much faster H2 scales than nitrogen.

Say we have 2 moles of each, for a total moles of 6. Add .1 mole of H2, nt is now 6.1

Q1=[2/6]^2     /    (2/6)(2/6)^3
Q2=[2/6.1]^2   /   (2/6.1)(2.1/6.1)^3

Compare the Q values.

[Borek]

If you increase the pressure by decreasing the volume of the reacting vessel and keeping temperature and total amount of substance fixed, Le Chataliers principle predicts that the equilibrium will shift to the right (where there are less gas molecules). If the equilibrium shifts (without adding more reactants or products) surely the ratio of C : AB has to change, meaning the equilibrium constant also changes.

Do you know the difference between Kp and Kc? See Gas Equilibrium Constants, Kc and Kp at USDavis wiki.