EQUILIBRIUM CONSTANTS and LE CHATELIER'S PRINCIPLE
This page looks at the relationship between equilibrium constants and Le Chatelier's Principle. Students often get confused about how it is possible for the position of equilibrium to change as you change the conditions of a reaction, although the equilibrium constant may remain the same.
Be warned that this page assumes a good understanding of Le Chatelier's Principle and how to write expressions for equilibrium constants.
Important: If you aren't happy about the basics of equilibrium, explore the equilibrium menu before you waste your time on this page.
This page should only be read when you are confident about everything else to do with equilibria.
Changing concentrations
The facts
Equilibrium constants aren't changed if you change the concentrations of things present in the equilibrium. The only thing that changes an equilibrium constant is a change of temperature.
The position of equilibrium is changed if you change the concentration of something present in the mixture. 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.
Suppose you have an equilibrium established between four substances A, B, C and D.
According to Le Chatelier's Principle, if you decrease the concentration of C, for example, the position of equilibrium will move to the right to increase the concentration again.
Note: The reason for choosing an equation with "2B" will become clearer when I deal with the effect of pressure further down the page.
Explanation in terms of the constancy of the equilibrium constant
The equilibrium constant, Kc for this reaction looks like this:
If you have moved the position of the equilibrium to the right (and so increased the amount of C and D), why hasn't the equilibrium constant increased?
This is actually the wrong question to ask! We need to look at it the other way...