Le Chatelier's Principle and Equilbrium constants
As we have seen, the temperature, concentration and pressures of the reaction each have an affect upon the overall equilibrium position. As such, it would be logical to assume that they would have an affect upon the equilibrium constant. However, this is not so.
Although the pressures and concentrations of the reactants and products have an affect upon the equilibrium position, however, it is also obvious that changing pressure or concentrations will directly affect the concentration of each species in the mixture. So although an increase in concentration of the products would shift the equilbrium to favour the reactants, the increase in concentration of reactants would mean that the equilibrium constant does not change.
This is a verbose way of saying that neither pressure nor concentration have any affect on the value for Kc.
By comparison, temperature has no direct affect upon the concentrations of the reactants or products. However, changes in temperature can force the equilibrium position to shift to minimise this change, this will indirectly change the ratios of products to reactants, hence altering the equilibrium constant.
Therefore, tempereature can affect the value for Kc.
If we apply this knowledge to the Haber process dicussed earlier, we can qualify these changes.
N2 (g) + 3H2 (g) 2NH3 (g)
Kc = [NH3]2 / [N2] [H2]3
As the reaction is exothermic in the forward direction, increasing the tempereature would shift the equilibrium position to the left. This would increase the concentrations of the reactants, and decrease the concentration of the product. Therefore, for the Haber process, an increase in temperature would result in a decrease in the value of Kc.
It is also worth noting that, even if we do not know the equilibrium constant, the fact that a change in pressure or concentration does not result in a change in Kc, an equation linking concentrations between the same reaction under different conditions can be written. For example:
[NH3]2 / [N2] [H2]3 = [NH3]12 / [N2]1 [H2]13
As as result, given enough data about the same reaction under different conditions, it is possible to calculate the concentration of one of the reactants or products.
Remember, the presence of a catalyst (iron in the case of the Haber process) does not affect either the equilibrium position of the reaction or the equilibirum constant.