For a reaction at equilibrium, it is useful to be able to write an equilibrium constant expression which relates the concentrations of the species when the system reaches equilibrium
For a generic reaction
Although this expression may look similar to the rate constant expression, especially the use of captial K for the equilibrium constant and little k for the rate constant, do not confuse the two- they are not related.
The equilibrium constant expression has a number of special properties.
Example: What is the equilibrium constant expression for the gas phase reaction
Solution: This reaction only has two species. If we check the generic reaction above, we will see that the expression should be
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