5. Equilibrium Constant K

The progress of a chemical reaction can be expressed by the reaction quotient Q.

a A + b B c C + d D

A equilibrium reaction proceeds until Q reaches a certain value (typical for that reaction). That value is called the equilibrium constant K of the reaction.

If the equilibrium constant K of an equilibrium reaction is small, the equilibrium state is positioned at the left. In this equilibrium state there are still lots of reactants A and B present and there are only small amounts of C and D. When K is very small (near to zero) it seems as if the reaction does not occur (concentrations of A and B allmost equal to the initial concentrations, concentrations of C and D allmost equal to zero).

If the equilibrium constant K of an equilibrium reaction is large, the equilibrium state is positioned at the right. In this equilibrium state there are only small amounts of reactants A and B present and there are large amounts of C and D. When K is very large (near to infinity) it seems as if the reaction proceeds to completion (concentrations of A and B allmost zero, concentrations of C and D allmost equal to maximal values).

Summary

K	Examples	Position of equilibrium state
very small	K < 10-30	completely at the left, reaction does not occur
small	10-30 < K < 10-3	at the left
average	10-3 < K < 103	in the middle, real chemical equilibrium
large	103 < K < 1030	at the right
very large	K > 1030	completely at the right, reaction proceeds to completion