Some reactions can go both in forward and reverse, while others can only go in one direction. For a reaction that can go both ways, the equilibrium constant describes the ratio of products to reactants. For the reaction:

The equilibrium constant would be:

For the reaction:

The equilibrium constant would be:

and for the reaction:

The equilibrium constant would be:

Reactions with a large equilibrium constant (K_eq > 1) favor formation of the products, while reactions with a small equilibrium constant (K_eq < 1)="" favor="" formation="" of="" the="">