In this chapter we have been looking at two of the most fundamental ideas of chemistry: spontaneity and chemical equilibrium. They are fundamental because they tell us when a reaction has an inherent tendency to occur (which is not to say that it will occur rapidly without help). If the forward and reverse reactions of a chemical process are occurring at the same rate, this condition of balance is equilibrium. A reaction that is not at equilibrium but is moving in that direction is spontaneous.

The higher the concentrations of reacting substances, the greater will be their tendency to react to form products. Conversely, as the concentration of products increases, the reverse reaction will be favored more and more over the forward process. At equilibrium the ratio of products to reactants has a characteristic numerical value known as the equilibrium constant, . For a general chemical reaction,

aA + bB cC + dD

the equilibrium-constant expression has the form