Law of Mass Action

For a general reaction of the form

aA  +   bB  →   cC  +   dD,

the law of mass action states that the equilibrium condition is expressed by the equation:

where Kc is a constant known as the concentration equilibrium constant. You may also see it
referred to as Keq. Its value changes depending on the reaction in question and on the temperature
at which the reaction is carried out, [C] denotes the concentration of C in moles per liter (molarity),
et cetera. (Even though molarity is a concept most commonly encountered when talking about
aqueous solutions, there is no reason why one cannot talk about the molarity of a gas in a mixture—
the number of moles of that gas divided by the volume of the container in liters.) Note that the
exponents of the concentrations are the same as the stoichiometric coefficients in the balanced
chemical equation.

Example: What   is  the expression  for the equilibrium constant    for the following   reaction?

3H 2    (g) +   N 2     (g) →   2NH 3   (g)

Solution: