chemical equilibrium

Condition in which the products of a chemical reaction are formed at the same rate at which they decompose back into the reactants, so that the concentration of each reactant and product remains constant. It is a reversible reaction; the reaction can happen in both directions.

For example, in the Haber process, nitrogen and hydrogen combine to form ammonia, but as the ammonia is formed, it breaks down again into nitrogen and hydrogen.

N₂ + 3H₂ ⇌ 2NH₃

At equilibrium, the forward and back reactions occur at the same rate. As fast as ammonia is being formed it is broken down into hydrogen and nitrogen, therefore, the amounts of nitrogen, hydrogen, and ammonia remain constant at equilibrium.

The amounts of reactant and product present at equilibrium are defined by the equilibrium constant for that reaction and specific temperature.