In a closed container, nitrogen and hydrogen react to form ammonia. But the reaction does not go to completion. Ammonia decomposes back into nitrogen and hydrogen. Eventually, the rates of the forward and reverse reactions become equal. The concentrations of all species stop changing. The system has reached chemical equilibrium. It is not static. Both reactions continue, but at the same rate. It is a dynamic balance.

Equilibrium is described by the equilibrium constant, K. For a reaction aA + bB cC + dD, K equals the product of the concentrations of the products raised to their coefficients, divided by the product of the concentrations of the reactants raised to their coefficients. A large K means the reaction favors products. A small K means it favors reactants. K depends only on temperature. It does not depend on initial concentrations or pressure.

Le Chatelier's principle predicts how a system at equilibrium responds to change. If you add more reactant, the system shifts to produce more product. If you remove product, it shifts to produce more. If you increase pressure, it shifts toward the side with fewer gas molecules. If you increase temperature, it shifts in the endothermic direction. These principles guide industrial processes, like the Haber-Bosch synthesis of ammonia, where conditions are optimized to maximize yield.

Equilibrium is not the same as completion. Many reactions are reversible, and equilibrium is the natural state. Understanding it is essential for predicting how reactions behave.