Le Chatelier's principle states that a system at equilibrium subjected to a stress will adjust to relieve the stress and restore equilibrium. For example, if the concentration of the reactants increases, the reaction will form more products. Equivalently, removing the products as soon as they are formed decreases the concentration of the products and causes more products to be formed. Removing products as they form is an effective way to force all of the reactants to react. In reactions that occur among gases, a change in pressure can drive the equation in a specific direction. A decrease in volume drives the reaction in a way such that the volume occupied by the reaction is reduced. Since the volume occupied depends on the number of molecules present, the reaction proceeds in a direction that minimizes the number of molecules. For example, in the decomposition of ozone:
two molecules of ozone form three molecules of oxygen. If the volume of the container decreases, the reaction will form ozone molecules to decrease the total number of molecules present. Le Chatelier's principle also describes how the equilibrium shifts when the temperature of a reaction is artificially changed. If a reaction that is endothermic in the forward direction is heated, the equilibrium will shift to favor the products to absorb heat and minimize the temperature change.
