Reversible reactions can proceed in both forward and reverse directions, forming a chemical equilibrium. Factors such as concentration, temperature, and catalysts can shift the equilibrium. Truly irreversible reactions are rare, and dynamic equilibrium is reached when the ratio of reactants to products stabilizes. Le Chatelier’s principle predicts how changes in reaction conditions will affect the equilibrium.
A reversible reaction is a chemical reaction that can proceed in both forward and reverse directions, thus producing a mixture of products and reactants. The mixture forms a chemical equilibrium in which products and reactants are produced and consumed at constant rates. Various factors such as the concentrations of various reactants, temperature, and the presence or absence of catalysts can cause the equilibrium to shift in favor of one side of the equilibrium or the other. The equilibrium of a reversible reaction also depends on an equilibrium constant called “K” in the equilibrium equations. This constant determines the direction an equilibrium will tend to favor under a given set of conditions.
Truly irreversible chemical reactions are quite rare. When a chemical reaction results in products that have significantly less energy than the reactants and are therefore much more stable than the reactants, it is sometimes considered irreversible. In reality, though, the reaction is generally a reversible reaction that greatly favors the products over the reactants; in theory, the reactants should still be present in very small quantities. In some reactions, one of the products may leave the reaction as a gas. When this occurs, it may be impossible to reform the reactants; such reactions can rightly be considered irreversible.
The equilibrium reached when the ratio of reactants to products in a reversible reaction stabilizes is called dynamic equilibrium. It is “dynamic” because products and reactants are still produced and consumed, but they do so at a constant rate that does not change the overall product-reactant ratio. Once equilibrium is reached in a reversible reaction, many different changes to internal or external conditions can alter the reaction to favor reactants or products. It is often important in science and industry to set conditions that ensure that a reaction favors products as much as possible.
Le Chatelier’s principle is a concept used by chemists to predict the change a given perturbation of reaction conditions will have on the equilibrium of a reversible reaction. The principle states that if any aspect of the reaction conditions are changed, the reaction equilibrium will shift to compensate for the change, thus forming a new, different concentration. If, for example, a particular reversible reaction requires a high input of energy to proceed, an increase in temperature will shift the reaction in the direction of the products because the heat will provide energy for the reaction.
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