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A reversible reaction is a chemical reaction that can proceed in both the forward and backward directions, thereby producing a mixture of products and reactants. The mixture forms a chemical equilibrium at which products and reactants are being produced and consumed at constant rates. Various factors such as the concentrations of the various reagents, the 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 referred to as "K" in equilibrium equations. This constant determines the direction that 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 lower 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 products over reactants; in theory, reactants should still be present in very small amounts. In some reactions, one of the products may leave the reaction in the form of a gas. When this occurs, it may be impossible for the reactants to reform; such reactions can rightly be considered irreversible.
The equilibrium achieved 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 being produced and consumed, but they are doing so at a constant rate that does not change the overall product-reactant ratio. Once equilibrium is achieved in a reversible reaction, many different changes to the internal or external conditions can alter the reaction to favor reactants or products. It is often important in science and in industry to set conditions that ensure a reaction favors products to the greatest possible degree.
Le Chatelier's principle is a concept used by chemists to predict the change that a given perturbation of reaction conditions will have on a reversible reaction's equilibrium. The principle states that if some aspect of the reaction conditions is changed, the equilibrium of the reaction will shift to compensate for the change, thereby forming a new, different concentration. If, for instance, a particular reversible reaction requires a high energy input to proceed, an increase in temperature will shift the reaction toward the direction of the products because the heat will provide energy for the reaction.