What is an equilibrium constant?

An equilibrium constant is a number that expresses the relationship between the concentrations of products and reactants at equilibrium.

In more detail, the equilibrium constant, often denoted as K, is a measure of the ratio of the concentrations of products to reactants for a chemical reaction at equilibrium. It is a crucial concept in physical chemistry, particularly in the study of reaction kinetics and thermodynamics.

The equilibrium constant is calculated using the law of mass action. This law states that the rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants, each raised to the power equal to its stoichiometric coefficient in the balanced chemical equation. For a general reaction where aA + bB ⇌ cC + dD, the equilibrium constant expression would be K = ([C]^c [D]^d) / ([A]^a [B]^b), where [A], [B], [C], and [D] represent the molar concentrations of the reactants and products at equilibrium.

The value of the equilibrium constant provides important information about a system at equilibrium. If K is greater than 1, the system has more products than reactants at equilibrium, indicating that the reaction tends to proceed towards the products. If K is less than 1, the system has more reactants than products at equilibrium, suggesting that the reaction tends to proceed towards the reactants.

It's important to note that the equilibrium constant is temperature-dependent. Changes in temperature will affect the value of K, which in turn affects the position of the equilibrium. However, the equilibrium constant is not affected by changes in the concentrations of reactants or products, or by the addition of a catalyst.

Understanding the equilibrium constant is key to predicting how a system will respond to changes in conditions, as described by Le Chatelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.