The conditions of a reaction equilibrium are described by the equilibrium constant, K. The equilibrium constant is a measure of the extent to which a reaction proceeds in the forward direction, compared to the reverse direction. It is defined as the ratio of the concentrations of the products to the concentrations of the reactants at equilibrium.

The equilibrium constant is a constant for a given reaction at a given temperature and pressure. It is not affected by the initial concentrations of the reactants or products. However, the equilibrium constant can change if the temperature or pressure is changed.

The equilibrium constant can be used to calculate the equilibrium concentrations of the reactants and products. It can also be used to predict the direction in which a reaction will proceed. If the equilibrium constant is greater than 1, the reaction will proceed in the forward direction. If the equilibrium constant is less than 1, the reaction will proceed in the reverse direction.