Chemical equilibrium involves reversible chemical reactions where the rates of the forward and reverse reactions are equal. The equilibrium constant, K, is calculated based on the concentrations or pressures of reactants and products at equilibrium. A reaction is at equilibrium when the reaction quotient, Q, equals the equilibrium constant, K. If Q is greater than K, the reaction proceeds in the forward direction, and if Q is less than K, the reaction proceeds in the reverse direction.