Q is the reaction quotient, while K is the equilibrium constant. They are both defined as ([C]ᶜ·[D]ᵈ) / ([A]ᵃ·[B]ᵇ): the ratio of the product of the concentrations of the reaction's products to the product of the concentrations of the reagents, each of them raised to the power of their relative stoichiometric coefficients. K is defined only at the equilibrium, while Q is defined during the whole reaction. They are equal at the equilibrium. Read more