Reaction quotient (Q)
The reaction quotient Q has the same form as Kc, but uses current concentrations instead of equilibrium concentrations. Comparing Q to Kc tells us which direction the reaction will shift:
Q = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
Q < Kc → forward | Q > Kc → reverse | Q = Kc → equilibrium