The isotherm can also be used at fixed temperature to describe the Law of Mass Action. When a reaction is at equilibrium, Q r = K eq and Δ Δ r G = 0 . Otherwise, the Van 't Hoff isotherm predicts the direction that the system must shift in order to achieve equilibrium; when Δ r G < 0, the reaction moves in the forward direction, whereas when Δ r G > 0, the reaction moves in the backwards direction. See Chemical equilibrium.