Entropy is equally essential in predicting the extent and direction of complex chemical reactions. For such applications, Δ Δ S must be incorporated in an expression that includes both the system and its surroundings: Δ Δ S u n i v e r s e = Δ Δ S s u r r o u n d i n g s + Δ Δ S s y s t e m Via additional steps this expression becomes the equation of Gibbs free energy change Δ Δ G for reactants and products in the system at the constant pressure and temperature T : Δ Δ G = Δ Δ H − − T Δ Δ S where Δ Δ H is the enthalpy change and Δ Δ S is the entropy change.