Heat (Enthalpy) of Adsorption

                        Adsorption is a surface phenomenon and adsorbate molecules are held to adsorbent due to attractive interactions. Since energy is always released during attractive interactions, adsorption is an exothermic reaction. The amount of heat evolved when 1 mole of an adsorbate gets adsorbed on the surface of an adsorbent is called Molar Heat or Molar Enthalpy of Adsorption.

                        In Chemisorption, molecules of adsorbate are held to surface of adsorbent by chemical bonds. On the other hand, in Physisorption, adsorbate molecules are held by the weak Vanderwaals forces. Hence, more energy is released during Chemisorption as compared to Physisorption.

Free Energy Change of Adsorption

                        Since adsorption is Exothermic Reaction. Therefore for adsorption DH is always negative and favours the process. Also adsorption leads to decrease in entropy i.e. DS is negative because the adsorbate molecules gets lesser opportunity to move about on the surface of adsorbent. According to Gibbs - Helmholtz Equation.

                                                DG = DH – T DS

                        Since adsorption is Spontaneous Process, hence DG is negative or DH > TDS. As adsorption continues, the difference between two opposing factors becomes lesser and lesser till they becomes equal i.e DH = TDS or DG = 0. At this stage, Equilibrium called Adsorption Equilibrium is established and hence no adsorption takes place at this stage.