Surface Chemistry is the branch of chemistry which deals with the phenomenon that occurs on the surfaces or interfaces , such phenomenon includes corrosion . catalysis , crystallisation , etc
Adsorption
Due to unbalanced attraction forces , accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption . The molecular species accumulates at the surface is termed as adsorbate and the material on the surface of which the adsorption takes place is called adsorbent , e . g ..
( i ) O2 , H2 , C12 , NB3 gases are adsorbed on the surface of charcoal . ( ii ) Silica gels adsorb water molecules from air .
Charcoal , silica gel , metals such as Ni , Cu , Ag , Pt and colloids are some adsorbents .
Important Characteristics of Adsorption
1 . It is specific and selective in nature .
2 . Adsorption is spontaneous process , therefore change in free energy ( ΔG ) is negative .
ΔG = ΔH – TΔS ,
For the negative value of ΔG , in a system , in which randomness decreases , ΔH must be negative . Hence , adsorption is always exothermic .
Adsorption of hydrogen over Pt is called occlusion .
Desorption
It is a process of removing an adsorbed substance from a surface on which it is adsorbed , is known as desorption .