CBSE Class 12 Chemistry Notes: Surface Chemistry – Introduction

Surface Chemistry:- is the branch of chemistry which deals with the study of the phenomena occurring at the surface or interface, at boundary separating two bulk phases.

• The results about surface studies can be obtained only by taking the clean surfaces.

Definitions of certain terms:-

Adsorption:- The phenomenon of attracting and retaining of the molecules of a substance on the surface of a liquid or solid into a higher concen ration of molecule on surface is called adsorption. The substance which is adsorbed on the surface is called adsorbate and the substance on which it is adsorbed is called adsorbent.

Desorption:- The reverse process; removal of the adsorbed substances from the surface is called. Desorption (it can be done by heating or reducing pressure)

Examples of adsorption in its Favor:-

1. To the solution of an organic dye such as methylene blue, add animal charcoal and stir it is observed that the intensity of the colour in the solution decreases showing some amount of dye has adsorbed.
2. Introduce Finely devided solid into a closed vessel containing a gas at low pressure. The pressure of the gas is found to decreasing showing adsorption.

Mechanism of Adsorption:-

• A molecule at the surface of a liquid is surrounded by larger no. of molecules in liquid phase and fewer molecules in vapor phase.
• Thus molecules laying at the surface experiences a net inward force. Because of these unbalanced inward forces of attraction or free valancies at the surface thus liquids and soilds and solids have property to adsorb the substances on their surface.

Adsorption is exothermic:-during adsorption the residual forces on the surface of adsorption decreases thus surface energy deceases. This appears in the form of heat called heat of adsorption thus adsorption is exothermic ΔH is always –ve.

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