NEET Chemistry Notes Surface Chemistry – Concept of Surface Chemistry
Concept of Surface Chemistry
Concept of Surface Chemistry
Surface chemistry deals with the phenomenon that occurs atn&bn surface or interfaces. There are several properties of substances, particularly of solids and liquids and solids and gases, which depend upon the nature of the surface of interface.
Concept of Adsorption
Retaining of the molecular species on the surface of the solid is known as adsorption. The solid substance on the surface of which adsorption occurs due to are called adsorbent. The molecular species that get adsorbed on the solid surface due to intermolecular attractions are called adsorbate.
Rate of adsorption decreased with time whereas rate of absorption remains constant with time.
Desorption and Sorption
The removal of the adsorbed substance from a surface is called desorption.
Sorption is a process in which adsorption and absorption take place simultaneously, e.g. dyes get adsorbed as well as absorbed on the cotton fibres.
Characteristics of Adsorption
Important characteristics of adsorption are as follows:
Specificity and Selectivity
Adsorption is specific and selective phenomenon, which strictly refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase.
Exothermic Process
Adsorption is accompanied with decrease in free energy of the system. In adsorption, there is a decrease in entropy of the system.
As,
Therefore, 
Hence, the adsorption is associated with decrease in enthalpy of the system. Thus, adsorption is always an exothermic process.
Residual Valency Bonds
The atoms or molecules of a solid surface have unbalanced or residual attractive forces on the surface, which can hold adsorbate particles together. Thus, the adsorbed atoms or molecule can be held on the surface of a metal by physical van der Waals’ forces or chemical forces due to residual valence bonds.
Extent of Adsorption
All solids adsorb gases to some extent. Adsorption is not very pronounced unless an adsorbent possesses a large surface area for a given mass. Various types of charcoals, silica gels, metal, etc., are used as adsorbents.
Appreciable adsorption also takes place on some smooth surfaces, such as those of platinum and glass. Charcoal and other solids gain increased power of adsorption on heating at low pressure with various gases or in air or vacuum at temperature varying from 350-1000°C. Such treated charcoal is called activated charcoal and the process is called activation.
On activation the hydrocarbons and other impurities are removed from charcoal. The surface area may increase due to chemical interaction with the oxidising agent which is used as activator. The degree of unsaturation of the surface atoms may be increased.
Types of Adsorption
Positive Adsorption
If the concentration of an adsorbate at the surface of adsorbent is more than in the bulk of the adjoining phases, it is called positive adsorption.
Negative Adsorption
If concentration of an adsorbate at the surface of adsorbent is less than in the bulk of the adjoining phases, it is called negative adsorption.
Physisorption and Chemisorption
In physisorption, the particles of the adsorbate are held to the surface of the adsorbent by physical forces such as van der Waals’ forces while in chemisorption, the molecules of the adsorbate are held to the surface of the adsorbate by chemical bonds.
