Chapter – 5
Surface Chemistry

ü  Introduction
ü  Adsorption
Ø  Adsorbent
Ø  Adsorbate
ü  Desorption
ü  Sorption
ü  Mechanism of Adsorption
ü  Types of Adsorption
Ø  Physisorption (Physical Adsorption)
Ø  Chemisorption (Chemical Adsorption)
ü  Adsorption Isotherm
ü  Applications of Adsorption
ü  Catalysis
Ø  Homogeneous and Heterogeneous catalysis
ü  Adsorption Theory of Heterogeneous Catalysis
ü  Shape Selective Catalysis
ü  Enzyme Catalysis
ü  Colloids
Ø  Classification of Colloids
ü  Preparation and Purification of Colloids
ü  Emulsion
ü  Colloids around us



Surface Chemistry is the branch of chemistry that deals with the study of the phenomenon occurring at the surface than bulk.

 ü  Adsorption: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed as adsorption.
Ø  Adsorbate: The substance which is adsorbed is called adsorbate.
Ø  Adsorbent: The substance whose surface on which adsorption takes place is called adsorbent.
The commonly used adsorbents are charcoal, silica gel, alumina gel, clay, colloids, metals in finely divided state etc.
Adsorption is a surface phenomenon. Some examples of adsorption are:
1.       Powdered charcoal adsorbs gases like H2, O2, CO, Cl2, NH3, SO2 etc.
2.       Silica gel adsorbs moisture
3.       Animal charcoal adsorbs colouring material from sugar solutions.

ü  Desorption: The removal of the adsorbed substance from a surface is called desorption.
ü  Sorption: If adsorption and absorption occur simultaneously, the process is called sorption.
ü  Distinction Between Adsorption and absorption:

                        
ü  Mechanism of Adsorption:
The surface particles of the adsorbent are not in the same environment as the particles inside the bulk (inner part). Inside the adsorbent, all the forces are mutually balanced. But at the surface, there is always some unbalanced or residual forces. These forces of the adsorbent are responsible for adsorption.
ü  Gibbs energy change during adsorption:
During adsorption, there is always decrease in residual force i.e., there is decrease in surface energy, which appears as heat. Therefore, adsorption is an exothermic process i.e. ΔH = – ve. Also, movement of the particles are restricted in this process. Therefore ΔS = – ve According to Gibbs Helmholtz equation:
ΔG = ΔH – TΔS,  or    ΔG = (–ΔH ) – T(–ΔS) for adsorption to occur.
ΔG must be negative which is possible only when ΔH >TΔS.

ü  Types of Adsorption:
Ø  Physical adsorption or Physisorption: When the particles of adsorbate are held to the surface of adsorbent by weak van der Waals forces.
Characteristics of physical adsorption: Lack of specificity, low enthalpy of adsorption, reversible in nature, no activation energy required, decrease with increase in temperature, multilayered phenominan.
Ø  Chemical adsorption or Chemisorption: When the molecules of adsorbate are held to the surface of adsorbent by strong chemical forces.
Characteristics of chemical adsorption: Highly specific in nature, high enthalpy of adsorption, Irreversible in nature, initially it increases with increase in temperature as it needs activation energy, very slow, monolayered phenominan.