Surface Chemistry

SYNOPSIS

1. Adsorption: The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or solid resulting in a higher concentration of the molecules on the surface.
2. Adsorbate: Substance adsorbed on the surface.
3. Adsorbent: Substance on which the adsorbate is adsorbed.
4. Desorption: Removal of the adsorbed substance from the surface.
5. Occlusion: Adsorption of gases on the surface of metals.
6. Entropy change during adsorption:
   is always negative. During adsorption, molecules are adsorbed on the surface of a solid adsorbent; hence, entropy decreases, i.e. ΔS is also negative. As ΔG = ΔH - TΔS, the process of adsorption to occur, ΔG must be negative.
7. Adsorption equilibrium:
8. Difference between adsorption and absorption:
   
   
   

9. Positive Adsorption:
   Concentration of the adsorbate is more on the surface of the adsorbent than in the bulk.

10. Negative Adsorption:
    Concentration of the adsorbate increases in the bulk after adsorption.
    
    

11. Physical Adsorption Vs Chemical Adsorption:
    
    

    Physical Adsorption	Chemical Adsorption
    Weak Van der Waals forces are present.	Strong chemical interactions are present.
    Energy of adsorption is low.	Energy of adsorption is high.
    Process takes place at low temperature.	Process takes place at high temperature.
    It is not specific.	It is highly specific.
    Process is reversible.	Process is irreversible.
    Occurs rapidly.	Occurs slowly.

12. Freundlich Adsorption Isotherm:
    A graph between the amount of the gas adsorbed per gram of the adsorbent (x/m) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm.
    
    
    
    

13. Catalysis: A catalyst is a substance which can alter the speed of a chemical reaction without itself undergoing any change in mass and its composition. This phenomenon is known as catalysis.

14. Positive catalyst: If the catalyst increases the speed of reaction, then it is called a positive catalyst, and the phenomenon is called positive catalysis.

15. Negative catalyst: If a catalyst decreases the rate of reaction, then it is called a negative catalyst, and the phenomenon is called negative catalysis.Types of catalysis:
    Homogeneous catalysis: If the catalyst is present in the same phase as the reactants, it is called homogeneous catalyst, and this type of catalysis is called homogeneous catalysis.
    Heterogeneous catalysis: If the catalyst is present in the different phase from that of the reactants, it is called a heterogeneous catalyst, and this type of catalysis is called heterogeneous catalysis.

• Modern Adsorption Theory:
  
  
  1. Diffusion of reactant molecules towards the surface of the catalyst.
  2. Adsorption of the reactant molecules on the surface of the catalyst by forming a loose bond with the catalyst due to the presence of free valencies.
  3. Occurrence of the chemical reaction between the reactants and the catalyst forming an intermediate.
  4. Desorption of the product molecule from the surface due to lack of its affinity for the catalyst surface, thereby making the surface free for fresh adsorption of reactant molecules.
  5. Diffusion of product molecules away from the surface of the catalyst.
• Important features of a solid catalyst:
  1. Activity: Capacity to increase the speed of a chemical reaction.
  2. Selectivity: Selectivity of a catalyst is the ability of a catalyst to direct the reaction to form a particular product excluding others.
  3. Specificity: The given substance can act as a catalyst only for specific reactions, not for all reactions

• Shape-selective catalysis by zeolites:
  The catalytic reaction which depends on the pore structure of the catalyst and the size of reactant and product molecules is called shape-selective catalysis.

• Enzyme catalysis:
  Enzymes are defined as biochemical catalysts and globular proteins with high molar mass.
  Characteristics of enzymes:
  

  1. Specificity
  2. Efficiency
  3. Small quantity
  4. Highly active under optimum temperature and pH
  5. Increasing activity in the presence of activators and co-enzymes
  6. Influence of inhibitors and poisons
     Mechanism:
     

• Colloids:
  A colloid is a homogeneous system in which one substance is dispersed as very fine particles in another substance called a dispersion medium.

• Classification of colloids based on the physical state of the dispersed phase and the dispersed medium:
  

  Dispersed phase	Dispersion medium	Type of colloid	Examples
  Solid	Solid	Solid sol	Some coloured glasses, gem stones
  Solid	Liquid	Sol	Paints, cell fluids
  Solid	Gas	Aerosol	Smoke, dust
  Liquid	Solid	Gel	Cheese, butter, jellies
  Liquid	Liquid	Emulsion	Milk, hair cream
  Liquid	Gas	Aerosol	Fog, mist, cloud
  Gas	Solid	Solid sol	Pumice stone, foam rubber
  Gas	Liquid	Foam	Froth, whipped cream, soap lather