Surface Chemistry PPT

Today's Goal

• Focus on Adsorption and its related concepts.

Surface Chemistry (NCERT-only)

• Importance of adsorption and its role in catalysis, colloidal solutions, and surface phenomena.

Terms Related to Adsorption and Absorption

Adsorption

• Definition: The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.

• Example: Water vapor (adsorbate) adsorbing onto the surface of silica gel (adsorbent).

• Key distinction: Adsorption is a surface phenomenon, contrasting with absorption, where substances penetrate the bulk material.

Adsorbate and Adsorbent

• Adsorbate: The molecular species or substance that adsorbs on the surface (e.g., water vapor).

• Adsorbent: The solid or liquid that the adsorbate adheres to (e.g., silica gel).

Desorption

• Definition: The process of removing an adsorbed substance from the adsorbent. At equilibrium, the rate of adsorption equals the rate of desorption.

• Characteristics: Typically, desorption is an exothermic process. When temperature increases or pressure decreases, desorption rate increases.

Absorption

• Definition: The process where atoms, molecules, or ions enter the bulk phase of a solid or liquid.

• Example: Water vapor is absorbed by anhydrous calcium chloride (CaCl2), illustrating how substances can diffuse throughout the medium.

Sorption

• Definition: The simultaneous occurrence of both adsorption and absorption.

• Example: Dyeing of fabric involves both processes, where dyes adsorb on the surface and penetrate into the fibers.

Thermodynamics of Adsorption

Enthalpy and Entropy

• Adsorption process is characterized by a negative enthalpy change (ΔH < 0), indicating it is exothermic.

• The process also typically involves a decrease in entropy (ΔS < 0), highlighting an ordering effect as gas molecules adhere to the surface of solids.

• The Gibbs free energy (ΔG) is negative for spontaneous adsorption processes. This includes the equation:

  • ΔG = ΔH - TΔS

Characteristics of Adsorption

1. Surface phenomenon.

2. Decrease in enthalpy.

3. Decrease in entropy.

4. Spontaneous process indicated by ΔG < 0.

Types of Adsorption

Physisorption vs. Chemisorption

1. Physisorption:

   • Low heat of adsorption (20-40 kJ/mol).

   • Involves van der Waals forces.

   • Reversible nature, typically occurs at low temperatures.

   • More liquefiable gases are adsorbed readily.

2. Chemisorption:

   • High heat of adsorption (80-240 kJ/mol).

   • Involves the formation of chemical bonds between adsorbate and adsorbent.

   • Generally irreversible and occurs at higher temperatures.

   • Forms monomolecular layers and requires activation energy.

Factors Affecting Adsorption

1. Nature of gas: Easily liquefiable gases tend to adsorb more.

2. Nature of adsorbent: Activated adsorbents increase adsorption.

3. Surface area: Increased surface area enhances adsorption capability.

4. Temperature: Generally, physisorption decreases with rising temperature, while chemisorption may increase.

5. Pressure: Increased pressure favors adsorption as there is a decrease in the volume of gas.

Adsorption Isotherms

Freundlich Isotherm

• Describes the adsorption process, particularly in physisorption, defined mathematically as:

  [ x/m = kp^{1/n} ]where x is the amount adsorbed, m is the mass of the adsorbent, and p is the pressure of the adsorbate.

Langmuir Isotherm

• A model for monolayer adsorption onto a surface with a finite number of identical sites:

  [ x/m = \frac{ap}{1+bp} ] where a and b are constants specific to the adsorbent-adsorbate pair.

Applications of Adsorption

1. Activated charcoal in gas masks: Removes toxins effectively.

2. Industrial processes: Catalysts often depend on adsorption principles.

3. Purification techniques: Used in chromatography and in water treatment processes.

4. Colloidal solutions: Utilized in food, medicine, and paint manufacturing.

Catalyst Overview

Definition

• A catalyst is a substance that alters the rate of a chemical reaction without undergoing permanent changes itself.

Types of Catalysts

1. Positive Catalysts: Increase the reaction rate.

2. Negative Catalysts: Decrease the reaction rate.

3. Auto catalysts: Products of a reaction act as catalysts.

4. Promoters: Enhance the activity of catalysts.

Mechanism of Heterogeneous Catalysis

1. Adsorption: Reactants bind to the catalyst surface.

2. Reaction: Reactants undergo a chemical transformation.

3. Desorption: Products are released from the catalyst.

4. Diffusion: Products diffuse away from the catalyst.