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.