Surface Chemistry Class 12 – Notes, NCERT Solutions, Formulas & CBSE PYQs | Unit 5 Chemistry Guide

Unit 5 – Surface Chemistry

5.1 Adsorption

5.2 Catalysis

5.3 Colloids

5.4 Classification of Colloids

5.5 Emulsions

5.6 Colloids Around Us

5.1 Adsorption

• Definition: Adsorption is the process by which atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to the surface of a solid or liquid (adsorbent), forming a film of the adsorbate on the surface.

• Types:

  • Physical Adsorption (Physisorption): Involves weak van der Waals forces; usually reversible.

  • Chemical Adsorption (Chemisorption): Involves stronger chemical bonds; often irreversible.

• Characteristics:

  • Rapid in initial stages, slows as equilibrium is approached.

  • Stronger with larger surface area of adsorbent.

  • Influenced by temperature and pressure.

  • Dye, gas, or pollutant removal often utilizes adsorption due to its high efficiency.

• Adsorbate: The substance being adsorbed (e.g., gas molecules).

• Adsorbent: The material on which adsorption occurs (e.g., charcoal, silica gel).

• Applications:

  • Gas masks (removal of toxic gases)

  • Water purification

  • Industrial gas separation

  • Heterogeneous catalysis

5.2 Catalysis

• Definition: Catalysis is a process where the rate of a chemical reaction is increased by a catalyst, which itself remains unchanged at the end of the process.

• Catalyst: A substance that changes (usually increases) the rate of a reaction without itself being consumed.

• Mechanism:

  1. Reactant molecules diffuse to the catalyst’s surface.

  2. Adsorption of reactants on the active sites of the catalyst.

  3. Formation of an activated complex.

  4. Formation of product and its desorption, regenerating the catalyst.

• Types:

  • Homogeneous Catalysis: Catalyst and reactants in the same phase.

    • E.g., Hydrolysis of ester by acid.

  • Heterogeneous Catalysis: Catalyst and reactants in different phases.

    • E.g., Haber process (N₂ + 3H₂ ⇌ 2NH₃ on Fe catalyst).

• Importance: Crucial in the chemical and petrochemical industries, pollution control (automobile catalytic converters), and biological systems (enzymes act as biological catalysts).

5.3 Colloids

• Definition: Colloids are mixtures where one substance (dispersed phase, particle size 1–1000nm) is uniformly spread in another (dispersion medium).

• Properties:

  • Heterogeneous: Consist of two phases.

  • Tyndall Effect: Scattering of light by colloidal particles.

  • Brownian Motion: Random movement of colloidal particles.

  • Stability: Colloids are stable and do not settle under gravity.

• Examples: Milk, blood, paint, fog.

5.4 Classification of Colloids

Colloids can be classified in several ways:

Based on Particle Nature

• Multimolecular Colloids: Aggregates of small molecules or ions.

  • E.g., Sulphur sol (S₈ molecules aggregate).

• Macromolecular Colloids: Large molecules act as colloidal particles themselves.

  • E.g., Starch, proteins, cellulose.

• Associated Colloids (Micelles): Behave as normal electrolytes at low concentration but form colloidal particles above a certain concentration.

  • E.g., Soaps and detergents.

Based on Interaction with Dispersion Medium

• Lyophilic Colloids: High affinity for the dispersion medium (solvent-loving).

  • E.g., Gum, gelatin, starch (easily reversible).

• Lyophobic Colloids: Little or no affinity for the medium (solvent-hating).

  • E.g., Metals, sulphur (irreversible sols), need stabilizers for preparation.

Based on Physical State

Dispersed Phase	Dispersion Medium	Colloid Type	Examples
Solid	Liquid	Sol	Paints, Mud
Liquid	Solid	Gel	Cheese, Jelly
Liquid	Liquid	Emulsion	Milk, Mayonnaise
Gas	Liquid	Foam	Froth, Whipped cream
Solid	Gas	Aerosol	Smoke, Dust
Liquid	Gas	Aerosol	Mist, Clouds

5.5 Emulsions

• Definition: Emulsions are colloidal systems in which both the dispersed phase and dispersion medium are liquids.

• Characteristics:

  • Consist of two immiscible liquids.

  • Unstable—tend to separate but stabilized by emulsifiers.

  • Exhibit Tyndall effect and Brownian motion.

  • More viscous than individual components.

• Types:

  • Oil-in-Water (O/W): Oil droplets in water (e.g., milk, vanishing cream).

  • Water-in-Oil (W/O): Water droplets in oil (e.g., butter, cold cream).

• Emulsifying Agents: Stabilize emulsions by preventing the coalescence of droplets (e.g., soaps, proteins, gum).

• Examples: Milk (O/W), butter (W/O), mayonnaise, lotions.

5.6 Colloids Around Us

• Natural Occurrence:

  • Atmosphere: Fog, mist, clouds, and rain are all colloidal in nature—tiny water droplets dispersed in air.

  • Food: Milk (emulsion), ice cream, butter.

  • Biological Systems: Blood is a colloidal solution (plasma proteins in water).

  • Environmental Phenomena: Blue color of the sky and sea due to scattering of light (Tyndall effect by colloidal particles).

  • Industrial and Domestic Uses:

    • Paints, ink, cosmetics (lotions, creams), waste treatment.

• Significance:

  • Artificial rain can be induced by spraying charged colloidal particles in clouds.

  • Colloidal medicines (colloidal gold, silver for drug delivery).

  • Soil fertility is maintained by colloidal clay and humic substances, which help in nutrient retention.

• What Is Adsorption?

  Adsorption is a surface phenomenon where atoms, ions, or molecules from a gas, liquid, or dissolved solid (adsorbate) adhere to the surface of another material (adsorbent), forming a thin film or layer. It is distinct from absorption, which involves a substance being distributed throughout another material.

  Mechanism of Adsorption

  Surface Energy Principle

  • Atoms and molecules inside the bulk of a substance are surrounded by other particles, so their attractive forces are balanced.

  • At the surface, particles lack neighbors on one side, leading to unbalanced, residual attractive forces.

  • Incoming adsorbate molecules are attracted by these unbalanced groups, becoming adhered to the surface, and this energy release drives the process2.

  Types of Adsorption Mechanisms

  Adsorption mechanisms are mainly categorized based on the nature of forces involved:

  1. Physical Adsorption (Physisorption)

  • Nature of Forces: Weak van der Waals forces.

  • Layers: Multi-molecular (multiple layers can be formed).

  • Reversibility: Typically reversible; molecules can be easily removed from the surface.

  • Specificity: Non-specific; occurs between any adsorbate and adsorbent.

  • Temperature Effect: Favored by low temperatures; decreases as temperature rises.

  • Activation Energy: Low (nearly negligible).

  • Enthalpy of Adsorption: Low (about 20–40kJ/mol).

  • Examples: Adsorption of N₂ or H₂ gases on charcoal.

  2. Chemical Adsorption (Chemisorption)

  • Nature of Forces: Formation of strong chemical bonds (covalent or ionic).

  • Layers: Unimolecular (usually forms only a monolayer).

  • Reversibility: Irreversible; the adsorbate is strongly bound.

  • Specificity: Highly specific; depends on chemical compatibility.

  • Temperature Effect: Favored at higher temperatures; increases with temperature up to a limit.

  • Activation Energy: High (energy required for bond formation).

  • Enthalpy of Adsorption: Higher (about 80–400kJ/mol).

  • Examples: Adsorption of hydrogen onto metal catalysts, formation of iron nitride on iron.

  Comparison Table: Physisorption vs Chemisorption

  Property	Physisorption	Chemisorption
  Forces Involved	van der Waals (weak)	Chemical bond (strong)
  Layer Formation	Multimolecular	Unimolecular (monolayer)
  Reversibility	Reversible	Irreversible
  Specificity	Non-specific	Highly specific
  Temperature	Favored at low temperature	Favored at high temperature up to limit
  Enthalpy of Adsorption	20–40kJ/mol	80–400kJ/mol
  Activation Energy	Low	High
  Example	N₂ on charcoal	H₂ on Ni, Fe with N₂

  Stepwise View of the Adsorption Mechanism

  1. Transport: Adsorbate molecules travel from the bulk phase to the adsorbent surface.

  2. Surface Attachment: Adsorbate attaches due to attractive forces (van der Waals or chemical).

  3. Energy Release: Attachment is exothermic (releases energy).

  4. Equilibrium: A dynamic balance is ultimately established between adsorption and desorption rates.

  Special Mechanisms and Models

  • Portal Site Mediated Adsorption: Certain "edge" or "corner" sites on catalysts serve as rapid entry points for adsorption, and molecules may then diffuse over the surface. This is relevant in heterogeneous catalysis and explains fast surface reactions in some bimetallic catalysts.

  • Isotherms: Mathematical models (like Langmuir and Freundlich isotherms) describe how the amount adsorbed varies with concentration at constant temperature.

  Thermodynamics of Adsorption

  • Exothermic Process: Adsorption typically releases energy (ΔH negative).

  • Spontaneity: Decrease in free energy (ΔG < 0).

  • Enthalpy & Entropy: Both enthalpy and entropy changes occur—entropy often decreases as molecules become ordered at the surface.

  Factors Influencing Adsorption Mechanisms

  • Surface Area: Greater surface area enhances adsorption.

  • Temperature & Pressure: Physisorption decreases with temperature, chemisorption increases up to an optimum.

  • Nature of Adsorbent & Adsorbate: Chemical compatibility, porosity, and polarity matter.

  • Activation Energy: Especially relevant in chemisorption, influencing the rate and extent.

  Summary of Key Points

  • Adsorption mechanisms involve surface phenomena driven by unbalanced forces at the interface.

  • Physisorption is driven by weak forces, is reversible, and non-specific.

  • Chemisorption involves chemical bond formation, is irreversible, and highly specific.

  • Adsorption is influenced by surface area, temperature, pressure, and the chemical nature of participants.

  These principles underpin a wide range of practical applications in industrial separation, catalysis, pollution control, and material science.

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  Adsorption Energy and Activation Processes

  • : Adsorption energy is the energy change associated with the attachment of atoms, ions, or molecules (the adsorbate) to the surface of a solid or liquid (the adsorbent).

  • : It quantifies how strongly an adsorbate is held at a surface, and reflects the stability of the adsorbed species. A more negative adsorption energy indicates a stronger, more stable adsorption.

  • : The adsorption energy is equal in magnitude (but opposite in sign) to the energy required to remove (desorb) the adsorbed molecule from the surface. Therefore, higher adsorption energies correspond to lower tendencies for desorption.

  • Physical vs. Chemical Adsorption:

    • Physisorption: Generally involves lower adsorption energies (20–40kJ/mol); mainly van der Waals forces.

    • Chemisorption: Involves higher adsorption energies (80–400kJ/mol); actual chemical bond formation.

  • : Adsorption energy depends on the chemical nature of both adsorbate and surface, the electronic and structural properties of the surface, and the interaction between them3.

  • Activation Energy of Adsorption:

    • Activation energy is the minimum energy barrier that must be overcome for adsorption to take place.

    • Physisorption usually has little to no activation energy; molecules adhere readily to the surface due to weak interactions.

    • Chemisorption typically involves a significant activation energy, as chemical bonds must form between the surface and the adsorbate.

  • Thermodynamic and Kinetic Aspects:

    • Physisorption: Low activation energies (often <40kJ/mol), usually rapid and can occur even at low temperatures.

    • Chemisorption: Higher activation energies (>40kJ/mol), temperature-dependent, and often requires initial energy to break bonds or rearrange atoms prior to adsorption.

  • Catalytic Surfaces and Activation:

    • In catalysis, adsorption helps concentrate reactants on the surface, facilitating reactions by lowering the overall activation energy of the process and sometimes providing energy when molecules bind (exothermic process).

    • The adsorbed state can enable unique reaction pathways that are not possible in the bulk phase.

  Type	Adsorption Energy	Activation Energy	Notes
  Physisorption	Low (20–40kJ/mol)	Low or None	Physical forces, multi-layers
  Chemisorption	High (80–400kJ/mol)	Moderate–High	Chemical bonds, monolayer

  • : Determines how stable the adsorbed species is on the surface, crucial for designing efficient adsorbents and catalysts.

  • : Indicates the energy barrier to adsorption; helps distinguish between physical and chemical adsorption and governs the temperature dependence of the process.

  • Catalytic adsorption lowers the activation energy for chemical reactions, thus increasing reaction rates

• The adsorption energy in catalysis is influenced by several key factors related to both the catalyst (adsorbent) and the adsorbate, as well as external conditions and surface interactions. Here are the main factors, with explanation relevant to catalysis:

  • Nature and Surface Area of the Catalyst (Adsorbent):

    • Catalysts with high surface area and porosity (like activated carbon or silica gel) provide more active sites, increasing adsorption energy and capacity. Porous and rough surfaces adsorb more due to greater exposure of active sites.

    • Finer particle size enhances surface area, leading to higher adsorption energy per unit mass.

  • Nature of the Adsorbate:

    • The chemical properties of the adsorbate, including size, polarity, and reactivity, impact its affinity for the catalyst surface. Polar and reactive molecules usually interact more strongly, resulting in higher adsorption energy

  • Temperature:

    • Generally, higher temperature reduces adsorption energy and the extent of adsorption for exothermic processes (common in physisorption and many catalytic systems). This is because increased kinetic energy favors desorption over adsorption

  • Pressure (for gases):

    • Increases in gas pressure raise the number of adsorbate molecules available on the surface, initially boosting adsorption energy and uptake. However, at high coverage, the energy per additional molecule may decrease due to repulsive interactions

  • Pore Structure of the Catalyst:

    • High porosity and optimal pore size support easier access of adsorbates to the interior surface, increasing overall adsorption and, in some cases, adsorption energy due to more effective interactions.

  • Surface Site Heterogeneity and Functional Groups:

    • Variations in surface atoms (such as defects, "dangling" bonds, or specific surface terminations) and chemical functional groups can create sites with higher affinity for adsorbates, boosting adsorption energyCoverage and Lateral Interactions (Induced Heterogeneity):

  • • As the surface becomes crowded, adsorbed species may repel each other, reducing the effective adsorption energy at high coverages. This leads to surface heterogeneity observed in thermal desorption and spectroscopic studies

  • Presence of Other Substances (Competitive Adsorption):

    • Other molecules in the environment can compete for the same sites, lowering the adsorption energy and capacity for the target species—an effect crucial in real-world catalytic processes

  • Ensemble and Ligand Effects:

    • The presence of other elements (alloying) or changes in local surface electronic properties can strengthen or weaken adsorption, as observed in catalysis by bimetallic surfaces. The ensemble effect (requirement for contiguous active atoms) and ligand effect (changes in chemical environment due to nearby atoms) can markedly alter adsorption energy

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