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Class 12 Chemistry Notes Chapter 5 (Surface chemistry) – Chemistry-I Book

Philoid

Philoid

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Chemistry-I
Detailed Notes with MCQs of Chapter 5: Surface Chemistry. This is an important chapter, not just for your board exams but also for various government competitive exams where fundamental chemistry concepts are tested. Surface chemistry deals with phenomena occurring at the surfaces or interfaces. An interface is typically represented by separating the bulk phases with a hyphen or a slash, e.g., solid-gas, liquid-liquid, solid-liquid etc.

We'll break this down into the main topics: Adsorption, Catalysis, and Colloids.

1. Adsorption

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

    • Adsorbate: The substance that gets adsorbed on the surface.
    • Adsorbent: The material on the surface of which adsorption takes place.
    • Examples: Adsorption of gases (like O₂, H₂, CO, Cl₂, NH₃) on charcoal; adsorption of dyes (like methylene blue) by animal charcoal; adsorption of water vapour by silica gel.

  • Distinction from Absorption: Absorption is a bulk phenomenon where the substance is uniformly distributed throughout the body of the solid or liquid (e.g., water absorbed by a sponge). Adsorption is a surface phenomenon.

  • Sorption: When both adsorption and absorption occur simultaneously.

  • Mechanism: Adsorption arises due to unbalanced residual forces on the surface atoms/molecules of the adsorbent. These forces attract and retain the adsorbate molecules.

  • Thermodynamics of Adsorption:

    • Adsorption is generally an exothermic process (ΔH = negative), as attractive forces are involved.
    • Entropy decreases (ΔS = negative) because the freedom of movement of adsorbate molecules becomes restricted.
    • For a process to be spontaneous, Gibbs free energy change (ΔG) must be negative. Since ΔG = ΔH - TΔS, adsorption occurs spontaneously. As adsorption proceeds, ΔH becomes less negative, and eventually, ΔH becomes equal to TΔS, so ΔG becomes zero, reaching equilibrium.

  • Types of Adsorption:

    Feature Physisorption (Physical Adsorption) Chemisorption (Chemical Adsorption)
    Forces Involved Weak van der Waals forces Strong chemical bonds (covalent or ionic)
    Nature Not specific; any gas can adsorb on any solid to some extent Highly specific; occurs only if chemical bonding is possible
    Reversibility Reversible Irreversible
    Enthalpy (ΔH) Low (20-40 kJ/mol) High (80-240 kJ/mol)
    Temperature Favoured at low temperatures Favoured at high temperatures (initially increases, then decreases)
    Activation Energy Not required Often requires activation energy
    Layers Formed Multimolecular layers Monomolecular layer
    Effect of Pressure Increases with pressure Increases with pressure (less pronounced effect)
    Example Adsorption of N₂ on mica Adsorption of H₂ on Ni

  • Factors Affecting Adsorption of Gases on Solids:

    • Nature of Adsorbate: Easily liquefiable gases (higher critical temperatures, stronger van der Waals forces) like NH₃, HCl, CO₂ are adsorbed more readily than gases like H₂, O₂, N₂.
    • Nature of Adsorbent: Porous materials and those with rough surfaces are better adsorbents due to larger surface area (e.g., activated charcoal, silica gel, alumina gel).
    • Surface Area: Adsorption increases with an increase in the surface area of the adsorbent. Finely divided substances are better adsorbents.
    • Pressure: At constant temperature, adsorption increases with pressure, reaching a saturation point at high pressure (especially in physisorption).
    • Temperature: Physisorption decreases with increasing temperature (Le Chatelier's principle, as it's exothermic). Chemisorption often increases initially (provides activation energy) and then decreases.

  • Adsorption Isotherms: A graph between the amount of gas adsorbed (x/m, where x is mass of adsorbate, m is mass of adsorbent) and the equilibrium pressure (P) at a constant temperature.

    • Freundlich Adsorption Isotherm (Empirical):
      • Equation: x/m = k * P^(1/n) (where k and n are constants depending on the nature of adsorbate, adsorbent, and temperature; n > 1)
      • At low pressure: 1/n ≈ 1, so x/m ∝ P (linear)
      • At high pressure: 1/n ≈ 0, so x/m ≈ k (constant, saturation)
      • At intermediate pressure: x/m depends on P^(1/n).
      • Logarithmic form: log(x/m) = log k + (1/n) log P (A plot of log(x/m) vs log P gives a straight line with slope 1/n and intercept log k).
      • Limitations: Empirical, fails at high pressure.
    • Langmuir Adsorption Isotherm (Theoretical): Assumes monolayer adsorption and equivalent adsorption sites. (Equation not typically required in detail for basic government exams, focus on Freundlich).

  • Applications of Adsorption:

    • Production of high vacuum.
    • Gas masks (adsorb poisonous gases).
    • Control of humidity (silica/alumina gels).
    • Removal of colouring matter from solutions (animal charcoal).
    • Heterogeneous catalysis.
    • Separation of inert gases.
    • Froth flotation process (concentration of sulphide ores).
    • Adsorption indicators.
    • Chromatographic analysis.

2. Catalysis

  • Definition: Substances that alter the rate of a chemical reaction without themselves undergoing any permanent chemical change are called catalysts, and the phenomenon is called catalysis.
    • Promoters: Substances that enhance the activity of a catalyst (e.g., Mo promotes Fe catalyst in Haber's process).
    • Poisons (Inhibitors): Substances that decrease the activity of a catalyst (e.g., CO poisons Fe catalyst in Haber's process).
  • Types of Catalysis:
    • Homogeneous Catalysis: Reactants and catalyst are in the same phase (e.g., Lead chamber process for H₂SO₄: 2SO₂(g) + O₂(g) --[NO(g)]--> 2SO₃(g)).
    • Heterogeneous Catalysis: Reactants and catalyst are in different phases (e.g., Haber's process for NH₃: N₂(g) + 3H₂(g) --[Fe(s)]--> 2NH₃(g)).
  • Adsorption Theory of Heterogeneous Catalysis (Modern Theory): Combines intermediate compound formation theory and adsorption theory. Steps:
    1. Diffusion of reactants to the catalyst surface.
    2. Adsorption of reactant molecules onto the catalyst surface (chemisorption).
    3. Occurrence of chemical reaction on the catalyst surface (formation of an intermediate).
    4. Desorption of product molecules from the catalyst surface.
    5. Diffusion of products away from the catalyst surface.
    • The active sites on the catalyst surface play a crucial role.
  • Important Features of Solid Catalysts:
    • Activity: The ability of a catalyst to increase the rate of reaction. Largely depends on the strength of chemisorption (should be reasonably strong but not too strong).
    • Selectivity: The ability of a catalyst to direct a reaction to yield a particular product (e.g., CO + H₂ gives different products with different catalysts like Ni, Cu/ZnO-Cr₂O₃, Cu).
  • Shape-Selective Catalysis: Catalytic reaction depends on the pore structure of the catalyst and the size/shape of reactant/product molecules.
    • Zeolites: Microporous aluminosilicates with a honeycomb-like structure. Used widely in petrochemical industries.
    • Example: ZSM-5 converts alcohols directly into gasoline (dehydration).
  • Enzyme Catalysis (Biochemical Catalysis):
    • Enzymes: Complex nitrogenous organic compounds (proteins) produced by living organisms that catalyse biochemical reactions.
    • Characteristics:
      • Highly efficient (millions of times faster).
      • Highly specific (catalyse only one or a specific set of reactions).
      • Active under optimum temperature (usually 298-310 K) and pH (usually 5-7).
      • Activity influenced by co-enzymes (activators) and inhibitors/poisons.
    • Mechanism: Lock and Key model. Enzyme (E) has active sites. Substrate (S) fits into the active site like a key into a lock, forming an enzyme-substrate complex (ES). This complex then decomposes to give the product (P) and regenerates the enzyme.
      E + S ⇌ ES → E + P

3. Colloids

  • Definition: A heterogeneous system in which one substance (dispersed phase, DP) is dispersed as very fine particles in another substance called the dispersion medium (DM). The particle size of the dispersed phase ranges from 1 nm to 1000 nm.

  • Types of Systems:

    • True Solutions: Particle size < 1 nm (homogeneous).
    • Colloidal Solutions: Particle size 1 nm - 1000 nm (heterogeneous).
    • Suspensions: Particle size > 1000 nm (heterogeneous, settle down).

  • Classification of Colloids:

    • Based on Physical State of DP and DM:

      DP DM Type of Colloid Example
      Solid Solid Solid Sol Gemstones, coloured glasses
      Solid Liquid Sol Paints, starch solution, ink
      Solid Gas Aerosol Smoke, dust
      Liquid Solid Gel Cheese, butter, jellies
      Liquid Liquid Emulsion Milk, hair cream
      Liquid Gas Aerosol Fog, mist, cloud, spray
      Gas Solid Solid Foam Pumice stone, foam rubber
      Gas Liquid Foam Froth, whipped cream, soap lather
      (Gas in Gas forms a true solution)

    • Based on Nature of Interaction between DP and DM:

      • Lyophilic Colloids (Solvent-loving): Strong affinity between DP and DM. Easily formed, stable, reversible. Examples: Starch, gelatin, proteins, rubber in suitable solvents.
      • Lyophobic Colloids (Solvent-hating): Little affinity between DP and DM. Difficult to prepare, unstable, require stabilizing agents, irreversible. Examples: Sols of metals (Au, Ag), metal sulphides (As₂S₃), metal hydroxides (Fe(OH)₃).

    • Based on Type of Particles of DP:

      • Multimolecular Colloids: Formed by aggregation of a large number of small molecules (diameter < 1 nm). Held by van der Waals forces. Examples: Sulphur sol (S₈ molecules), Gold sol.
      • Macromolecular Colloids: Formed by large molecules (macromolecules) having dimensions in the colloidal range. Stable, often resemble lyophilic sols. Examples: Starch, cellulose, proteins, synthetic polymers (nylon, polythene).
      • Associated Colloids (Micelles): Substances which behave as normal electrolytes at low concentrations but exhibit colloidal behaviour at higher concentrations due to the formation of aggregates (micelles).
        • Micelle Formation: Occurs above a particular temperature called Kraft Temperature (Tk) and above a particular concentration called Critical Micelle Concentration (CMC).
        • Examples: Soaps (sodium stearate C₁₇H₃₅COONa), detergents.
        • Mechanism: Soap molecule has a hydrophilic head (-COO⁻Na⁺) and a hydrophobic tail (C₁₇H₃₅-). Above CMC, anions aggregate with hydrophobic tails pointing inwards and hydrophilic heads outwards into the water.
        • Cleansing Action of Soap: Micelles entrap oily grease (hydrophobic interaction) in their core. The hydrophilic heads interact with water, allowing the grease droplet to be washed away.

  • Preparation of Colloids:

    • Chemical Methods: Double decomposition, oxidation, reduction, hydrolysis (e.g., As₂O₃ + 3H₂S → As₂S₃(sol) + 3H₂O; FeCl₃ + 3H₂O → Fe(OH)₃(sol) + 3HCl).
    • Electrical Disintegration (Bredig's Arc Method): For metal sols (Au, Ag, Pt). An electric arc is struck between metal electrodes immersed in the dispersion medium (kept cool). Intense heat vaporizes metal, which then condenses into colloidal particles.
    • Peptization: Process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium in the presence of a small amount of electrolyte (peptizing agent). The electrolyte provides ions that are preferentially adsorbed onto the precipitate particles, giving them a charge and causing repulsion. (e.g., adding FeCl₃ solution to Fe(OH)₃ precipitate).

  • Purification of Colloidal Solutions: Removal of excess electrolytes and soluble impurities.

    • Dialysis: Using a semipermeable membrane (parchment paper, cellophane) that allows ions/small molecules to pass but retains colloidal particles.
    • Electrodialysis: Dialysis carried out under the influence of an electric field to speed up the removal of ionic impurities.
    • Ultrafiltration: Using special filter papers (ultrafilters) with very fine pores that allow the dispersion medium and true solution solutes to pass through but retain colloidal particles. Pores are often reduced by impregnating with collodion.

  • Properties of Colloidal Solutions:

    • Colligative Properties: Show lower values than true solutions of the same concentration due to the larger size and smaller number of particles.
    • Tyndall Effect (Optical Property): Scattering of light by colloidal particles when a beam of light passes through the sol, making the path of light visible. Condition: Diameter of dispersed particles is not much smaller than the wavelength of light used; refractive indices of DP and DM differ greatly. Used to distinguish colloids from true solutions. Seen in ultramicroscope.
    • Brownian Movement (Kinetic Property): Continuous, random, zig-zag motion of colloidal particles due to unbalanced bombardment by the molecules of the dispersion medium. Provides stability against settling.
    • Charge on Colloidal Particles: Colloidal particles carry a uniform electric charge (either positive or negative). This charge arises due to:
      • Selective adsorption of common ions from the medium (e.g., AgI sol prepared with excess KI adsorbs I⁻ → negatively charged; AgI sol with excess AgNO₃ adsorbs Ag⁺ → positively charged).
      • Electron capture during Bredig's arc method.
      • Dissociation of surface molecules (e.g., proteins).
      • Electrical Double Layer: A combination of a fixed layer (adsorbed ions) and a diffuse layer (counter-ions) around the charged colloidal particle (Helmholtz or Stern double layer). The potential difference between these layers is called the Zeta Potential (or Electrokinetic Potential). Higher zeta potential indicates greater stability.
    • Electrophoresis (Electrical Property): Movement of colloidal particles towards the oppositely charged electrode under the influence of an applied electric field. Confirms the charge on particles. Used for coagulation and determining charge.
    • Coagulation or Precipitation (Flocculation): Process of settling down of colloidal particles by the addition of an excess of electrolyte, boiling, persistent dialysis, or mutual precipitation. The electrolyte provides ions opposite in charge to the colloidal particles, neutralizing their charge and allowing them to aggregate and settle.
      • Hardy-Schulze Rule: The greater the valency of the oppositely charged ion (coagulating ion) of the electrolyte added, the greater is its power to cause coagulation.
        • For negative sols (e.g., As₂S₃): Coagulating power Al³⁺ > Ba²⁺ > Na⁺
        • For positive sols (e.g., Fe(OH)₃): Coagulating power [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻
      • Coagulating Value: The minimum concentration of an electrolyte (in millimoles per litre) required to cause coagulation of a sol in two hours. Lower coagulating value means higher coagulating power.

  • Protection of Colloids:

    • Lyophobic sols can be stabilized by adding a small amount of a lyophilic sol (protective colloid).
    • The lyophilic colloid forms a protective layer around the lyophobic particles, preventing them from coming close and coagulating upon addition of electrolytes.
    • Gold Number: A measure of the protective power of a lyophilic colloid. Defined as the minimum weight (in milligrams) of the protective colloid required to just prevent the coagulation of 10 mL of a standard gold sol when 1 mL of 10% NaCl solution is added to it. Smaller the gold number, greater the protective power (e.g., Gelatin has a very low gold number).

  • Emulsions: Colloidal systems where both DP and DM are liquids. Generally unstable, require an emulsifying agent (emulsifier) for stabilization.

    • Types:
      • Oil in Water (O/W): Oil is DP, Water is DM (e.g., Milk, vanishing cream). Stabilized by proteins, gums, soaps.
      • Water in Oil (W/O): Water is DP, Oil is DM (e.g., Butter, cold cream). Stabilized by heavy metal salts of fatty acids, long-chain alcohols.
    • Test for Emulsion Type: Dilution test (O/W miscible with water), conductivity test (O/W conducts more), dye test.
    • Emulsifying Agent: Stabilizes the emulsion by forming an interfacial film between suspended particles and the medium. Examples: Soaps, detergents, proteins, gums.
    • Demulsification: Breaking an emulsion into its constituent liquids (by heating, freezing, centrifuging, adding electrolytes).

  • Applications of Colloids: Very wide range: Food items (milk, butter, ice cream), Medicines (colloidal silver - Argyrol, milk of magnesia), Sewage disposal (coagulation), Purification of drinking water (coagulation with alum), Cleansing action of soaps/detergents, Rubber industry (latex coagulation), Tanning of leather, Artificial rain, Smoke precipitation (Cottrell precipitator), Formation of delta.

Multiple Choice Questions (MCQs):

  1. Which of the following processes is responsible for the cleansing action of soap?
    (a) Adsorption
    (b) Emulsification
    (c) Peptization
    (d) Dialysis

  2. According to the Freundlich adsorption isotherm, at high pressure, the extent of adsorption (x/m) becomes:
    (a) Directly proportional to P
    (b) Inversely proportional to P
    (c) Directly proportional to P^(1/n)
    (d) Independent of P

  3. Which property of colloids is NOT dependent on the charge on colloidal particles?
    (a) Electrophoresis
    (b) Coagulation
    (c) Tyndall effect
    (d) Electro-osmosis

  4. Which of the following electrolytes will have the highest coagulating power for an As₂S₃ sol (negatively charged)?
    (a) NaCl
    (b) BaCl₂
    (c) AlCl₃
    (d) K₄[Fe(CN)₆]

  5. Fog is a colloidal solution of:
    (a) Gas in liquid
    (b) Liquid in gas
    (c) Solid in gas
    (d) Gas in solid

  6. The process of converting a freshly prepared precipitate into a colloidal solution by adding a suitable electrolyte is called:
    (a) Coagulation
    (b) Peptization
    (c) Dialysis
    (d) Electrophoresis

  7. Which of the following represents chemisorption?
    (a) Low enthalpy of adsorption
    (b) Reversible nature
    (c) Formation of multimolecular layers
    (d) High specificity and high activation energy

  8. ZSM-5 is an example of:
    (a) An enzyme catalyst
    (b) A homogeneous catalyst
    (c) A shape-selective catalyst (Zeolite)
    (d) An adsorption indicator

  9. The Brownian movement in colloidal solutions is due to:
    (a) Attraction and repulsion between charged particles
    (b) Convection currents in the medium
    (c) Unequal bombardment of colloidal particles by molecules of the dispersion medium
    (d) Scattering of light by colloidal particles

  10. Gold number is associated with:
    (a) Amount of gold present in a colloid
    (b) Protective power of lyophilic colloids
    (c) Purity of gold
    (d) Electrophoresis of gold sol

Answer Key for MCQs:

  1. (b) Emulsification (specifically micelle formation leading to emulsification of grease)
  2. (d) Independent of P
  3. (c) Tyndall effect (depends on particle size and refractive indices)
  4. (c) AlCl₃ (Al³⁺ has the highest positive charge among Na⁺, Ba²⁺, Al³⁺)
  5. (b) Liquid in gas
  6. (b) Peptization
  7. (d) High specificity and high activation energy
  8. (c) A shape-selective catalyst (Zeolite)
  9. (c) Unequal bombardment of colloidal particles by molecules of the dispersion medium
  10. (b) Protective power of lyophilic colloids

Remember to thoroughly revise these concepts, focusing on definitions, differences (like physisorption vs chemisorption, lyophilic vs lyophobic), key laws/rules (Hardy-Schulze), and applications. Good luck with your preparation!

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