Class 12 Chemistry Notes Chapter 5 (Surface Chemistry) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 5, 'Surface Chemistry'. This is an important chapter, not just for board exams but also frequently features in various government competitive exams. It deals with phenomena occurring at surfaces or interfaces. We'll break it down systematically, focusing on concepts often tested.

Chapter 5: Surface Chemistry - Detailed Notes for Exam Preparation

1. Introduction
Surface chemistry deals with phenomena that occur at the surfaces or interfaces. The interface represents the boundary separating two bulk phases (e.g., solid-gas, solid-liquid, liquid-liquid, liquid-gas).

2. Adsorption
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 substance on the surface of which adsorption takes place.
• Desorption: The process of removing an adsorbed substance from a surface.
• Sorption: When adsorption and absorption (substance uniformly distributed throughout the bulk) occur simultaneously.

Types of Adsorption:

Factors Affecting Adsorption of Gases on Solids:

1. Nature of Adsorbate (Gas): Easily liquefiable gases (higher critical temperatures) like NH₃, HCl, CO₂ are adsorbed more readily than gases like H₂, O₂, N₂ because van der Waals forces are stronger near critical temperatures.
2. Nature of Adsorbent: Porous materials and those with rough surfaces are better adsorbents due to larger surface area (e.g., charcoal, silica gel, alumina gel).
3. Surface Area of Adsorbent: Adsorption is a surface phenomenon, so ↑Surface Area → ↑Adsorption. Finely divided substances are good adsorbents.
4. Temperature: Adsorption is generally exothermic (ΔH is negative).
   • Physisorption: Decreases with increase in temperature (Le Chatelier's principle).
   • Chemisorption: Initially increases with temperature (provides activation energy) and then decreases.
5. Pressure (for Gases): Adsorption increases with an increase in pressure at constant temperature. At high pressure, saturation may be reached, especially in chemisorption (monolayer formation).
6. Activation of Adsorbent: Increasing the adsorbing power by methods like heating in vacuum (removes adsorbed gases), breaking into smaller pieces (increases surface area).

Adsorption Isotherms:
A graph between the amount of gas adsorbed per gram of adsorbent (x/m) and the equilibrium pressure (P) at a constant temperature.

• Freundlich Adsorption Isotherm (Empirical):

  • x/m = k * P^(1/n) (where n > 1)
  • log(x/m) = log k + (1/n) log P
  • Applicable at moderate pressures. Fails at high pressure (becomes independent of P) and very low pressure (x/m ∝ P).
  • Plot of log(x/m) vs log P is a straight line with slope 1/n and intercept log k.

• Langmuir Adsorption Isotherm (Theoretical):

  • Assumes monolayer adsorption and equivalent adsorption sites.
  • Derivation leads to relationships explaining behavior at low and high pressures. (Detailed derivation less common, but understanding the monolayer concept is key).

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 floatation process (concentration of sulphide ores)
• Adsorption indicators
• Chromatographic analysis

3. Catalysis
Substances that alter the rate of a chemical reaction without themselves undergoing any permanent chemical change. This phenomenon is called catalysis.

• Promoters: Substances that enhance the activity of a catalyst (e.g., Mo in Haber's process).
• Poisons: Substances that decrease the activity of a catalyst (e.g., CO poisoning Haemoglobin's O₂ carrying capacity, Arsenic compounds poisoning Pt catalyst).

Types of Catalysis:

1. 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)).
2. 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. Steps:

1. Diffusion of reactants to the catalyst surface.
2. Adsorption of reactant molecules onto the catalyst surface.
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 reaction occurs at 'active centres' on the catalyst surface)

Important Features of Solid Catalysts:

1. Activity: The ability of a catalyst to increase the rate of reaction. Largely depends on the strength of chemisorption (should be intermediate - not too strong, not too weak).
2. Selectivity: The ability of a catalyst to direct a reaction to yield a particular product (e.g., CO + H₂ can give CH₄, CH₃OH, or HCHO depending on the catalyst used).

Shape-Selective Catalysis by Zeolites:

• Catalytic reaction depends on the pore structure of the catalyst and the size/shape of reactant/product molecules.
• Zeolites are microporous aluminosilicates with a honeycomb-like structure.
• Example: ZSM-5 converts alcohols directly into gasoline (hydrocarbons).

Enzyme Catalysis (Biochemical Catalysis):

• Enzymes are complex nitrogenous organic compounds (proteins) that catalyse biochemical reactions.
• Characteristics: High efficiency, high specificity, active under optimum temperature (~298-310 K) and pH (~5-7), activity influenced by activators (co-enzymes) and inhibitors/poisons.
• Mechanism: Lock and Key model (substrate fits into the active site of the enzyme).

4. Colloids
A state in which the size of the solute particles is intermediate between those in true solutions and suspensions (Diameter: 1 nm to 1000 nm). It's a heterogeneous system.

• Dispersed Phase (DP): The component present in small proportion (like solute).
• Dispersion Medium (DM): The component in excess (like solvent).

Classification of Colloids:

1. Based on Physical State of DP and DM:

   • Solid in Gas: Aerosol (Smoke, dust)
   • Liquid in Gas: Aerosol (Fog, mist, cloud)
   • Gas in Solid: Solid Sol (Pumice stone, foam rubber)
   • Gas in Liquid: Foam (Whipped cream, soap lather)
   • Solid in Liquid: Sol (Paints, cell fluids, starch sol, gold sol)
   • Liquid in Liquid: Emulsion (Milk, hair cream)
   • Solid in Solid: Solid Sol (Coloured glasses, gemstones)
   • Liquid in Solid: Gel (Cheese, butter, jellies)
   • (Gas in Gas forms a true solution)

2. Based on Nature of Interaction between DP and DM:

   • Lyophilic Colloids (Solvent-loving): DP has great affinity for DM. Easily formed, stable, reversible. Examples: Gelatin, starch, gum, proteins in water.
   • Lyophobic Colloids (Solvent-hating): DP has little affinity for DM. Difficult to prepare, unstable, require stabilising agents, irreversible. Examples: Metals (Au, Ag sols), metal sulphides (As₂S₃ sol), metal hydroxides (Fe(OH)₃ sol).

3. Based on Type of Particles of the Dispersed Phase:

   • Multimolecular Colloids: On dissolution, a large number of small molecules aggregate to form species having size in the colloidal range (e.g., Sulphur sol, Gold sol). Held by van der Waals forces.
   • Macromolecular Colloids: Substances having large molecules (macromolecules) whose dimensions are in the colloidal range. Stable, resemble true solutions in some aspects. Examples: Starch, cellulose, proteins, nylon, polythene in suitable solvents.
   • 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). Examples: Soaps and detergents.
     • Micelle Formation: Occurs above a particular temperature (Kraft Temperature, T<0xE2><0x82><0x96>) and concentration (Critical Micelle Concentration, CMC).
     • Mechanism (Soap - Sodium Stearate C₁₇H₃₅COONa): Has a polar head (-COO⁻Na⁺) - hydrophilic, and a long non-polar tail (C₁₇H₃₅-) - hydrophobic. Above CMC, anions aggregate with tails inward and heads outward in water.

Preparation of Colloids (Lyophobic Sols):

• Dispersion Methods: Breaking down larger particles.
  • Mechanical Dispersion: Using a colloid mill.
  • Electrical Dispersion (Bredig’s Arc Method): For metal sols (Au, Ag, Pt). Arc struck between metal electrodes under the dispersion medium (water + stabiliser like KOH). Intense heat vaporises metal, which then condenses to form colloidal particles.
  • Peptization: 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 by the precipitate particles, giving them a charge and causing repulsion. (e.g., Fe(OH)₃ ppt + FeCl₃ solution → Fe(OH)₃ sol).
• Condensation Methods: Building up from smaller particles (ions/molecules).
  • Chemical Methods: Double decomposition (As₂O₃ + 3H₂S → As₂S₃(sol) + 3H₂O), Oxidation (SO₂ + 2H₂S → 3S(sol) + 2H₂O), Reduction (AuCl₃ + HCHO + H₂O → Au(sol) + HCOOH + HCl), Hydrolysis (FeCl₃ + 3H₂O → Fe(OH)₃(sol) + 3HCl).

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

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

Properties of Colloidal Solutions:

1. Colligative Properties: Exhibit properties like osmotic pressure, lowering of vapour pressure etc., but the values are small due to the large size/fewer number of particles compared to true solutions.
2. Tyndall Effect: Scattering of light by colloidal particles when a beam of light passes through the sol. The path of light becomes visible. True solutions do not show this. Conditions: (i) Diameter of dispersed particles not much smaller than wavelength of light used. (ii) Refractive indices of DP and DM differ greatly. Used to distinguish colloids from true solutions. (Observed via ultramicroscope).
3. Brownian Movement: Continuous zig-zag motion of colloidal particles due to unbalanced bombardment by the molecules of the dispersion medium. Responsible for the stability of sols (counteracts gravity).
4. Charge on Colloidal Particles: Colloidal particles carry either a positive or negative charge. The dispersion medium has an equal and opposite charge, making the system electrically neutral overall.
   • Origin of Charge: Preferential adsorption of ions from the medium, or dissociation of surface molecules.
   • Examples:
     • Positively charged sols: Metal hydroxides (Fe(OH)₃, Al(OH)₃), basic dyes (methylene blue), haemoglobin.
     • Negatively charged sols: Metal sols (Au, Ag, Cu), metal sulphides (As₂S₃, CdS), acid dyes (eosin, congo red), sols of starch, gum, gelatin, clay.
   • Helmholtz Electrical Double Layer: A layer of fixed oppositely charged ions forms around the charged colloidal particle, followed by a diffuse mobile layer of counter-ions. The potential difference between the fixed layer and the diffuse layer is called the Zeta Potential (or Electrokinetic Potential). Higher zeta potential indicates greater stability.
5. Electrophoresis (Cataphoresis): Movement of colloidal particles towards the oppositely charged electrode under the influence of an applied electric field. Confirms the charge on colloidal particles. If movement is prevented, the medium moves (Electroosmosis).
6. Coagulation or Precipitation (Flocculation): The process of settling down of colloidal particles by the aggregation of smaller particles. Can be caused by:
   • Electrophoresis: Particles move to electrode, get discharged, and precipitate.
   • Mixing opposite sols: Mutual coagulation occurs.
   • Boiling: Increases collisions, disturbs adsorbed layer.
   • Persistent dialysis: Removes stabilizing electrolytes.
   • Adding electrolytes: The ions carrying charge opposite to that of the sol particles cause neutralisation and coagulation.
   • Hardy-Schulze Rule: The greater the valency of the flocculating ion (oppositely charged ion), the greater its coagulating power.
     • For negative sols (e.g., As₂S₃): Al³⁺ > Ba²⁺ > Na⁺
     • For positive sols (e.g., Fe(OH)₃): [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻
   • Coagulating Value: Minimum concentration of an electrolyte (in millimoles per litre) required to cause coagulation of a sol in 2 hours. Smaller coagulating value means higher coagulating power.

Protection of Colloids:

• Lyophobic sols are unstable and easily coagulated.
• Lyophilic sols are much more stable. They can be used to protect lyophobic sols from coagulation by electrolytes.
• The process of protecting a lyophobic sol by adding a lyophilic sol is called protection. The lyophilic sol forms a layer around the lyophobic particles.
• Gold Number: (Introduced by Zsigmondy) The minimum amount of protective colloid (in milligrams) required to 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, starch has higher).

5. Emulsions
Colloidal systems in which both the dispersed phase and dispersion medium are liquids (immiscible or partially miscible).

• Types:
  • Oil in Water (O/W): Oil is DP, Water is DM (e.g., Milk, vanishing cream).
  • Water in Oil (W/O): Water is DP, Oil is DM (e.g., Butter, cold cream).
• Detection: Dilution test (emulsion miscible with DM), conductivity test (O/W conducts more), dye test.
• Emulsifiers (Emulsifying Agents): Substances added to stabilize emulsions. They form an interfacial film between DP and DM. Examples: Soaps, detergents (for O/W); long-chain alcohols, lampblack (for W/O); proteins, gums.
• Demulsification: Process of breaking an emulsion into its constituent liquids (e.g., by heating, freezing, centrifuging, adding electrolytes).

6. Colloids Around Us (Applications)

• Food: Milk, butter, ice cream, fruit juices.
• Medicines: Colloidal silver (Argyrol), milk of magnesia, antibiotic injections.
• Industry: Rubber plating, tanning of leather, paints, inks, synthetic plastics.
• Natural Phenomena: Blue colour of the sky (Tyndall effect by dust/water particles), fog, mist, rain, fertile soil (colloidal humus), delta formation (coagulation of river clay by sea electrolytes).
• Technical Applications: Sewage disposal (coagulation of dirt), smoke precipitation (Cottrell precipitator - uses electrophoresis), purification of drinking water (coagulation of suspended impurities by alum).

Multiple Choice Questions (MCQs)

1. Which of the following is an example of chemisorption?
   (a) Adsorption of N₂ on mica at low temperature
   (b) Adsorption of H₂ on Nickel surface
   (c) Adsorption of water vapour on silica gel
   (d) Formation of multilayers of gas on a solid surface

2. According to the Hardy-Schulze rule, the coagulating power of ions for a negatively charged Arsenious sulphide (As₂S₃) sol follows the order:
   (a) Na⁺ > Ba²⁺ > Al³⁺
   (b) Al³⁺ > Ba²⁺ > Na⁺
   (c) Cl⁻ > SO₄²⁻ > PO₄³⁻
   (d) PO₄³⁻ > SO₄²⁻ > Cl⁻

3. Which property of colloids is utilized in the Cottrell smoke precipitator?
   (a) Tyndall effect
   (b) Brownian movement
   (c) Electrophoresis
   (d) Adsorption

4. The formation of micelles takes place only above:
   (a) Inversion temperature
   (b) Boyle temperature
   (c) Critical temperature
   (d) Kraft temperature and CMC

5. Which of the following represents a multimolecular colloid?
   (a) Starch sol
   (b) Sulphur sol
   (c) Soap solution
   (d) Protein solution

6. The ability of a catalyst to direct a reaction to yield a particular product is termed as:
   (a) Activity
   (b) Selectivity
   (c) Reactivity
   (d) Specificity

7. Gold number is a measure of:
   (a) The amount of gold in a colloid
   (b) The stability of a colloid
   (c) The protective power of a lyophilic colloid
   (d) The charge on a gold sol particle

8. Bredig's Arc method is used for the preparation of:
   (a) Metal sulphide sols
   (b) Metal hydroxide sols
   (c) Metal sols (e.g., Au, Ag)
   (d) Emulsions

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

10. In the Freundlich adsorption isotherm equation, log(x/m) = log k + (1/n) log P, the value of 'n' is generally:
    (a) Equal to 1
    (b) Less than 1
    (c) Greater than 1
    (d) Zero

Answer Key for MCQs:

1. (b) - Chemisorption involves chemical bond formation, like H₂ on Ni. Others are examples of physisorption or its characteristics.
2. (b) - As₂S₃ is a negative sol. Coagulation depends on the charge of the positive ion (counter-ion). Higher the positive charge, greater the coagulating power (Hardy-Schulze rule).
3. (c) - Cottrell precipitator works on the principle of electrophoresis, where charged smoke particles are attracted to oppositely charged electrodes and precipitated.
4. (d) - Micelles form only above the Kraft temperature (T<0xE2><0x82><0x96>) and Critical Micelle Concentration (CMC).
5. (b) - Sulphur sol consists of aggregates of many small S₈ molecules. Starch and protein are macromolecular; soap is an associated colloid.
6. (b) - Selectivity refers to the catalyst's ability to favour the formation of a specific product among possible options.
7. (c) - Gold number specifically measures the protective action of a lyophilic colloid towards a lyophobic sol (standard gold sol). Lower gold number means better protection.
8. (c) - Bredig's Arc method involves striking an electric arc between metal electrodes immersed in the dispersion medium, suitable for preparing metal sols.
9. (b) - Fog is an aerosol where the dispersed phase is liquid (water droplets) and the dispersion medium is gas (air).
10. (c) - In the Freundlich equation x/m = k * P^(1/n), the exponent 1/n typically ranges between 0 and 1, which means n is generally greater than 1.

Remember to revise these concepts thoroughly, paying attention to definitions, differences (like physisorption vs chemisorption, lyophilic vs lyophobic), specific examples, and rules like Hardy-Schulze. Good luck with your preparation!