NEET Chemistry: Surface Chemistry – Detailed Notes and Practice Questions

Chapter 5: Surface Chemistry

1. Introduction to Surface Chemistry

• Surface Chemistry deals with phenomena that occur at the surfaces or interfaces of two phases. These phenomena include adsorption, catalysis, and colloid formation.
• An interface is the boundary between two bulk phases (e.g., solid-liquid, liquid-gas, solid-gas, liquid-liquid). When phases are miscible, no interface exists.

2. Adsorption

• Adsorption is the phenomenon of accumulation of molecular species at the surface rather than in the bulk of a solid or liquid.
• The substance that gets adsorbed is called the adsorbate.
• The surface on which adsorption takes place is called the adsorbent.
• Adsorption vs. Absorption:
  • Adsorption: Surface phenomenon (e.g., water vapor on silica gel).
  • Absorption: Bulk phenomenon, where the substance is uniformly distributed throughout the bulk of the solid or liquid (e.g., water vapor in anhydrous calcium chloride).
• Sorption: When both adsorption and absorption occur simultaneously (e.g., dyes on cotton).
• Desorption: The process of removing an adsorbed substance from a surface.

A. Mechanism of Adsorption:

• Adsorption arises due to the presence of unbalanced (residual) attractive forces at the surface of the adsorbent.
• Inside the bulk, forces acting on the particles are mutually balanced. At the surface, particles are not surrounded by similar atoms/molecules from all sides, resulting in residual attractive forces.

B. Enthalpy Change in Adsorption:

• When a gas is adsorbed on a solid surface, its freedom of movement is restricted, leading to a decrease in entropy (ΔS<0).
• For the process to be spontaneous, according to Gibbs-Helmholtz equation (ΔG=ΔH−TΔS), ΔG must be negative. Since ΔS is negative, ΔH must be negative (exothermic) and sufficiently large to make ΔG negative.
• Thus, adsorption is always an exothermic process (ΔH<0).

C. Types of Adsorption:

1. Physisorption (Physical Adsorption):
   • Mechanism: Adsorbate held to the adsorbent by weak van der Waals forces.
   • Nature: Non-specific.
   • Enthalpy of adsorption: Low (20−40kJ/mol).
   • Reversibility: Reversible (desorption occurs by increasing temperature or decreasing pressure).
   • Activation Energy: Low or almost negligible.
   • Layers: Multimolecular layers formed on adsorbent surface.
   • Effect of Temperature: Decreases with increasing temperature.
   • Effect of Pressure: Increases with increasing pressure.
   • Nature of Adsorbent: Not specific, any gas can be adsorbed on any solid. Favoured by easily liquefiable gases.
2. Chemisorption (Chemical Adsorption):
   • Mechanism: Adsorbate held to the adsorbent by strong chemical bonds (covalent or ionic).
   • Nature: Highly specific.
   • Enthalpy of adsorption: High (80−240kJ/mol). Comparable to chemical bond enthalpies.
   • Reversibility: Irreversible (forms compounds).
   • Activation Energy: High activation energy often required.
   • Layers: Unimolecular layer formed on adsorbent surface.
   • Effect of Temperature: Increases with increase in temperature (up to a certain point, then decreases due to desorption).
   • Effect of Pressure: Increases with increasing pressure.
   • Nature of Adsorbent: Highly specific, requires chemical bond formation.

D. Factors Affecting Adsorption of Gases on Solids:

1. Nature of Adsorbate: Easily liquefiable gases (high critical temperature) are more readily adsorbed (e.g., SO2​,NH3​,Cl2​) because van der Waals forces are stronger.
2. Nature of Adsorbent: Surface area is crucial. Porous and finely divided solids are good adsorbents.
3. Temperature: Adsorption is an exothermic process, so it decreases with increasing temperature (Le Chatelier’s Principle).
4. Pressure: Adsorption increases with increasing pressure (for physisorption, up to saturation).

E. Adsorption Isotherms:

• A graph that shows the relationship between the amount of adsorbate adsorbed by an adsorbent and the pressure (or concentration) at constant temperature.

1. Freundlich Adsorption Isotherm:
   • An empirical relationship: x/m=kP1/n (n>1)
     • x: mass of adsorbate, m: mass of adsorbent, P: pressure.
     • k and n are constants at a given temperature.
   • In logarithmic form: log(x/m)=logk+(1/n)logP
   • Valid for a limited range of pressure. Fails at high pressure where adsorption reaches saturation.
2. Langmuir Adsorption Isotherm:
   • Based on theoretical assumptions (monolayer adsorption, constant heat of adsorption, dynamic equilibrium).
   • Equation: x/m=(aP)/(1+bP)
     • a and b are constants.
   • Provides a better fit for adsorption data over a wider range of pressure.
   • Explains saturation at high pressure.

F. Adsorption from Solution Phase:

• Solids can adsorb solutes from solutions.
• Factors: Temperature (decreases with increase), nature of adsorbate (structure, polarity), nature of adsorbent (surface area), concentration of solute.

3. Catalysis

• Catalysis: The process that alters the rate of a chemical reaction by adding a substance called a catalyst.
• Catalyst: A substance that increases the rate of a reaction without itself being consumed in the reaction. It participates in the reaction but is regenerated at the end.
• Promoters: Substances that enhance the activity of a catalyst. Example: Mo in Haber’s process (Fe+Mo→NH3​).
• Poisons: Substances that decrease or destroy the activity of a catalyst. Example: CO in Haber’s process, H2​S for metal catalysts.

A. Types of Catalysis:

1. Homogeneous Catalysis: Catalyst and reactants are in the same phase (gas or liquid).
   • Examples:
     • Oxidation of SO2​ to SO3​ by NO in lead chamber process: 2SO2​(g)+O2​(g)NO(g)​2SO3​(g)
     • Hydrolysis of ester by acid: CH3​COOCH3​(l)+H2​O(l)H+(aq)​CH3​COOH(aq)+CH3​OH(aq)
2. Heterogeneous Catalysis: Catalyst and reactants are in different phases.
   • Examples:
     • Haber’s process for NH3​ synthesis: N2​(g)+3H2​(g)Fe(s)​2NH3​(g)
     • Contact process for H2​SO4​ synthesis: 2SO2​(g)+O2​(g)V2​O5​(s)​2SO3​(g)

B. Adsorption Theory of Heterogeneous Catalysis:

• Explains why heterogeneous catalysts work by providing a surface for reactions.
• Steps:
  1. Diffusion: Reactants diffuse to the catalyst surface.
  2. Adsorption: Reactant molecules get adsorbed on the active sites of the catalyst surface.
  3. Reaction: Adsorbed reactant molecules react to form products (often via intermediate formation).
  4. Desorption: Product molecules desorb from the surface.
  5. Diffusion: Products diffuse away from the catalyst surface.
• Active Sites: Specific sites on the catalyst surface that have high catalytic activity due to unsaturation.

C. Shape-Selective Catalysis by Zeolites:

• Zeolites: Aluminosilicates with a honeycomb-like porous structure.
• Shape-selective: Their catalytic activity depends on the pore structure of the catalyst and the size/shape of the reactant and product molecules.
• Example: ZSM-5 (used to convert alcohols directly into gasoline).

D. Enzymes Catalysis:

• Enzymes: Complex nitrogenous organic compounds (proteins) produced by living organisms. They are highly efficient biochemical catalysts.
• Characteristics: Highly specific (each enzyme catalyzes a specific reaction), highly efficient (accelerate reactions up to 1020 times), sensitive to temperature (optimum temperature 298−310K) and pH (optimum pH 5−7).
• Mechanism (Lock and Key Hypothesis):
  1. Enzyme-Substrate Binding: The substrate molecule (reactant) fits into the specific “active site” of the enzyme, forming an enzyme-substrate complex. (Like a key fitting into a lock).
  2. Catalysis: The enzyme facilitates the reaction, transforming the substrate into product.
  3. Product Release: The product molecules leave the active site, and the enzyme is ready for another cycle.

4. Colloids

• Colloids (Dispersions): Heterogeneous systems in which one substance is dispersed (dispersed phase) in another substance (dispersion medium).
• Particle Size: The size of dispersed particles is intermediate between true solutions and suspensions (diameter 1nm to 1000nm).
• True Solution: Homogeneous, particle size <1nm. Particles cannot be seen, do not scatter light, do not settle.
• Suspension: Heterogeneous, particle size >1000nm. Particles visible, settle on standing.

A. Classification of Colloids:

1. Based on Physical State of Dispersed Phase and Dispersion Medium:
   • Solid in Solid: Solid Sol (e.g., coloured glass, gemstones)
   • Solid in Liquid: Sol (e.g., paints, cell fluids)
   • Solid in Gas: Aerosol (e.g., smoke, dust)
   • Liquid in Solid: Gel (e.g., cheese, jellies)
   • Liquid in Liquid: Emulsion (e.g., milk, butter)
   • Liquid in Gas: Aerosol (e.g., fog, mist, clouds)
   • Gas in Solid: Solid Foam (e.g., pumice stone, foam rubber)
   • Gas in Liquid: Foam (e.g., whipped cream, soap lather)
2. Based on Nature of Interaction between Dispersed Phase and Dispersion Medium:
   • Lyophilic Colloids (Solvent-loving):
     • Strong affinity between dispersed phase and dispersion medium.
     • Easily formed by directly mixing.
     • Reversible (can be reformed by just mixing if medium is separated).
     • Stable.
     • Examples: Starch solution, gum, proteins, gelatin.
   • Lyophobic Colloids (Solvent-hating):
     • Little or no affinity between dispersed phase and dispersion medium.
     • Special methods needed for preparation.
     • Irreversible.
     • Less stable, easily coagulated.
     • Examples: Metal sols (Au sol, Ag sol), metal sulphides (As$_2S_3$ sol), metal hydroxides.
3. Based on Type of Particles of the Dispersed Phase:
   • Multimolecular Colloids: Formed by aggregation of a large number of atoms or small molecules (diameter <1nm) to form particles in colloidal range. Held by van der Waals forces.
     • Example: Gold sol (particles may contain hundreds of Au atoms), Sulphur sol (S8​ units aggregate).
   • Macromolecular Colloids: Large molecules (macromolecules) having dimensions in the colloidal range. High molecular masses.
     • Example: Starch, cellulose, proteins, nylon, polythene.
   • Associated Colloids (Micelles): Formed by substances that act as strong electrolytes at low concentrations but exhibit colloidal behavior at higher concentrations (due to aggregation).
     • Micelles: Aggregates of molecules (like soaps or detergents) that form above a certain concentration called Critical Micelle Concentration (CMC) and a certain temperature called Kraft temperature (Tk​).
     • Soaps/Detergents: Have a polar head (hydrophilic) and a non-polar hydrocarbon tail (hydrophobic).
     • Formation in water: Hydrophobic tails aggregate inward, forming a non-polar core, while hydrophilic heads face outward towards water, forming a sphere.
     • Cleansing Action of Soaps: Micelles encapsulate oily/greasy dirt particles in their non-polar core, suspending them in water for removal.

B. Preparation of Colloids:

1. Condensation Methods: Aggregation of smaller particles.
   • Chemical methods (oxidation, reduction, hydrolysis, double decomposition).
   • Bredig’s Arc Method (for metal sols: Au, Ag, Pt – uses electric arc).
   • Peptization (conversion of fresh precipitate into colloidal sol by adding an electrolyte (peptizing agent)).
2. Dispersion Methods: Breaking down larger particles.
   • Mechanical dispersion (colloid mill).
   • Electrical disintegration (Bredig’s Arc method is also a dispersion method).
   • Ultrasonic dispersion.

C. Purification of Colloidal Solutions:

1. Dialysis: Removal of dissolved substances (crystalloids) from colloidal solutions by diffusion through a selectively permeable membrane.
2. Electrodialysis: Dialysis in the presence of an electric field to speed up the process.
3. Ultrafiltration: Filtering colloidal solutions through special filters (e.g., collodion membranes) that are permeable to all substances except colloidal particles.

D. Properties of Colloidal Solutions:

1. Tyndall Effect: Scattering of light by colloidal particles when a beam of light is passed through the colloidal solution. Makes the path of light visible. (Observed in true solutions only if highly concentrated, not in suspensions).
2. Brownian Movement: Continuous, random zigzag motion of colloidal particles in the dispersion medium. Due to unbalanced collisions with the molecules of the dispersion medium. Provides stability to sol (prevents settling).
3. Electrophoresis (Cataphoresis): Movement of colloidal particles under the influence of an electric field towards the oppositely charged electrode. Proves that colloidal particles carry electric charge.
4. Coagulation (Flocculation): The process of settling down of colloidal particles, forming a precipitate. Occurs when the charge on colloidal particles is neutralized.
   • Hardy-Schulze Rule: The coagulating power of an electrolyte is directly proportional to the valency of the active ion (the ion carrying charge opposite to that of the colloidal particles).
     • For negatively charged sol: Coagulating power order: Al3+>Ba2+>Na+
     • For positively charged sol: Coagulating power order: Fe(CN)64−​>SO42−​>Cl−
   • Precipitation by Electrophoresis: Colloidal particles move to oppositely charged electrodes and get discharged, leading to coagulation.
   • Mutual Coagulation: Mixing two oppositely charged sols.
   • Salting Out: Addition of large amount of electrolyte.

E. Emulsions:

• Colloidal systems in which both the dispersed phase and the dispersion medium are liquids.
• Types:
  1. Oil in Water (O/W) type: Oil is the dispersed phase, water is the dispersion medium. (e.g., milk, vanishing cream).
  2. Water in Oil (W/O) type: Water is the dispersed phase, oil is the dispersion medium. (e.g., butter, cod liver oil).
• Emulsifying Agents: Substances added to stabilize an emulsion. They form an interfacial film between the dispersed phase and dispersion medium.
  • Examples: Proteins, gums, soaps, detergents.
• Demulsification: Breaking an emulsion into its constituent liquids (e.g., by heating, centrifugation, freezing, adding electrolytes).

5. Applications of Colloids

1. Purification of Water: Alum (potassium aluminium sulphate) is used to coagulate suspended impurities in water.
2. Medicines: Colloidal medicines are more effective due to their large surface area (e.g., Argyrol (Ag sol) for eye lotion, Colloidal Antimony for kala-azar, Milk of Magnesia for stomach disorders).
3. Tanning of Leather: Animal hides (positively charged protein) are treated with tannin (negatively charged colloid) to produce hardened leather (mutual coagulation).
4. Rubber Industry: Latex (negatively charged rubber particles) is coagulated by adding acetic acid to produce rubber.
5. Industrial Products: Paints, inks, synthetic plastics, rubber, graphite lubricants, varnishes are colloidal in nature.
6. Sewage Disposal: Colloidal impurities in sewage are removed by electrophoresis (charged particles move to electrodes and coagulate).
7. Smog and Dust Precipitation (Cottrell Precipitator): Smoke particles (charged colloids) are passed through charged plates, where they get discharged and precipitate.
8. Artificial Rain: Spraying oppositely charged colloidal dust or sand particles over clouds (which are colloids of water droplets) causes coagulation and leads to rain.

NEET Chemistry: Surface Chemistry – Practice Questions

I. Multiple Choice Questions (MCQs)

1. Question: Which of the following is an example of absorption? a) Water vapor on silica gel b) Water vapor in anhydrous calcium chloride c) Hydrogen gas on charcoal d) Dye on cotton fabric

2. Question: Adsorption is always an exothermic process because: a) ΔH>0 b) ΔS<0 c) ΔG<0 d) Both (b) and (c) are prerequisites for spontaneity, and for ΔG<0 with ΔS<0, ΔH must be negative.

3. Question: Which of the following is a characteristic of physisorption? a) High enthalpy of adsorption (80−240kJ/mol) b) Highly specific nature c) Formation of multimolecular layers d) Irreversible process

4. Question: According to Freundlich adsorption isotherm, the relationship between the amount of adsorbate (x/m) and pressure (P) at constant temperature is: a) x/m=kP b) x/m=kP1/n c) x/m=kPn d) x/m=(aP)/(1+bP)

5. Question: Which of the following is an example of homogeneous catalysis? a) Haber’s process for NH3​ synthesis b) Contact process for H2​SO4​ synthesis c) Oxidation of SO2​ to SO3​ by NO in lead chamber process d) Hydrogenation of oils using Ni

6. Question: The substance that enhances the activity of a catalyst is called a: a) Poison b) Promoter c) Substrate d) Adsorbate

7. Question: Which of the following is not a characteristic of enzyme catalysis? a) Highly specific b) Sensitive to temperature and pH c) Forms an enzyme-substrate complex d) Works effectively over a wide range of temperatures

8. Question: The size range of colloidal particles is approximately: a) <1nm b) 1nm to 1000nm c) >1000nm d) 10μm to 100μm

9. Question: Milk is an example of which type of colloid? a) Solid in Liquid (Sol) b) Liquid in Liquid (Emulsion) c) Liquid in Solid (Gel) d) Gas in Liquid (Foam)

10. Question: Which of the following is a lyophobic colloid? a) Starch solution b) Gold sol c) Gelatin d) Gum solution

11. Question: Micelles are examples of: a) Multimolecular colloids b) Macromolecular colloids c) Associated colloids d) Lyophilic colloids

12. Question: The process of converting a freshly prepared precipitate into a colloidal sol by adding a small amount of electrolyte is called: a) Coagulation b) Dialysis c) Peptization d) Electrophoresis

13. Question: Which property of colloidal solutions makes the path of light visible? a) Brownian movement b) Electrophoresis c) Tyndall effect d) Coagulation

14. Question: According to Hardy-Schulze rule, for the coagulation of a negatively charged sol, the most effective ion would be: a) Na+ b) K+ c) Ba2+ d) Al3+

15. Question: Which of the following is an O/W (oil in water) emulsion? a) Butter b) Vanishing cream c) Cod liver oil d) Cold cream

II. Assertion-Reason Type Questions

Directions: In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R). Choose the correct option. a) Both A and R are true and R is the correct explanation of A. b) Both A and R are true but R is NOT the correct explanation of A. c) A is true but R is false. d) A is false but R is true.

16. Assertion (A): Physisorption decreases with an increase in temperature. Reason (R): Physisorption is an exothermic process.

17. Assertion (A): Colloidal solutions show Brownian movement. Reason (R): Brownian movement is due to the unbalanced collisions of colloidal particles with the molecules of the dispersion medium.

18. Assertion (A): Zeolites are shape-selective catalysts. Reason (R): Their catalytic activity depends on the pore structure of the catalyst and the size and shape of reactant and product molecules.

19. Assertion (A): Gold sol is a lyophilic colloid. Reason (R): Lyophilic colloids are easily prepared by simple mixing and are highly stable.

20. Assertion (A): Micelles are formed by soap solutions above a certain concentration. Reason (R): Micelles are aggregates of molecules where hydrophobic parts are directed inwards and hydrophilic parts outwards.

III. Short Answer / Conceptual Questions

21. Question: Differentiate between adsorption and absorption. Give one example for each.

22. Question: What is the significance of ΔG<0 and ΔH<0 for adsorption to be a spontaneous process?

23. Question: Explain the Lock and Key hypothesis for enzyme catalysis.

24. Question: Define Critical Micelle Concentration (CMC) and Kraft Temperature (Tk​). What are their roles in micelle formation?

25. Question: Explain the cleansing action of soap.

26. Question: How can colloidal solutions be purified using dialysis? What is electrodialysis?

27. Question: State Hardy-Schulze rule. Give an example of its application for the coagulation of a positive sol.

28. Question: Differentiate between multimolecular and macromolecular colloids with one example each.

29. Question: What are emulsifying agents? Give two examples and their function.

30. Question: List any three applications of colloids in daily life or industry.

Answers and Explanations

I. Multiple Choice Questions (MCQs) – Answers

1. Answer: b) Water vapor in anhydrous calcium chloride Explanation: Absorption is a bulk phenomenon where the substance is uniformly distributed throughout the solid/liquid. Water vapor in anhydrous calcium chloride is an example of absorption. Water vapor on silica gel is adsorption. Hydrogen on charcoal is adsorption. Dye on cotton is sorption (both adsorption and absorption).

2. Answer: d) Both (b) and (c) are prerequisites for spontaneity, and for ΔG<0 with ΔS<0, ΔH must be negative. Explanation: For a process to be spontaneous, ΔG must be negative. Adsorption involves a decrease in randomness (entropy, ΔS<0). According to the Gibbs-Helmholtz equation (ΔG=ΔH−TΔS), for ΔG to be negative when ΔS is negative, ΔH must be negative (exothermic) and its magnitude must be greater than TΔS.

3. Answer: c) Formation of multimolecular layers Explanation: Physisorption involves weak van der Waals forces, allowing the formation of multiple layers of adsorbate molecules on the adsorbent surface. High enthalpy, high specificity, and irreversibility are characteristics of chemisorption.

4. Answer: b) x/m=kP1/n Explanation: This is the empirical mathematical expression for the Freundlich adsorption isotherm, where x is the mass of adsorbate, m is the mass of adsorbent, P is pressure, and k,n are constants.

5. Answer: c) Oxidation of SO2​ to SO3​ by NO in lead chamber process Explanation: In homogeneous catalysis, the catalyst and reactants are in the same phase. In the lead chamber process, SO2​(g), O2​(g), and NO(g) (catalyst) are all in the gaseous phase. The other examples are heterogeneous catalysis.

6. Answer: b) Promoter Explanation: A promoter is a substance that enhances the activity of a catalyst. A poison decreases or destroys catalyst activity. A substrate is a reactant in enzymatic reactions. An adsorbate is the substance that gets adsorbed.

7. Question: d) Works effectively over a wide range of temperatures Explanation: Enzymes are highly sensitive to temperature and pH, showing optimum activity within a narrow range. They get denatured outside this range. They are highly specific and form enzyme-substrate complexes.

8. Question: b) 1nm to 1000nm Explanation: Colloidal particles have diameters ranging from approximately 1 nanometer to 1000 nanometers. Particles smaller than 1 nm form true solutions, and those larger than 1000 nm form suspensions.

9. Question: b) Liquid in Liquid (Emulsion) Explanation: Milk is an emulsion, specifically an oil-in-water (O/W) type emulsion, where tiny droplets of fat (oil) are dispersed in water.

10. Answer: b) Gold sol Explanation: Gold sol is an example of a lyophobic (solvent-hating) colloid. Lyophobic colloids require special methods for preparation and are less stable. Starch, gelatin, and gum solutions are examples of lyophilic (solvent-loving) colloids.

11. Answer: c) Associated colloids Explanation: Micelles are formed by the aggregation of a large number of ions or molecules (like soaps and detergents) above a certain concentration (CMC) and temperature (Kraft temperature). They are a type of associated colloid.

12. Answer: c) Peptization Explanation: Peptization is the process used to convert a freshly prepared precipitate into a colloidal sol by adding a small amount of an electrolyte called a peptizing agent.

13. Answer: c) Tyndall effect Explanation: The Tyndall effect is the phenomenon of scattering of light by colloidal particles, making the path of the light beam visible. This occurs because the size of colloidal particles is comparable to the wavelength of visible light.

14. Answer: d) Al3+ Explanation: According to Hardy-Schulze rule, the coagulating power of an ion is directly proportional to its valency. For a negatively charged sol, the effectiveness of cations in causing coagulation increases with increasing positive charge. So, Al3+ (valency +3) is most effective, followed by Ba2+ (valency +2), and then Na+ and K+ (valency +1).

15. Answer: b) Vanishing cream Explanation: Vanishing cream is an oil-in-water (O/W) emulsion, meaning oil droplets are dispersed in a continuous water phase. Butter, cod liver oil, and cold cream are examples of water-in-oil (W/O) emulsions.

II. Assertion-Reason Type Questions – Answers

16. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Physisorption is an exothermic process (ΔH<0). According to Le Chatelier’s principle, for an exothermic process, increasing the temperature shifts the equilibrium towards desorption (the reverse process), thus decreasing the amount of physisorption. So, Reason (R) correctly explains Assertion (A).

17. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Colloidal solutions exhibit Brownian movement, which is the continuous, random zigzag motion of colloidal particles. This motion is caused by the unbalanced collisions of the colloidal particles with the rapidly moving molecules of the dispersion medium. This continuous motion helps in stabilizing the sol by preventing the particles from settling down due to gravity.

18. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Zeolites are aluminosilicates with a specific porous structure. Their catalytic activity is indeed highly shape-selective, meaning that only reactant molecules of a particular size and shape can enter the pores and react, and only product molecules of a certain size and shape can exit. Reason (R) accurately explains the phenomenon of shape-selective catalysis.

19. Answer: d) A is false but R is true. Explanation: Assertion (A) is false: Gold sol is a lyophobic (solvent-hating) colloid, not lyophilic. Lyophobic colloids are difficult to prepare directly and are less stable. Reason (R) is true: Lyophilic colloids (like starch, gum) are easily prepared by simple mixing and are highly stable (reversible).

20. Answer: a) Both A and R are true and R is the correct explanation of A. Explanation: Micelles are formed by associated colloids (like soaps and detergents) above a certain concentration (CMC) and temperature. In an aqueous solution, the hydrophobic (non-polar) hydrocarbon tails of the soap/detergent molecules aggregate inwards to form a core, while the hydrophilic (polar) heads face outwards towards the water, forming a spherical aggregate, which is exactly how micelles are structured and why they form.

III. Short Answer / Conceptual Questions – Answers

21. Question:

• Adsorption: It is a surface phenomenon where molecular species accumulate only at the surface of a solid or liquid, rather than penetrating into the bulk.
  • Example: Water vapor adsorbing on the surface of silica gel.
• Absorption: It is a bulk phenomenon where molecular species are uniformly distributed throughout the bulk of the solid or liquid. The substance penetrates the entire volume of the material.
  • Example: Water vapor being absorbed by anhydrous calcium chloride, leading to a uniform distribution of water molecules throughout the salt.

22. Question: For a process to be spontaneous, the change in Gibbs free energy (ΔG) must be negative (ΔG<0). The Gibbs-Helmholtz equation is: ΔG=ΔH−TΔS

• In adsorption, molecules move from a freer state (gas/solution) to a more restricted state on the surface. This leads to a decrease in entropy (ΔS<0) of the system.
• For ΔG to be negative when ΔS is negative, the enthalpy change (ΔH) must also be negative (i.e., the process must be exothermic), and its magnitude must be sufficiently large to overcome the positive TΔS term. Since ΔS is negative, −TΔS becomes positive. To make ΔG negative, ΔH must be negative and ∣ΔH∣>∣TΔS∣. Therefore, adsorption is always an exothermic process, releasing heat.

23. Question: The Lock and Key hypothesis is a model proposed by Emil Fischer to explain the specificity and mechanism of enzyme catalysis:

1. Specific Active Site (The Lock): Each enzyme has a unique three-dimensional structure with a specific region called the active site. This active site has a precise shape and chemical environment (due to specific amino acid residues) that is complementary to the shape of its specific substrate molecule. It acts like a “lock.”
2. Substrate Binding (The Key): The reactant molecule, called the substrate, has a shape that precisely fits into the active site of the enzyme, similar to how a unique “key” fits into a specific “lock.”
3. Enzyme-Substrate Complex Formation: When the substrate binds to the active site, it forms an intermediate structure called the enzyme-substrate complex (ES complex).
4. Catalysis and Product Release: Within the ES complex, the enzyme facilitates the chemical reaction, converting the substrate(s) into product(s). Once the products are formed, they no longer fit the active site’s shape and are released, leaving the enzyme free to bind with another substrate molecule and repeat the catalytic cycle. This hypothesis explains why enzymes are highly specific, generally catalyzing only one or a very limited type of reaction.

24. Question:

• Critical Micelle Concentration (CMC): It is the minimum concentration of a surfactant (like soap or detergent) in a solution above which micelle formation begins. Below the CMC, surfactant molecules exist individually in the solution. Above CMC, they aggregate to form micelles.
• Kraft Temperature (Tk​): It is the minimum temperature above which micelle formation can occur for a given surfactant solution. Below the Kraft temperature, surfactant molecules exist as individual ions/molecules regardless of concentration, because the thermal energy is not sufficient to overcome the forces preventing micelle formation.
• Role in Micelle Formation: Micelles will only form in a surfactant solution if the concentration of the surfactant is above its CMC AND the temperature of the solution is above its Kraft temperature (Tk​). Both conditions must be met for stable micelle formation.

25. Question: The cleansing action of soap is based on its ability to form micelles and emulsify greasy or oily dirt.

1. Structure of Soap: Soap molecules are sodium or potassium salts of long-chain fatty acids. They have two distinct parts:
   • A long non-polar hydrocarbon tail (hydrophobic, “water-hating”) that is soluble in oil and grease.
   • A short polar ionic head (hydrophilic, “water-loving”, COO−Na+) that is soluble in water.
2. Micelle Formation: When soap is added to water, the molecules orient themselves with their hydrophilic heads towards the water and hydrophobic tails away from the water. Above the Critical Micelle Concentration (CMC), these molecules aggregate to form spherical structures called micelles. In a micelle, the hydrophobic tails cluster inwards, forming a non-polar core, while the hydrophilic heads remain on the outer surface in contact with water.
3. Emulsification of Dirt: When a dirty (greasy/oily) cloth is put into soap solution:
   • The non-polar hydrocarbon tails of the soap molecules penetrate into the oil/grease droplets because they are soluble in oil.
   • The polar ionic heads remain exposed to the water on the surface of the oil droplet.
   • This arrangement encapsulates the oil/grease droplet within a layer of soap molecules, forming an emulsion. The negatively charged heads on the surface of these droplets repel each other, preventing the aggregation of dirt particles.
4. Washing Away: The emulsified oil/grease droplets, now surrounded by hydrophilic heads, are suspended in water and can be easily washed away with rinsing. This effectively removes the dirt from the surface.

26. Answer: Dialysis: Dialysis is a process used to purify colloidal solutions by removing dissolved impurities (crystalloids like ions or small molecules) from them. It works on the principle of diffusion through a selectively permeable membrane.

• Process: The impure colloidal solution is placed in a bag made of a selectively permeable membrane (e.g., cellophane, parchment paper, or animal bladder). This bag is suspended in a vessel through which fresh distilled water (or a suitable solvent) is continuously flowing. The dissolved impurities (crystalloids) are small enough to diffuse through the pores of the membrane into the flowing water, while the larger colloidal particles remain inside the bag due to their size.
• Electrodialysis: Electrodialysis is an accelerated version of dialysis. It is used when the impurities are electrolytes (ions). An electric field is applied across the dialyzing membrane. The ions (impurities) being charged, migrate faster towards the oppositely charged electrodes, thereby speeding up their removal from the colloidal solution.

27. Answer: Hardy-Schulze Rule: This rule states that the effectiveness (coagulating power) of an electrolyte in coagulating a colloidal sol is directly proportional to the valency (charge) of the active ion (the ion that carries the charge opposite to that of the colloidal particles). Example of its application for the coagulation of a positive sol: Let’s consider a positively charged sol, such as ferric hydroxide sol (Fe(OH)3​ sol). To coagulate this sol, we need negatively charged ions (anions). According to Hardy-Schulze rule, the coagulating power of anions will increase with their valency.

• For example, consider the ions: Cl−,SO42−​,[Fe(CN)6​]4−.
• Their valencies (magnitudes of charge) are: 1, 2, and 4 respectively.
• Therefore, the increasing order of coagulating power for Fe(OH)3​ sol would be: Cl−<SO42−​<[Fe(CN)6​]4− This means K4​[Fe(CN)6​] would be most effective in coagulating the positive ferric hydroxide sol.

28. Answer: Colloids can be classified based on the type of particles of the dispersed phase:

1. Multimolecular Colloids:
   • Definition: These colloids are formed by the aggregation of a large number of atoms or small molecules (typically with a diameter less than 1 nm) to form particles whose size falls within the colloidal range (1nm−1000nm).
   • Interactions: The individual atoms or molecules are held together by weak van der Waals forces.
   • Example: A gold sol consists of aggregates of many gold atoms. A sulphur sol contains many S8​ molecules aggregated together to form colloidal particles.
2. Macromolecular Colloids:
   • Definition: These colloids are formed when large molecules (macromolecules) themselves have dimensions that fall within the colloidal range. They usually have very high molecular masses.
   • Interactions: The individual atoms within the macromolecule are held by strong covalent bonds. The molecules themselves are large enough to be in the colloidal size.
   • Example: Solutions of starch, cellulose, proteins, enzymes, synthetic polymers like nylon and polythene are examples of macromolecular colloids. They behave like true solutions in many respects but are colloidal in size.

29. Answer:

• Emulsifying Agents (Emulsifiers or Stabilizers): These are substances that are added to an emulsion to stabilize it, i.e., to prevent the separation of the two immiscible liquid phases. They do this by reducing the interfacial tension between the two liquids and forming a protective film around the dispersed phase droplets.
• Two Examples and their function:
  1. Soaps and Detergents: These are common emulsifying agents. Their molecules have a dual nature (hydrophilic head and hydrophobic tail). They form an interfacial film around the oil droplets (in O/W emulsions) or water droplets (in W/O emulsions), which prevents the droplets from coalescing. The charged heads on the surface of the droplets also create electrostatic repulsion, further stabilizing the emulsion.
  2. Proteins and Gums (e.g., Arabic gum, gelatin): These are effective emulsifying agents, particularly for oil-in-water (O/W) emulsions. They form a protective viscous layer around the dispersed phase particles, preventing them from coming together and separating. For example, proteins in milk act as natural emulsifiers, keeping the fat globules dispersed in water.

30. Question: Three important applications of colloids in daily life or industry:

1. Purification of Water: Alum (potassium aluminium sulphate, K2​SO4​⋅Al2​(SO4​)3​⋅24H2​O) is widely used to purify water. The Al3+ ions from alum neutralize the negative charge on suspended clay and other colloidal impurities in water, causing them to coagulate and settle down, making the water clear.
2. Medicines: Many medicines are prepared in colloidal form because colloidal particles have a very large surface area, making them more effective and easily assimilated by the body. Examples include:
   • Argyrol (a silver sol) is used as an eye lotion.
   • Colloidal antimony is used in the treatment of kala-azar.
   • Milk of Magnesia (a colloidal suspension of magnesium hydroxide) is used as an antacid for stomach disorders.
3. Smoke Precipitation (Cottrell Precipitator): Smoke is a colloidal dispersion of solid carbon particles in air. Industrial smoke often contains charged particles that pollute the environment. In a Cottrell precipitator, smoke is passed through a chamber containing highly charged plates. The charged smoke particles are attracted to the oppositely charged plates, get discharged, coagulate, and precipitate, thus reducing air pollution. (Other applications: Tanning of leather, sewage disposal, rubber industry, artificial rain, photographic plates, etc.)