Questions & explanations
1. Which of the following correctly describes the effect of temperature on the extent of adsorption?
- Physisorption increases with temperature; chemisorption decreases with temperature.
- Physisorption decreases with temperature; chemisorption first increases then decreases with temperature.
- Physisorption decreases with temperature; chemisorption increases with temperature.
- Both physisorption and chemisorption decrease with increase in temperature.
Answer: Physisorption decreases with temperature; chemisorption first increases then decreases with temperature.
Adsorption is exothermic. By Le Chatelier's principle, increasing temperature shifts equilibrium backward, decreasing physisorption. Chemisorption requires activation energy; initially, temperature rise increases the number of molecules with sufficient energy to adsorb, so extent increases. At very high temperatures, desorption dominates, causing a decrease. Hence, chemisorption first increases then decreases.
2. Which colloid type has particles that are aggregates of hundreds of atoms and is lyophobic?
- Emulsion
- Macromolecular colloid
- Associated colloid
- Multimolecular colloid
Answer: Multimolecular colloid
Multimolecular colloids, like gold sol, consist of aggregates of many small atoms or molecules held by van der Waals forces. They are lyophobic (solvent-hating) and require stabilizers. Macromolecular colloids are single large molecules and lyophilic. Associated colloids form micelles above CMC and are lyophilic.
3. Which protective colloid has the smallest gold number?
- Gum arabic
- Gelatin
- Dextrin
- Egg albumin
Answer: Gelatin
Gold number is the minimum mass (in mg) of protective colloid needed to prevent coagulation of 10 mL of gold sol. A smaller gold number indicates greater protective power. Gelatin has a gold number of 0.005-0.01 mg, which is the smallest among the given options, so it has the strongest protective action.
4. Which of the following is NOT an application of the Hardy-Schulze rule?
- Formation of micelles by soap above CMC.
- Stopping bleeding by applying FeCl3 to a cut.
- Coagulation of clay in river deltas by seawater cations.
- Precipitation of smoke particles in Cottrell precipitator.
Answer: Formation of micelles by soap above CMC.
The Hardy-Schulze rule states that the ion with opposite charge to the colloid causes coagulation, and its coagulating power increases with charge. Micelle formation is a self-assembly process driven by hydrophobic interactions, not by addition of electrolytes; it does not involve the Hardy-Schulze rule.
5. Why is osmotic pressure preferred over boiling point elevation for determining molar mass of a macromolecular colloid?
- Osmotic pressure is independent of temperature
- Osmotic pressure does not depend on the number of particles
- Boiling point elevation is too small to measure accurately for colloids
- Boiling point elevation requires the solute to be volatile
Answer: Boiling point elevation is too small to measure accurately for colloids
Colligative properties depend on the number of particles. In a macromolecular colloid, the number of particles is very small for a given mass, so boiling point elevation is negligible. Osmotic pressure, however, is measurable even at low concentrations, making it suitable for molar mass determination.
6. For a negative sol of As2S3, which electrolyte has the smallest coagulation value?
- AlCl3
- MgCl2
- NaCl
- Na3PO4
Answer: AlCl3
For a negative sol, the coagulating ions are cations. According to the Hardy-Schulze rule, the coagulating power increases with the charge on the cation. Al3+ has the highest charge among the given cations, so AlCl3 has the smallest coagulation value (minimum concentration needed for coagulation).
7. Which of the following best describes the role of zeolites in shape-selective catalysis?
- They are amorphous silica gels with variable pore sizes that allow all molecules to adsorb.
- They are microporous crystalline aluminosilicates with uniform channels that allow only molecules of specific size to react.
- They are layered clay minerals that intercalate reactants between sheets for catalysis.
- They are porous carbon materials that act as molecular sieves by size exclusion.
Answer: They are microporous crystalline aluminosilicates with uniform channels that allow only molecules of specific size to react.
Zeolites are microporous crystalline aluminosilicates with a three-dimensional framework of SiO4 and AlO4 tetrahedra, forming uniform channels of molecular dimensions (260–740 pm). This allows only molecules of appropriate size to enter and react, enabling shape-selective catalysis.
8. Freshly precipitated Fe(OH)3 is shaken with a few drops of FeCl3 solution to form a colloidal sol. This process is called:
- Dialysis
- Peptization
- Coagulation
- Electrophoresis
Answer: Peptization
Peptization is the process of converting a fresh precipitate into a colloidal sol by adding a suitable electrolyte (peptizing agent). FeCl3 provides Fe3+ ions that adsorb on Fe(OH)3 particles, giving them a positive charge, causing repulsion and breakdown into colloidal size.
9. Which of the following is NOT affected by the addition of a catalyst to a reaction at equilibrium?
- Activation energy of the forward reaction
- Rate of attainment of equilibrium
- Both forward and reverse reaction rates
- Equilibrium constant of the reaction
Answer: Equilibrium constant of the reaction
A catalyst lowers the activation energy for both forward and reverse reactions equally, thereby speeding up the attainment of equilibrium. However, it does not change the equilibrium constant (Keq), which depends only on the standard Gibbs free energy change (ΔG°).
10. For a negative As₂S₃ sol, the coagulation values of NaCl, MgCl₂, and AlCl₃ are 51, 0.72, and 0.093 mmol/L respectively. What is the coagulating power ratio of AlCl₃ to NaCl?
- 1:548
- 548:1
- 1:71
- 71:1
Answer: 548:1
Coagulating power is inversely proportional to coagulation value. For AlCl₃, power ∝ 1/0.093 ≈ 10.75; for NaCl, ∝ 1/51 ≈ 0.0196. Ratio = 10.75/0.0196 ≈ 548:1. This follows Hardy-Schulze rule: higher charge on coagulating ion (Al³⁺ vs Na⁺) gives higher power.
11. At constant temperature, the extent of adsorption (x/m) of a gas on a solid increases with pressure and eventually becomes constant. This behaviour is best described by which curve?
- x/m ∝ P at all pressures
- x/m ∝ P^(1/n) at all pressures
- x/m decreases with pressure
- x/m increases linearly then saturates
Answer: x/m increases linearly then saturates
The adsorption isotherm shows that at low pressure, x/m is proportional to P (linear region), and at high pressure, x/m becomes independent of P (saturation). This is the correct description of the variation of x/m with pressure at constant temperature.
12. Which of the following is an application of adsorption?
- Use of calcium carbonate as an antacid
- Use of sodium chloride for preserving food
- Use of potassium permanganate for disinfection
- Use of silica gel for controlling humidity
Answer: Use of silica gel for controlling humidity
Silica gel adsorbs moisture from the air, thus controlling humidity. This is a classic application of adsorption. The other options involve different chemical processes: preservation by osmosis, disinfection by oxidation, and neutralisation of acid.