Solution: Surface Chemistry (CBSE Class 12)
(a) (i) Coagulation: The process of settling of colloidal particles by the addition of an electrolyte is called coagulation or flocculation.
(ii) Hardy-Schulze Rule: Greater the valency of the flocculating ion added, the greater is its power to cause precipitation. For example, for a negatively charged sol, the coagulating power of cations follows the order: $Al^{3+} > Ba^{2+} > Na^+$.
(iii) Electrophoresis: The movement of colloidal particles under an applied electric field is called electrophoresis. If the particles accumulate near the negative electrode, they are positively charged, and vice versa.
(b) Differences between Physisorption and Chemisorption:
- Step 1 (Nature of Forces): [0.5 Mark] Physisorption arises due to weak van der Waals forces, whereas Chemisorption arises due to strong chemical bonds.
- Step 2 (Specificity and Reversibility): [0.75 Mark] Physisorption is not specific in nature and is reversible, whereas Chemisorption is highly specific and is irreversible.
- Step 3 (Enthalpy and Activation Energy): [0.75 Mark] Physisorption has low enthalpy of adsorption ($20-40 \text{ kJ/mol}$) and requires low activation energy. Chemisorption has high enthalpy of adsorption ($80-240 \text{ kJ/mol}$) and requires high activation energy.
💡 EL10 Concept Insight: Think of Physisorption like a sticky note that you can easily peel off (weak bond), while Chemisorption is like super-gluing two pieces of paper together—once they bond, they become a new substance and are very hard to separate!