Learning Objectives
- Understand adsorption and its types
- Differentiate between physisorption and chemisorption
- Study catalysis and its types
- Learn about colloidal solutions and their properties
- Understand emulsions and their applications
Key Concepts
Adsorption
Accumulation of molecules on a surface. Adsorbate: substance adsorbed. Adsorbent: surface on which adsorption occurs.
Adsorption is exothermic (ΔH < 0) and decreases entropy (ΔS < 0). Since ΔG = ΔH - TΔS must be negative, |ΔH| > |TΔS|.
Physisorption: Weak van der Waals forces. Low enthalpy (20-40 kJ/mol). Reversible. Multilayer. Not specific. Increases with pressure, decreases with temperature.
Chemisorption: Chemical bond formation. High enthalpy (80-240 kJ/mol). Irreversible. Monolayer. Specific. First increases then decreases with temperature.
Freundlich isotherm: x/m = kP^(1/n) or log(x/m) = log k + (1/n) log P (1/n between 0 and 1).
Langmuir isotherm: x/m = aP/(1 + bP) (monolayer adsorption model).
Catalysis
Homogeneous catalysis: Catalyst and reactants in same phase. Example: NO(g) in oxidation of SO₂(g); H₂SO₄ in esterification.
Heterogeneous catalysis: Catalyst in different phase. Example: Fe in Haber process (N₂ + 3H₂ → 2NH₃); V₂O₅ in contact process (2SO₂ + O₂ → 2SO₃); Ni in hydrogenation.
Enzyme catalysis: Biological catalysts (proteins). Highly specific. Michaelis-Menten kinetics.
Features of catalysts: Do not change equilibrium, lower activation energy, are specific, small amount needed, not consumed.
Promoters: Enhance catalyst activity (Mo in Haber process). Poisons: Reduce activity (As₂O₃ in contact process).
Colloidal Solutions (Colloids)
Particle size: 1-1000 nm (between true solution and suspension).
Classification by dispersed phase/medium: Sol (solid in liquid), Aerosol (liquid/solid in gas), Emulsion (liquid in liquid), Foam (gas in liquid), Gel (liquid in solid).
Lyophilic colloids: Solvent-loving. Reversible. Self-stabilising. Example: starch, gelatin, gum.
Lyophobic colloids: Solvent-hating. Irreversible. Need stabilisation. Example: gold sol, Fe(OH)₃ sol, As₂S₃ sol.
Preparation: Bredig's arc method (metal sols), peptization (adding electrolyte to precipitate), chemical methods (double decomposition, oxidation, reduction).
Purification: Dialysis (semi-permeable membrane removes dissolved substances), electrodialysis, ultrafiltration.
Properties of Colloids
- Tyndall effect: Scattering of light by colloidal particles. Visible beam path.
- Brownian motion: Zigzag motion due to bombardment by solvent molecules.
- Charge: Colloidal particles carry charge. Positively charged: Fe(OH)₃, Al(OH)₃. Negatively charged: As₂S₃, Au, starch, clay.
- Electrophoresis: Migration of colloidal particles under electric field.
- Coagulation/Flocculation: Settling of colloidal particles by adding electrolyte. Hardy-Schulze rule: higher valency of coagulating ion → greater coagulating power.
Emulsions
Oil-in-water (O/W): Oil droplets in water. Example: milk, vanishing cream.
Water-in-oil (W/O): Water droplets in oil. Example: butter, cold cream.
Emulsifiers: Stabilise emulsions (soaps, detergents, proteins).
Summary
Surface chemistry involves adsorption, catalysis, and colloid science. Adsorption is classified as physisorption or chemisorption. Catalysts increase reaction rates by lowering activation energy. Colloids have unique properties (Tyndall effect, Brownian motion, electrophoresis) due to their particle size. Emulsions are colloidal systems of two immiscible liquids.
Important Terms
- Adsorption: Accumulation on surface; opposite of absorption
- Physisorption: Physical adsorption by van der Waals forces
- Chemisorption: Chemical adsorption by bond formation
- Tyndall Effect: Light scattering by colloidal particles
- Coagulation: Settling of colloid by electrolyte addition
- Emulsion: Colloidal dispersion of one liquid in another
Quick Revision
- Physisorption: weak, reversible, multilayer; Chemisorption: strong, irreversible, monolayer
- Freundlich: x/m = kP^(1/n); Langmuir: x/m = aP/(1+bP)
- Lyophilic: reversible, stable; Lyophobic: irreversible, need stabilisation
- Tyndall, Brownian motion, electrophoresis: colloidal properties
- Hardy-Schulze: higher valency ion → stronger coagulating power
- O/W emulsion: milk; W/O emulsion: butter