Surface Chemistry
🧪 Chapter 5: Surface Chemistry
1. Introduction
Surface chemistry deals with phenomena occurring at the surfaces or interfaces of two phases — solid-liquid, solid-gas, liquid-gas, etc. It covers adsorption, colloids, and catalysis, all of which depend heavily on surface area and surface properties.
2. Adsorption
Adsorption: The phenomenon of accumulation of molecules of a substance (adsorbate) on the surface of another substance (adsorbent), rather than in the bulk.
Absorption: A bulk phenomenon where the substance is uniformly distributed throughout the body of the solid or liquid.
Sorption: When adsorption and absorption occur simultaneously.
| Physisorption | Chemisorption |
|---|---|
| Caused by weak Van der Waals forces | Caused by strong chemical bond formation |
| Low enthalpy of adsorption (20–40 kJ/mol) | High enthalpy of adsorption (80–240 kJ/mol) |
| Reversible in nature | Usually irreversible |
| Not specific — occurs on any surface | Highly specific |
| Multilayer formation possible | Only monolayer formation |
| Decreases with increase in temperature | First increases then decreases with temperature |
3. Factors Affecting Adsorption of Gases on Solids
- Nature of gas: Easily liquefiable gases (like NH₃, CO₂, Cl₂) are adsorbed more than permanent gases (H₂, N₂).
- Surface area of adsorbent: Greater surface area means greater adsorption — this is why porous and finely divided solids are good adsorbents.
- Pressure: Adsorption increases with pressure (at constant temperature) until saturation.
- Temperature: Physisorption decreases with increasing temperature (exothermic process).
- Activation of adsorbent: Increasing surface roughness/porosity increases adsorption capacity.
4. Adsorption Isotherms
The variation of adsorption with pressure at constant temperature is called an adsorption isotherm.
Freundlich Adsorption Isotherm:
x/m = k·P^(1/n) (n > 1)
where x = mass of gas adsorbed, m = mass of adsorbent, P = pressure, k and n are constants depending on temperature and nature of adsorbent/adsorbate. This is an empirical relationship valid only over a limited pressure range.
5. Catalysis
Catalyst: A substance that alters (usually increases) the rate of a chemical reaction without itself undergoing any permanent chemical change, by providing an alternate pathway with lower activation energy.
Homogeneous catalysis: Catalyst is in the same phase as the reactants. Example: oxidation of SO₂ to SO₃ using NO(g) as catalyst.
Heterogeneous catalysis: Catalyst is in a different phase from the reactants. Example: Haber's process (Fe catalyst, gaseous reactants), hydrogenation of oils using finely divided Ni.
Mechanism of heterogeneous catalysis (adsorption theory) — 5 steps:
- Diffusion of reactants to the catalyst surface
- Adsorption of reactant molecules on the surface
- Occurrence of the chemical reaction on the surface (formation of an activated complex)
- Desorption of products from the surface
- Diffusion of products away from the surface
Shape-selective catalysis: Zeolites act as catalysts based on their pore structure and molecular size, used widely in petrochemical industries for converting alcohols directly into gasoline.
6. Enzyme Catalysis
Enzymes are complex protein molecules with three-dimensional structures that act as highly efficient and specific biological catalysts. They possess "active sites" that bind to specific substrate molecules (lock-and-key mechanism).
Characteristics of enzyme catalysis:
- Highly specific to a particular reaction/substrate
- Most efficient — increase reaction rates enormously (10⁸ to 10²⁰ times)
- Active only within a narrow temperature range (optimum around body temperature)
- Active only within a specific pH range
- Activity can be increased by the presence of coenzymes/activators
7. Colloids
A colloidal solution is a heterogeneous mixture in which particle size lies between that of a true solution (<1 nm) and a suspension (>1000 nm), typically 1 nm to 1000 nm.
Components:
- Dispersed phase: The component present in small proportion (like solute)
- Dispersion medium: The component present in bulk (like solvent)
| Dispersed Phase | Dispersion Medium | Type | Example |
|---|---|---|---|
| Solid | Gas | Solid sol / Aerosol | Smoke |
| Liquid | Gas | Aerosol | Fog, mist |
| Solid | Liquid | Sol | Paints, muddy water |
| Liquid | Liquid | Emulsion | Milk |
| Gas | Liquid | Foam | Whipped cream, soap lather |
| Solid | Solid | Solid sol | Coloured glass, gemstones |
| Liquid | Solid | Gel | Cheese, jelly |
| Gas | Solid | Solid foam | Pumice stone, foam rubber |
8. Classification of Colloids
Based on interaction between dispersed phase and dispersion medium:
- Lyophilic (solvent-loving) sols: Strong affinity between dispersed phase and medium; stable, reversible. E.g. gum, gelatin, starch.
- Lyophobic (solvent-hating) sols: Little or no affinity; unstable, irreversible, need stabilisers. E.g. metal sols, sulphur sol.
Based on type of particles of dispersed phase:
- Multimolecular colloids: Aggregates of atoms/small molecules with diameter <1nm (e.g. gold sol, sulphur sol).
- Macromolecular colloids: Single large molecules forming a colloidal size (e.g. proteins, starch, cellulose).
- Associated colloids (micelles): Substances behaving as normal electrolytes at low concentration but forming aggregates (micelles) above the Critical Micelle Concentration (CMC), e.g. soaps and detergents.
9. Properties of Colloidal Solutions
| Property | Description |
|---|---|
| Tyndall effect | Scattering of light by colloidal particles, making the path of light visible when passed through a colloidal solution. |
| Brownian movement | Continuous, random, zig-zag movement of colloidal particles due to unequal bombardment by dispersion medium molecules; provides stability against gravity. |
| Electrophoresis | Movement of charged colloidal particles under an applied electric field towards the oppositely charged electrode. |
| Coagulation/Flocculation | Precipitation of colloidal particles due to loss of charge, often by adding electrolytes. |
💡 Hardy-Schulze Rule: The greater the valency of the flocculating ion, the greater is its coagulating/precipitating power. Coagulating power order: Al³⁺ > Ba²⁺ > Na⁺
10. Emulsions
Colloidal systems in which both dispersed phase and dispersion medium are liquids. Two types:
- Oil in water (O/W): Oil is dispersed in water. E.g. milk, vanishing cream.
- Water in oil (W/O): Water is dispersed in oil. E.g. butter, cold cream.
Emulsifying agent: A substance (like soap) added to stabilise an emulsion by forming an interfacial film between the two liquids.
11. Important Formula/Concept Summary
| Concept | Key Point |
|---|---|
| Freundlich isotherm | x/m = kP^(1/n) |
| Colloidal particle size | 1 nm – 1000 nm |
| True solution particle size | < 1 nm |
| Suspension particle size | > 1000 nm |
| Coagulating power order | Al³⁺ > Ba²⁺ > Na⁺ (Hardy-Schulze rule) |
12. Quick Revision Points
- Adsorption is a surface phenomenon; absorption is a bulk phenomenon.
- Physisorption is reversible with weak forces; chemisorption is irreversible with strong bonds.
- Catalysts lower activation energy — they do not shift equilibrium, only help reach it faster.
- Enzymes are the most efficient and specific biological catalysts.
- Colloidal particle size range: 1 nm to 1000 nm (between true solutions and suspensions).
- Tyndall effect, Brownian movement, and electrophoresis are hallmark colloidal properties.
- Lyophilic sols are self-stabilising; lyophobic sols need stabilisers and are easily coagulated.
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