Surface Chemistry — Revision Notes

⚡ 30-Second Revision

Adsorption: — Surface phenomenon. Adsorbate (sticks), Adsorbent (surface).
Physisorption: — Weak van der Waals, non-specific, reversible, multimolecular, low $Delta H_{ads}$ (20-40 kJ/mol), decreases with T.
Chemisorption: — Strong chemical bonds, specific, irreversible, monolayer, high $Delta H_{ads}$ (80-240 kJ/mol), increases then decreases with T (requires $E_a$).
Adsorption Isotherms: — $rac{x}{m} = kP^{1/n}$ (Freundlich), $rac{x}{m} = \frac{aP}{1+bP}$ (Langmuir).
Catalysis: — Alters reaction rate, lowers $E_a$, unchanged chemically. Homogeneous (same phase), Heterogeneous (different phases).
Catalyst Properties: — Activity, Selectivity, Specificity. Promoters enhance, Poisons inhibit.
Colloids: — Particle size 1-1000 nm. Heterogeneous. Show Tyndall effect, Brownian movement, Electrophoresis.
Lyophilic Sols: — Solvent-loving, stable, reversible (e.g., starch, gum).
Lyophobic Sols: — Solvent-hating, unstable, irreversible (e.g., metal sols).
Associated Colloids (Micelles): — Form above Critical Micelle Concentration (CMC) (e.g., soaps).
Coagulation: — Precipitation of colloids. Hardy-Schulze Rule: Higher valency of oppositely charged ion = greater coagulating power.
Emulsions: — Liquid-liquid colloids. O/W (oil in water, e.g., milk), W/O (water in oil, e.g., butter). Stabilized by emulsifying agents.

2-Minute Revision

Surface chemistry focuses on phenomena at interfaces. Adsorption is key, where molecules (adsorbate) accumulate on a surface (adsorbent). Distinguish between physisorption (weak van der Waals forces, reversible, multilayer, low heat, decreases with temperature) and chemisorption (strong chemical bonds, irreversible, monolayer, high heat, initially increases with temperature).

Adsorption isotherms like Freundlich and Langmuir describe the extent of adsorption with pressure. Catalysis involves catalysts speeding up reactions by providing an alternative pathway with lower activation energy, without being consumed.

Catalysts are specific and selective. Colloids are heterogeneous mixtures with particle sizes 1-1000 nm, exhibiting unique properties like the Tyndall effect (light scattering) and Brownian movement (random motion).

They are classified as lyophilic (stable, solvent-loving) or lyophobic (unstable, solvent-hating). Associated colloids, or micelles, form above a Critical Micelle Concentration (CMC). Colloidal stability is often due to particle charge, and they can be coagulated by adding electrolytes, following the Hardy-Schulze rule (higher valency of oppositely charged ion, greater coagulating power).

Emulsions are liquid-liquid colloids, either oil-in-water (O/W) or water-in-oil (W/O), stabilized by emulsifying agents.

5-Minute Revision

Surface chemistry is the study of interfacial phenomena. The most fundamental concept is adsorption, the accumulation of molecules on a surface. It differs from absorption, which is bulk penetration.

Adsorption is primarily of two types: Physisorption (physical adsorption) and Chemisorption (chemical adsorption). Physisorption is driven by weak van der Waals forces, is non-specific, reversible, forms multimolecular layers, has a low enthalpy of adsorption (20-40 kJ/mol), and decreases with increasing temperature.

Chemisorption involves strong chemical bonds, is highly specific, irreversible, forms a monolayer, has a high enthalpy of adsorption (80-240 kJ/mol), and typically increases with temperature initially (due to activation energy) before decreasing.

The extent of adsorption is described by adsorption isotherms, such as the empirical Freundlich isotherm ($rac{x}{m} = kP^{1/n}$) and the theoretical Langmuir isotherm ($rac{x}{m} = \frac{aP}{1+bP}$).

Factors like surface area, temperature, and pressure significantly influence adsorption.

Catalysis is another crucial aspect, where a catalyst alters the rate of a chemical reaction by lowering its activation energy, without being consumed. Catalysts are characterized by their activity, selectivity, and specificity.

They can be homogeneous (same phase as reactants) or heterogeneous (different phase, typically solid catalyst with gaseous reactants). Heterogeneous catalysis often proceeds via adsorption of reactants on the catalyst surface, followed by reaction and desorption of products.

Promoters enhance catalyst activity, while poisons inhibit it.

Colloids are heterogeneous systems with particle sizes ranging from 1 nm to 1000 nm. They are intermediate between true solutions (<1 nm) and suspensions (>1000 nm). Colloids exhibit several unique properties: Tyndall effect (scattering of light), Brownian movement (random zigzag motion), and electrophoresis (movement in an electric field due to particle charge).

They are classified as lyophilic (solvent-loving, stable, reversible, e.g., starch) or lyophobic (solvent-hating, unstable, irreversible, e.g., metal sols). Some substances form associated colloids or micelles above a certain Critical Micelle Concentration (CMC), like soaps and detergents.

The stability of colloids is often due to the electric charge on their particles. Coagulation is the process of precipitating colloidal particles, typically by adding electrolytes. The Hardy-Schulze rule states that the coagulating power of an ion increases with its valency and must be opposite to the charge of the sol.

Emulsions are liquid-liquid colloids, either oil-in-water (O/W, e.g., milk) or water-in-oil (W/O, e.g., butter), stabilized by emulsifying agents. Understanding these concepts and their applications is vital for NEET.

Prelims Revision Notes

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Adsorption vs. Absorption: — Adsorption is a surface phenomenon (e.g., gas on charcoal); absorption is a bulk phenomenon (e.g., water in sponge). Adsorption is exothermic.
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Physisorption (Physical Adsorption):

* Weak van der Waals forces. * Non-specific. * Reversible. * Low enthalpy of adsorption (20-40 kJ/mol). * Multimolecular layers. * Decreases with increasing temperature. * Increases with increasing pressure.

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Chemisorption (Chemical Adsorption):

* Strong chemical bonds (covalent/ionic). * Highly specific. * Irreversible. * High enthalpy of adsorption (80-240 kJ/mol). * Monolayer formation. * Initially increases with temperature (requires activation energy), then decreases. * Increases with increasing pressure, then saturates.

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Adsorption Isotherms: — Graphical representation of $x/m$ vs. P at constant T.

* Freundlich: $rac{x}{m} = kP^{1/n}$ (empirical, $n>1$). * Langmuir: $rac{x}{m} = \frac{aP}{1+bP}$ (theoretical, monolayer).

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Catalysis: — Alters reaction rate, lowers activation energy ($E_a$), does not change $Delta G$ or equilibrium constant ($K$). Chemically unchanged.

* Homogeneous: Reactants and catalyst in same phase. * Heterogeneous: Reactants and catalyst in different phases (e.g., solid catalyst, gaseous reactants). * Mechanism (Heterogeneous): Diffusion $o$ Adsorption $o$ Reaction $o$ Desorption $o$ Diffusion. * Characteristics: Activity, Selectivity, Specificity. * Promoters: Enhance catalyst activity (e.g., Mo in Haber process). * Poisons: Decrease catalyst activity (e.g., CO for Fe catalyst).

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Colloids: — Particle size 1-1000 nm. Heterogeneous systems.

* True Solution (<1 nm): Homogeneous, no Tyndall, no settling. * Suspension (>1000 nm): Heterogeneous, visible particles, settles.

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Classification of Colloids:

* Lyophilic (solvent-loving): Stable, reversible, easily prepared (starch, gum, proteins). * Lyophobic (solvent-hating): Unstable, irreversible, special preparation (metal sols, metal sulphides). * Multimolecular: Aggregates of atoms/small molecules (sulfur sol). * Macromolecular: Large molecules of colloidal size (starch, proteins). * Associated (Micelles): Form above Critical Micelle Concentration (CMC) (soaps, detergents).

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Properties of Colloids:

* Tyndall Effect: Scattering of light, visible path. * Brownian Movement: Random zigzag motion. * Charge on Colloidal Particles: Due to preferential adsorption of ions. Responsible for stability. * Electrophoresis: Movement of charged particles in electric field. * Coagulation (Flocculation): Precipitation of colloidal particles. * Hardy-Schulze Rule: Coagulating power $propto$ (valency of effective ion)$^x$. Effective ion has opposite charge to sol.

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Emulsions: — Liquid-liquid colloids.

* O/W (Oil in Water): Oil dispersed phase, water dispersion medium (milk, vanishing cream). * W/O (Water in Oil): Water dispersed phase, oil dispersion medium (butter, cold cream). * Emulsifying Agents: Stabilize emulsions (e.g., soap, gum, proteins).

Vyyuha Quick Recall

All Cats Can Eat Mice:

Adsorption: Surface phenomenon.
Catalysis: Speeds up reactions, lowers $E_a$.
Colloids: 1-1000 nm particles, Tyndall effect.
Emulsions: Liquid-liquid colloids (O/W, W/O).
Micelles: Form above CMC, associated colloids.