NEET Chemistry - Chapter 10

Surface Chemistry

Fresh NEET surface chemistry notes on adsorption, catalysis, colloids, emulsions, micelles, and colloidal properties.

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Concept Block

1. Adsorption vs Absorption, and Adsorption Terminology

Adsorption is accumulation of molecules on a surface (2D phenomenon). Absorption is uniform distribution of a substance throughout the bulk of another (3D phenomenon). Sorption means both occur simultaneously.

Feature	Adsorption	Absorption
Location	On the surface	Throughout the bulk
Nature	Surface (2D) phenomenon	Bulk (3D) phenomenon
Rate of attainment	Increases with time then equilibrium	Uniform from start
Example	Silica gel adsorbs water vapour	Anhydrous CaCl $_2$ absorbs water

Key terms: Adsorbate = the substance adsorbed. Adsorbent = the solid surface. Factors increasing adsorption: high surface area of adsorbent, low temperature (physisorption), suitable chemical affinity (chemisorption).

Concept Block

2. Physisorption vs Chemisorption — Complete Comparison

The most important table in Surface Chemistry for NEET. Every single parameter here is individually tested.

Parameter	Physisorption	Chemisorption
Forces	van der Waals (weak)	Chemical bonds (strong)
Enthalpy ( $\Delta H_{ads}$ )	Low (20–40 kJ mol $^{-1}$ )	High (40–400 kJ mol $^{-1}$ )
Reversibility	Easily reversible	Irreversible (mostly)
Specificity	Non-specific (all adsorbents)	Highly specific
Layers	Multilayer	Monolayer only
Temperature effect	Decreases with ↑ T	May increase initially then decreases
Activation energy	Not required	Required (activated adsorption)

Freundlich Isotherm:

x/m = kP^{1/n}

(at constant T). Plot of

\log(x/m)

\log P

is linear with slope

1/n

and intercept

\log k

Concept Block

3. Catalysis — Mechanism, Types, Promoters, and Poisons

A catalyst provides an alternative pathway with lower activation energy. It is neither consumed nor appears as a product.

Steps in heterogeneous catalysis (Langmuir mechanism):

Reactant molecules diffuse to the catalyst surface
Adsorption of reactants on the surface (chemisorption)
Chemical reaction occurs on the surface — activated complex forms
Products desorb from the surface
Products diffuse away

Types of Catalysis

Homogeneous: Catalyst and reactants in same phase. Example: H $^+$ (aq) catalysing ester hydrolysis.
Heterogeneous: Catalyst and reactants in different phases. Example: V $_2$ O $_5$ (solid) in SO $_2$ oxidation (gas).
Enzyme (biocatalysis): Enzymes are proteins that catalyse biological reactions with high specificity (lock-and-key model).

Promoters (activators) enhance catalyst activity without being catalysts themselves. Example: Mo in Haber process. Catalyst poisons block active sites and reduce activity. Example: CO poisons Fe catalyst in Haber process; lead poisons Pt catalyst in car converters.

Concept Block

4. Colloids — Classification, Preparation, Properties, and Purification

Colloids have particle diameter between 1 nm and 1000 nm (1 µm) — intermediate between true solutions (<1 nm) and suspensions (>1000 nm).

Property	True Solution	Colloid	Suspension
Particle size	<1 nm	1–1000 nm	>1000 nm
Tyndall effect	Absent	Present	Present (but settles)
Filtration	Passes filter paper	Passes filter paper; stopped by semipermeable membrane	Stopped by filter paper

Key colloidal phenomena:

Tyndall effect: Scattering of light by colloidal particles — sky is blue, sunset is red, milk is white.
Brownian motion: Random zigzag motion of colloidal particles due to collisions from solvent molecules. Prevents settling.
Electrophoresis: Migration of colloidal particles toward oppositely charged electrode under electric field.
Dialysis: Purification of colloids by removing dissolved ions through a semipermeable membrane.

Concept Block

5. Coagulation, Hardy-Schulze Rule, Emulsions, and Micelles

Coagulation (flocculation) is the precipitation of colloidal particles by neutralising their charge. Methods: adding electrolyte, heating, by oppositely charged colloid, and by persistent dialysis.

Hardy-Schulze Rule

The coagulating power of an ion is proportional to its valency. Higher the valency of the counter-ion, greater the coagulating power:

For a negative sol (e.g., As $_2$ S $_3$ ): coagulating power Al $^{3+}$ > Mg $^{2+}$ > Na $^+$
For a positive sol (e.g., Fe(OH) $_3$ ): coagulating power PO $_4^{3-}$ > SO $_4^{2-}$ > Cl $^-$

Emulsions: Colloidal systems of two immiscible liquids (one dispersed in the other). O/W emulsion (oil in water) = milk, cold cream. W/O emulsion (water in oil) = butter, margarine. Emulsifying agents (soaps, detergents) stabilise emulsions by forming films at the oil-water interface.

Micelles: Aggregates of soap/detergent molecules in water above the critical micelle concentration (CMC). The hydrophobic tails point inward (toward the oil droplet) and hydrophilic heads point outward (toward water). This structure entraps and removes grease and dirt.

NEET fast track: Charge on common colloids — Fe(OH)

_3

sol: positive (Fe

^{3+}

ions adsorbed). As

_2

_3

sol: negative (S

^{2-}

adsorbed). Starch: negative. Clay in water: negative.

Practice Tests

5 Chapter Tests of 25 Questions Each

Each test is original, NEET-aligned, and answer-backed. Use them as sectional revision instead of a single long mock so your weak subtopics become easier to identify quickly.

Test 1: Adsorption Basics

Adsorption, absorption, physisorption, and chemisorption.

Test 2: Catalysis

Catalyst action, active surface, promoters, poisons, and enzymes.

Test 3: Colloids

Colloidal size, Tyndall effect, Brownian motion, and classification.

Test 4: Coagulation and Electrophoresis

Charge on colloids, coagulation, and Hardy-Schulze rule.

Test 5: Mixed NEET Drill

Integrated practice across adsorption, catalysis, colloids, and micelles.

Open Practice Tests

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