Surface Chemistry

Concept Core

Adsorption, colloids, emulsions, and catalysis — the complete surface chemistry framework.

Adsorption — Physisorption vs Chemisorption

Feature	Physisorption	Chemisorption
Force	van der Waals (weak)	Chemical bond (strong)
Reversibility	Easily reversible	Irreversible
Heat evolved	Low (20–40 kJ/mol)	High (40–400 kJ/mol)
Layers	Multilayers	Monolayer only
Temperature effect	Decreases with ↑T	Increases then decreases
Specificity	Non-specific	Highly specific

Freundlich isotherm: x/m = kP^(1/n) (0 < 1/n < 1) log(x/m) = log k + (1/n) log P [linear form; slope = 1/n]

Adsorption is always exothermic (ΔH < 0). Entropy decreases (ΔS < 0). Spontaneous at low temperatures (ΔG = ΔH − TΔS < 0).

Colloids — Types and Size

Colloidal particle size: 1 nm to 1000 nm (between true solution <1 nm and suspension >1000 nm).

Lyophilic (solvent-loving): starch, gum, gelatin — stable, reversible, solvation layer protects particles.

Lyophobic (solvent-fearing): metal sols, Fe(OH)₃ — unstable, irreversible, stabilised only by surface charge.

Properties of Colloidal Solutions

Tyndall effect: Colloid particles (1–1000 nm) scatter visible light. True solutions do NOT show Tyndall effect. Diagnostic test to distinguish colloid from true solution.

Brownian motion: Zig-zag random movement due to unequal bombardment by solvent molecules. Evidence for kinetic theory.

Electrophoresis: Migration of charged colloidal particles toward opposite electrode in electric field.

Dialysis: Removal of crystalloids (small ions) from colloid using semi-permeable membrane.

Coagulation & Hardy-Schulze Rule

Hardy-Schulze rule: coagulating power ∝ valence of the oppositely charged ion For negatively charged sol (As₂S₃): Al³⁺ > Ca²⁺ > Na⁺ For positively charged sol (Fe(OH)₃): PO₄³⁻ > SO₄²⁻ > Cl⁻

Adding electrolyte neutralises the charge on colloidal particles → van der Waals attraction takes over → aggregation and settling.

Emulsions & Gels

Emulsion: Liquid dispersed in liquid. Two types:

• O/W (oil-in-water): milk, cream, vanishing cream — oil droplets in water

• W/O (water-in-oil): butter, cold cream — water droplets in oil

Stabilised by emulsifying agents (soaps, proteins) that reduce interfacial tension.

Gel: Solid network in liquid — gelatin in water (reversible), silica gel (irreversible).

Catalysis — Homogeneous & Heterogeneous

Homogeneous catalysis: Catalyst and reactants in same phase. E.g., H₂SO₄ catalysing ester hydrolysis (all liquid phase).

Heterogeneous catalysis: Different phases. Mechanism: adsorption → activation → reaction → desorption.

Key examples: Fe/Mo (Haber process: N₂+3H₂→2NH₃); V₂O₅ (Contact process: 2SO₂+O₂→2SO₃); Ni (hydrogenation of oils); Pt/Rh (catalytic converters).

Enzyme catalysis: Highly specific biological catalysts. Lock-and-key model. Optimum at 37°C and pH 7–8.

Formula Vault

Surface chemistry key facts and formulas for EAPCET.

Freundlich Isotherm

x/m = kP^(1/n)

x = mass adsorbed; m = adsorbent mass

Freundlich Log Form

log(x/m) = log k + (1/n) log P

Slope = 1/n; y-intercept = log k

Colloidal Size Range

1 nm to 1000 nm

Between solution and suspension

Hardy-Schulze (−ve sol)

Al³⁺ > Ca²⁺ > Na⁺

Higher valence cation coagulates more

Hardy-Schulze (+ve sol)

PO₄³⁻ > SO₄²⁻ > Cl⁻

Higher valence anion coagulates more

Physisorption Heat

20–40 kJ/mol (low)

van der Waals; reversible

Chemisorption Heat

40–400 kJ/mol (high)

Chemical bond; monolayer; irreversible

Tyndall Effect

Light scattering by colloid particles

True solution: no Tyndall effect

Worked Examples

5 problems — Hardy-Schulze, Freundlich, adsorption trends, Tyndall, and catalysis.

EasyWhich coagulates As₂S₃ sol better: NaCl or AlCl₃?▾

As₂S₃ forms a negatively charged colloid. Which coagulates it more effectively: NaCl or AlCl₃?

As₂S₃ sol is negatively charged → coagulating ion is the cation (opposite charge).

NaCl provides Na⁺ (valence 1); AlCl₃ provides Al³⁺ (valence 3).

Hardy-Schulze: higher valence → greater coagulating power.

AlCl₃ is more effective.

✓ AlCl₃ (Al³⁺ has valence 3, much greater coagulating power than Na⁺)

EasyFreundlich isotherm: if 1/n = 0.5, what is n?▾

From the Freundlich isotherm log(x/m) = log k + (1/n) log P, the slope is found to be 0.5. Find n.

Slope = 1/n = 0.5

n = 1/0.5 = 2

✓ n = 2

MediumDoes adsorption of a gas increase or decrease with temperature? Explain.▾

Explain the effect of temperature on physisorption of gases on solid surfaces.

Physisorption is exothermic (ΔH < 0). By Le Chatelier's principle, increasing temperature shifts equilibrium toward desorption (endothermic direction).

Additionally, higher temperature gives gas molecules more kinetic energy — harder to hold them on the surface.

Result: adsorption decreases with increasing temperature.

✓ Decreases — physisorption is exothermic; higher T favours desorption (Le Chatelier)

EAPCET LevelDistinguish a colloidal starch solution from a true NaCl solution using Tyndall effect.▾

How would you experimentally distinguish a starch solution (colloid) from a NaCl solution (true solution)?

Pass a beam of light through both solutions in a dark room.

Starch colloid: particles (1–1000 nm) scatter visible light → bright cone of light visible (Tyndall effect).

NaCl solution: ions (~0.1 nm) too small to scatter visible light → no Tyndall effect, beam invisible.

Conclusion: Tyndall effect is positive → colloid. Negative → true solution.

✓ Starch shows Tyndall effect (light scattering); NaCl solution does not

Trap QuestionLyophobic colloids are more stable than lyophilic — True or False?▾

Compare stability of lyophilic colloids (e.g., starch in water) and lyophobic colloids (e.g., Au sol).

The trap: Students often think lyophobic (with surface charge) is more stable.

Lyophilic: Solvent molecules surround particles (solvation/hydration shell). This thick layer prevents aggregation. Stable even without charge. Adding electrolyte does NOT easily coagulate them.

Lyophobic: Stabilised only by surface electrical charge. Adding electrolyte neutralises charge → coagulation occurs immediately. Much less stable.

Lyophilic > Lyophobic in stability.

✓ False — lyophilic colloids are MORE stable (solvation shell + charge vs charge only for lyophobic)

Mistake DNA

3 surface chemistry errors from EAPCET distractor analysis.

🌡️

Physisorption Increases with Temperature

Physisorption is exothermic — it DECREASES with rising temperature. Chemisorption may first increase (needs activation energy) then decrease.

❌ Wrong

Heating solid adsorbent: more gas adsorbed ✗ (opposite is true for physi)

✓ Correct

Physisorption ↑T → ↓ adsorption ✓ (Le Chatelier: exothermic process shifts left with ↑T)

Remember: adsorption is always exothermic (gas becomes ordered on surface, ΔS < 0; for spontaneity ΔH must be negative). Higher T disfavours exothermic process.

🔋

Hardy-Schulze: Same-Charge Ion Coagulates

Only oppositely charged ions coagulate a colloid. Ions with the SAME charge as the colloidal particle have no coagulating effect.

❌ Wrong

Fe(OH)₃ sol (+ve): coagulated by Na⁺ (cation) ✗ (same charge as colloid!)

✓ Correct

Fe(OH)₃ (+ve sol): coagulated by anions ✓ Cl⁻ < SO₄²⁻ < PO₄³⁻ ✓

Rule: identify charge on colloid first. Then pick the ion of OPPOSITE charge. Higher valence of that ion = more effective coagulation.

🔬

Tyndall Effect is Shown by True Solutions

Only colloidal dispersions show the Tyndall effect (particle size 1–1000 nm scatters visible light). True solution ions are too small.

❌ Wrong

CuSO₄ solution (blue): shows Tyndall effect ✗ (true solution; ions ~0.1 nm)

✓ Correct

CuSO₄ = true solution: no Tyndall ✓ Fe(OH)₃ sol = colloid: Tyndall ✓ Particle size determines this

Only particles in the colloidal size range (1–1000 nm) are large enough to scatter visible light. Ions in true solutions are far too small. This is used as a diagnostic test.

Chapter Intelligence

Surface chemistry is conceptual — understand mechanisms rather than memorising isolated facts.

EAPCET Weightage (2019–2024)

Hardy-Schulze and coagulation

Lyophilic vs lyophobic stability

Tyndall effect and Brownian motion

Adsorption isotherms (Freundlich)

Catalysis types and examples

High-Yield PYQ Patterns

Which electrolyte coagulates faster?Tyndall effect: colloid vs solutionLyophilic/lyophobic: which more stable?Freundlich: slope and n meaningAdsorption decreases with temperatureHaber/Contact process catalystsPhysisorption vs chemisorption table

Exam Strategy

Hardy-Schulze: identify colloid charge → pick opposite ion → higher valence = more powerful. Al³⁺ (for −ve sol) always wins over Ca²⁺ or Na⁺.
Tyndall = scattering of light by colloidal particles. True solutions (ions) show NO Tyndall. This is the standard diagnostic test in EAPCET questions.
Adsorption: always exothermic → always decreases with temperature for physisorption. A very direct 1-mark question.
Haber: Fe catalyst. Contact: V₂O₅. Hydrogenation: Ni. Catalytic converter: Pt/Rh. These four pairs are tested every few years.

Concept Core

Formula Vault

Worked Examples

Mistake DNA

Chapter Intelligence

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