ChemistryModerate Weightage ★★★Class 12
Surface Chemistry
Adsorption (physisorption vs chemisorption), Freundlich isotherm, colloids (Tyndall, Brownian, Hardy-Schulze), emulsions, and catalysis — expect 2–3 EAPCET questions.
2–3Questions in EAPCET
~2%Paper Weightage
6Key Concepts
3Mistake Traps
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?
1
As₂S₃ sol is negatively charged → coagulating ion is the cation (opposite charge).
2
NaCl provides Na⁺ (valence 1); AlCl₃ provides Al³⁺ (valence 3).
3
Hardy-Schulze: higher valence → greater coagulating power.
4
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.
✓ 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.
1
Physisorption is exothermic (ΔH < 0). By Le Chatelier's principle, increasing temperature shifts equilibrium toward desorption (endothermic direction).
2
Additionally, higher temperature gives gas molecules more kinetic energy — harder to hold them on the surface.
3
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)?
1
Pass a beam of light through both solutions in a dark room.
2
Starch colloid: particles (1–1000 nm) scatter visible light → bright cone of light visible (Tyndall effect).
3
NaCl solution: ions (~0.1 nm) too small to scatter visible light → no Tyndall effect, beam invisible.
4
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).
1
The trap: Students often think lyophobic (with surface charge) is more stable.
2
Lyophilic: Solvent molecules surround particles (solvation/hydration shell). This thick layer prevents aggregation. Stable even without charge. Adding electrolyte does NOT easily coagulate them.
3
Lyophobic: Stabilised only by surface electrical charge. Adding electrolyte neutralises charge → coagulation occurs immediately. Much less stable.
4
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~7 Lyophilic vs lyophobic stability~6 Tyndall effect and Brownian motion~5 Adsorption isotherms (Freundlich)~5 Catalysis types and examples~3 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.