JEE Chem — Surface Chemistry Deep Dive

Chapter Overview & Weightage

Surface Chemistry is a relatively low-weightage chapter (3-5% in JEE Main) but the questions are often pure recall — easy marks if you’ve memorised the facts. The chapter rarely makes JEE Advanced.

Year	JEE Main Qs	JEE Advanced Qs
2024	1	0
2023	1	0
2022	2	1
2021	1	0

Key Concepts You Must Know

Adsorption: accumulation of molecules at a surface. Different from absorption (bulk uptake).
Physisorption (van der Waals adsorption): low enthalpy ( $-20$ to $-40$ kJ/mol), reversible, multilayer, decreases with temperature, no specificity.
Chemisorption: high enthalpy ( $-80$ to $-240$ kJ/mol), irreversible (mostly), monolayer, increases initially with temperature then decreases, specific.
Adsorption isotherms: Freundlich ( $x/m = kP^{1/n}$ ), Langmuir (monolayer model).
Catalysis: positive (speeds up), negative (slows), homogeneous (same phase), heterogeneous (different phases).
Enzymes: biological catalysts, highly specific, optimal temperature and pH.
Colloids: dispersed phase + dispersion medium. Particle size 1-1000 nm.
Classification of colloids: lyophilic vs lyophobic, sol/gel/emulsion/foam/aerosol.
Methods of preparation: dispersion (Bredig’s arc, peptization) and condensation (oxidation, reduction, exchange).
Properties of colloids: Tyndall effect, Brownian motion, electrophoresis, coagulation.
Hardy-Schulze rule: higher charge of counter-ion → more effective coagulation.

Important Frameworks

$\frac{x}{m} = kP^{1/n}, \quad n > 1$

Take log: $\log(x/m) = \log k + (1/n)\log P$ . Plot of $\log(x/m)$ vs $\log P$ gives slope $1/n$ , intercept $\log k$ .

When to use: adsorption from solution or gas onto a solid surface.

$\frac{x}{m} = \frac{aP}{1 + bP}$

At low pressure: $x/m \approx aP$ (linear). At high pressure: $x/m \approx a/b$ (saturation, monolayer complete).

When to use: monolayer adsorption with limited active sites.

Solved Previous Year Questions

PYQ 1 (JEE Main 2024)

Which of the following is NOT a property of physisorption?

(a) Multilayer adsorption (b) Specific (c) Low enthalpy (d) Reversible

Solution: (b) Specific. Physisorption is non-specific — any gas can physisorb on any surface (depending on temperature/pressure). Chemisorption is the specific one.

PYQ 2 (JEE Main 2023)

For a coagulation experiment, which of the following ions has the highest coagulating power for a positively charged sol?

(a) Na⁺ (b) Mg²⁺ (c) Cl⁻ (d) PO₄³⁻

Solution: For a positively charged sol, the negative ions cause coagulation (Hardy-Schulze rule). Among Cl⁻ and PO₄³⁻, the higher charge wins → PO₄³⁻ (3-).

PYQ 3 (JEE Advanced 2022)

The catalyst used in the contact process for $\text{H}_2\text{SO}_4$ manufacture is:

Solution: V₂O₅ (vanadium pentoxide). Replaced earlier Pt catalyst due to lower cost and resistance to poisoning.

Memorise the catalysts for major industrial processes:

Haber’s process (NH₃): Fe with Mo promoter.
Contact process (H₂SO₄): V₂O₅.
Ostwald’s process (HNO₃): Pt-Rh gauze.
Hydrogenation of oils: Ni.

Difficulty Distribution

Easy (60%): Direct recall (catalyst names, isotherm equations, definitions).
Medium (30%): Application of Hardy-Schulze rule, distinguishing physisorption from chemisorption in given contexts.
Hard (10%): Mathematical isotherm problems with given data.

Expert Strategy

Make a single comparison table for physisorption vs chemisorption with rows: nature of forces, enthalpy, specificity, reversibility, temperature effect, pressure effect, layer formation, activation energy. Memorise this table; it answers ~70% of the chapter’s questions.

For colloids, learn the four common categories with examples: aerosol (fog, smoke), emulsion (milk), sol (paint), gel (jelly, butter). Examiners love asking “milk is what kind of colloid” type questions.

Common Traps

Trap 1: Hardy-Schulze rule direction. For a positive sol, negative ions cause coagulation. For a negative sol, positive ions do. The rule is about the opposite-sign counter-ion’s charge magnitude.

Trap 2: Confusing peptization with dialysis. Peptization is converting a fresh precipitate into a colloid by adding an electrolyte (which gets adsorbed). Dialysis is purifying a colloid by separating dissolved electrolytes through a semipermeable membrane.

Trap 3: Lyophilic vs lyophobic. Lyophilic = solvent-loving, naturally stable, reversible (e.g., starch sol). Lyophobic = solvent-hating, unstable, needs stabilisers (e.g., metal sols).

JEE Main 2022 had a question on the Brownian motion of colloidal particles. Brownian motion arises from molecular collisions of the dispersion medium with colloidal particles — gives evidence for molecular motion and is independent of the chemical nature of the colloid.