Surface Chemistry

Surface Chemistry

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Surface chemistry studies phenomena at the interface of two phases: adsorption, colloids, catalysis, and emulsions.
• Adsorption is the accumulation of a species (adsorbate) at the surface of a solid/liquid (adsorbent); it is exothermic, so by Le Chatelier’s principle, lower temperature favours it (ΔH ≈ 20–40 kJ/mol for physisorption, 80–240 kJ/mol for chemisorption).
• Freundlich isotherm (empirical): x/m = kP^(1/n); linear form log(x/m) = log k + (1/n) log P, valid at intermediate pressures.
• Langmuir isotherm (monolayer): x/m = (aP)/(1 + bP), assumes a homogeneous surface and no lateral interaction among adsorbed molecules.
• Catalyst lowers activation energy (Eₐ) without being consumed; heterogeneous catalysis operates at solid–gas/liquid interfaces via surface adsorption.
• CUET pointer: expect 1–2 MCQs on distinguishing physisorption vs chemisorption and applying the Hardy–Schulze rule for coagulation.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Adsorption vs Absorption

Sorption is the umbrella term. Adsorption concentrates a substance on a surface (a surface phenomenon); absorption involves penetration into the bulk of the material. A solid that takes up water on its surface but not inside is exhibiting adsorption only. The heat released during adsorption of a gas on a solid is called the heat of adsorption.

Physisorption vs Chemisorption

Feature	Physisorption	Chemisorption
Bonding	Van der Waals	Chemical (covalent/ionic)
Specificity	Non-specific	Highly specific
Layers	Multilayer possible	Monolayer only
ΔH (kJ/mol)	20–40	80–240
Reversibility	Reversible	Often irreversible
Activation energy	Low	Appreciable

Both types can occur simultaneously, and the apparent activation energy for chemisorption often decreases as surface coverage rises because of heterogeneity in surface sites.

Adsorption Isotherms

The Freundlich isotherm x/m = kP^(1/n) fits experimental data well at intermediate pressure but fails at very high P (predicts infinite uptake). The Langmuir isotherm x/m = (aP)/(1 + bP) assumes (i) monolayer coverage, (ii) equivalent sites, and (iii) no interaction between adsorbed species; at very low P, x/m ∝ P, and at high P, x/m saturates to a/b, representing complete monolayer formation. The BET isotherm extends Langmuir to multilayer adsorption and is the basis for measuring surface area of porous solids.

Catalysis

A catalyst accelerates a reaction by providing an alternate pathway with lower Eₐ. From the Arrhenius form k = A·exp(−Eₐ/RT), the ratio k₂/k₁ = exp[−(Eₐ₂−Eₐ₁)/RT] = A·exp(−Eₐ/RT). In heterogeneous catalysis, reactants adsorb on active sites, react, and products desorb — the basis of contact catalysis (e.g., Pt in H₂SO₄ manufacture, Fe in Haber process). Shape-selective catalysts such as ZSM-5 zeolite restrict reaction to molecules of a specific geometry, raising selectivity. Catalyst poisoning by impurities (e.g., H₂S poisoning Pt, CO poisoning Fe) destroys active sites.

Colloids

A colloid is a heterogeneous dispersion with particle diameter 1–1000 nm — large enough to scatter light (Tyndall effect) yet small enough not to settle. Lyophilic sols (e.g., gum, starch) are solvent-loving, self-stabilized, and reversible; lyophobic sols (e.g., metals, metal sulphides) need stabilizers and are irreversible. Brownian motion of particles counters gravity, helping stability. Coagulation follows the Hardy–Schulze rule: the coagulating power of an electrolyte ion rises sharply with its charge; trivalent > divalent > monovalent. The coagulation value (or flocculation value) is the minimum millimoles of electrolyte per litre needed to coagulate a sol.

Emulsions and Gels

Emulsions are colloidal dispersions of two immiscible liquids (O/W or W/O) stabilized by emulsifiers/surfactants; gels are semi-solid systems where the dispersed phase forms a continuous network trapping the medium (e.g., jelly, silicic acid gel).

CUET Exam Pattern

Surface Chemistry is allotted ~2% weightage in CUET Chemistry — typically one MCQ. Most-tested areas: isotherm equations, Hardy–Schulze rule, identification of Tyndall/Brownian phenomena, and heterogeneous catalysis examples.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Mechanism of Heterogeneous Catalysis

The widely accepted sequence is: (1) diffusion of reactants to the surface, (2) adsorption on active sites, (3) surface reaction between adsorbed species (Langmuir–Hinshelwood mechanism) or between an adsorbed and a gas-phase species (Eley–Rideal), (4) desorption of products. The rate-determining step is usually either adsorption or surface reaction; product desorption, if slow, can also poison the catalyst by blocking sites — a phenomenon called product inhibition. Promoters enhance activity without themselves being catalysts (e.g., Mo in Fe catalyst for Haber), while poisons reduce activity.

Edge Cases in Isotherms

• At very low P, both Freundlich and Langmuir reduce to x/m ∝ P, but Freundlich has no saturation limit, so it is invalid at high P.
• The BET equation [P/(V(P₀−P)) = 1/(VₘC) + (C−1)/(VₘC)·P/P₀] gives the monolayer capacity Vₘ, from which specific surface area is computed via A = (Vₘ·Nₐ·σ)/(V_molar) where σ is the cross-sectional area of the adsorbate (0.162 nm² for N₂ at 77 K).
• For chemisorption with dissociation (e.g., H₂ on Ni), the Langmuir equation is modified to include a √P dependence.

Worked Example

A 2.0 g sample of charcoal adsorbs 10 mL of N₂ at STP when the pressure is 1.0 atm. Using Freundlich with k = 2.5 (units: mL g⁻¹ atm⁻¹·ⁿ) and 1/n = 0.5, predict uptake at 4.0 atm: x/m at 1 atm = 10/2 = 5 mL/g = 2.5·(1)^0.5 ✓ At 4 atm: x/m = 2.5·(4)^0.5 = 2.5·2 = 5.0 mL/g → 10 mL of N₂ adsorbed on 2 g.

Common Mistakes

• Writing the Freundlich exponent as “1/n” without specifying it is between 0 and 1 (0 < 1/n < 1 for favourable adsorption).
• Treating gel as a true solid — it is a colloidal state with a continuous dispersed-phase network holding the medium.
• Forgetting that emulsifiers reduce interfacial tension; without them, emulsions coalesce rapidly.

Practice Prompts

1. A gas adsorbs on a surface with ΔH = −120 kJ/mol. Predict whether physisorption or chemisorption dominates and justify using the ΔH range.
2. The coagulation values of NaCl, BaCl₂, and AlCl₃ for a negatively charged sol are 50, 0.8, and 0.05 mmol/L. Verify Hardy–Schulze and identify the charge sign on the sol particles.

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