Exam Master Review Sheet - Surface Chemistry

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EXAM MASTER REVIEW SHEET

Physical Chemistry: Surface Chemistry

Adsorption, Catalysis & Colloidal State

1. Physisorption vs Chemisorption

Adsorption is a surface phenomenon where substance accumulates at the interface.

Property	Physisorption	Chemisorption
Forces	Weak van der Waals forces.	Strong Chemical bonds.
Specificity	Non-specific.	Highly specific.
Enthalpy ($\Delta H_{ads}$)	Low (20–40 kJ/mol).	High (80–240 kJ/mol).
Layers	Multimolecular.	Unimolecular.
Temp. Effect	Favoured by low temperature.	Favoured by high temperature.

2. Freundlich Adsorption Isotherm

Relationship between the amount of gas adsorbed per unit mass of adsorbent ($x/m$) and pressure ($P$) at constant temperature.

$$ \frac{x}{m} = k \cdot P^{1/n} \quad (n > 1) $$

Logarithmic Form: $\log(x/m) = \log k + \frac{1}{n} \log P$

• At low $P$: $x/m \propto P^1$

• At high $P$: $x/m \propto P^0$ (Saturation)

3. Catalysis & Enzyme Actions

Enzyme Catalysis

Bio-catalysts. Lock and Key mechanism.

• Highly efficient and specific.
• Optimum pH ($\approx$ 7) and Temp (310 K).

Zeolites

Shape-selective catalysis. Honeycomb-like structure.

Example: ZSM-5 (Alcohols $\rightarrow$ Gasoline).

4. Classification of Colloids

Lyophilic vs Lyophobic

• Lyophilic (Solvent loving): Reversible, stable, easy to prepare (e.g., starch, gum).
• Lyophobic (Solvent hating): Irreversible, unstable, require stabilizers (e.g., metals, metal sulphides).

Associated Colloids (Micelles)

Formed by surface active agents (Soaps/Detergents) above:
1. Kraft Temperature ($T_k$)
2. Critical Micelle Concentration (CMC)

5. Properties & Coagulation Rules

Optical & Kinetic

• Tyndall Effect: Scattering of light by colloidal particles.

• Brownian Motion: Zig-zag motion; provides stability by opposing gravity.

Hardy-Schulze Rule

Coagulating power $\propto$ (Valency of active ion)$^4$

For Negative Sol: $Al^{3+} > Mg^{2+} > Na^+$

For Positive Sol: $[Fe(CN)_6]^{4-} > PO_4^{3-} > Cl^-$

Protection & Gold Number

Lyophilic colloids protect lyophobic ones from coagulation.

$\text{Protective Power} \propto \frac{1}{\text{Gold Number}}$

Gelatin has the smallest Gold Number (highest protective power).

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