Exam Master Review Sheet - Surface Chemistry

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EXAM MASTER FORMULA SHEET

Physical Chemistry: Surface Chemistry

Adsorption, Catalysis & Colloidal State (JEE & NEET)

1. Physisorption vs Chemisorption

Adsorption is a surface phenomenon ($\Delta G < 0, \Delta S < 0, \Delta H < 0$).

Property	Physisorption (Physical)	Chemisorption (Chemical)
Forces	Weak van der Waals forces	Strong Chemical bonds
Specificity & Nature	Non-specific, Reversible	Highly specific, Irreversible
Enthalpy of Adsorption ($\Delta H$)	Low (20–40 kJ/mol)	High (80–240 kJ/mol)
Temperature Effect	Decreases with increase in Temp	First increases, then decreases (requires $E_a$)
Adsorption Layers	Multimolecular layers (at high P)	Unimolecular layer only

2. Adsorption Isotherms

Freundlich Isotherm

Empirical relation for adsorption of gases on solid surfaces.

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

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

At low P: $x/m \propto P^1$ | At high P: $x/m \propto P^0$

Langmuir Isotherm

Theoretical relation assuming unimolecular layer adsorption.

\[ \frac{x}{m} = \frac{a P}{1 + b P} \]

$a, b$ are Langmuir constants.

At very high P: $x/m = a/b$ (Saturation)

3. Catalysis & Zeolites

Homogeneous vs Heterogeneous

• Homogeneous: Catalyst & reactants are in the same phase. E.g., Hydrolysis of ester catalyzed by $H^+(aq)$.
• Heterogeneous: Catalyst is in a different phase. E.g., Haber's process ($N_2 + 3H_2 \to 2NH_3$ over solid $Fe$).

Zeolites (Shape-Selective)

Microporous aluminosilicates with honeycomb structures. Catalysis depends on pore size and reactant molecule size.

Famous Example: ZSM-5 (Converts alcohols directly into gasoline/petrol).

Enzyme Catalysis (Bio-catalysts)

Complex nitrogenous organic compounds (Proteins). Work via the Lock and Key mechanism.

• Highly specific and extraordinarily efficient.
• Highly active under optimum temperature ($\approx 298 - 310$ K) and optimum pH ($\approx 5 - 7$).
• Activity increases in presence of coenzymes or activators ($Na^+, Mn^{2+}, Co^{2+}$).

4. Colloidal State & Classification

Particle size ranges from $1 \text{ nm}$ to $1000 \text{ nm}$ ($10^{-9}$ to $10^{-6}$ m).

Lyophilic Sols (Liquid-Loving)

• • High affinity between dispersed phase and dispersion medium.
• • Reversible in nature. Very stable.
• • Formed directly by mixing (e.g., Starch, Gum, Gelatin, Protein in water).

Lyophobic Sols (Liquid-Hating)

• • No affinity between phases.
• • Irreversible and easily coagulated by adding electrolytes.
• • Require stabilizing agents (e.g., Metal sols like $Au$, Metal sulphides like $As_2S_3$).

Associated Colloids (Micelles)

Substances (like soaps/detergents) that behave as normal electrolytes at low concentrations but form aggregates (micelles) at high concentrations.

• • Kraft Temperature ($T_k$): The temperature above which micelle formation takes place.
• • Critical Micelle Concentration (CMC): The concentration above which micelles form. (For soaps, CMC $\approx 10^{-4} \text{ to } 10^{-3} \text{ mol/L}$).

5. Properties, Coagulation & Protection

Properties of Colloids

• Tyndall Effect (Optical): Scattering of light by colloidal particles, making the path of light visible. Condition: Diameter of particles is not much smaller than wavelength of light used.
• Brownian Motion (Kinetic): Continuous zig-zag motion. Responsible for stability (counters gravity).
• Electrophoresis (Electrical): Movement of colloidal particles under applied electric field. Determines charge on sol.

Hardy-Schulze Rule (Coagulation)

The coagulating (flocculating) power of an ion depends on its valency. Higher the valency, greater the coagulating power.

For Negative Sols (e.g., $As_2S_3$):

$\ce{Al^{3+} > Ba^{2+} > Na+}$

For Positive Sols (e.g., $Fe(OH)_3$):

$\ce{[Fe(CN)6]^{4-} > PO4^{3-} > SO4^{2-} > Cl-}$

Protection & Gold Number

Lyophilic sols are added to lyophobic sols to protect them from coagulation.

Gold Number: The minimum mass (in mg) of a protective lyophilic colloid that must be added to 10 mL of standard red gold sol to prevent its coagulation by 1 mL of 10% $\ce{NaCl}$ solution.

\[ \text{Protective Power} \propto \frac{1}{\text{Gold Number}} \]

Gelatin has the smallest Gold Number ($\approx 0.005 - 0.01$), hence maximum protective power.

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