Board exam focus — Surface Chemistry (CBSE & HBSE)

Adsorption (physisorption vs chemisorption), adsorption isotherms, catalysis (homogeneous and heterogeneous), enzyme catalysis, colloids (preparation, properties, coagulation) and emulsions.

Adsorption and Adsorption Isotherms

Surface Chemistry

Adsorption vs Absorption

Adsorption: Accumulation of substances on the SURFACE of a material (surface phenomenon). Absorption: Uniform distribution throughout the BULK of a material (bulk phenomenon). Sorption: Both adsorption and absorption occurring simultaneously.

Adsorbent: Material that does the adsorbing (charcoal, silica gel, zeolite). Adsorbate: Substance adsorbed on surface.

Types of Adsorption

Adsorption Isotherms

Relationship between amount adsorbed (x/m) and pressure at constant temperature.

Freundlich Adsorption Isotherm: x/m = k x p^(1/n) (n > 1, usually 1/n < 1) log(x/m) = log k + (1/n) log p Linear plot: log(x/m) vs log p; slope = 1/n, intercept = log k. Approximate; works at intermediate pressures.

Langmuir Adsorption Isotherm: (p/x/m) = (1/aK) + (p/a) (a = max adsorption) Linear plot: p/(x/m) vs p; slope = 1/a, intercept = 1/(aK). Assumes monolayer, no interaction between adsorbed molecules.

Factors Affecting Adsorption

1. Nature of adsorbate: Easily liquefied gases adsorb more (higher Tc = more adsorption)
2. Nature of adsorbent: Activated charcoal > silica gel. More surface area = more adsorption.
3. Temperature: Physisorption decreases with T; chemisorption has a maximum.
4. Pressure: Adsorption increases with pressure (then reaches saturation).
5. Surface area: Finely divided adsorbent has more area; porous materials (zeolites, charcoal) excellent.

Applications of Adsorption

1. Gas masks: Charcoal adsorbs toxic gases
2. Decolorization: Bone char removes color from cane sugar
3. Drying: Silica gel adsorbs water vapor
4. Heterogeneous catalysis
5. Separation by column chromatography

Catalysis

Catalysis

Catalyst

A substance that increases the rate of a reaction without being consumed. It lowers the activation energy (Ea) by providing an alternative pathway. Catalyst doesn't change equilibrium constant (both forward and reverse rates increase equally).

Types of Catalysis

1. Homogeneous Catalysis: Catalyst and reactants are in the SAME phase.

Examples:

• H+ (aq) catalysis of ester hydrolysis: CH3COOC2H5 + H2O → CH3COOH + C2H5OH
• NO catalysis of SO2 oxidation (Lead Chamber process):

2SO2 + 2NO + O2 → 2NO2 + 2SO2 → 2H2SO4 (net: 2SO2 + O2 → 2SO3)

• I- in decomposition of H2O2

2. Heterogeneous Catalysis: Catalyst and reactants are in DIFFERENT phases (most industrial).

Examples:

Mechanism of Heterogeneous Catalysis

1. Reactants adsorb on catalyst surface (chemisorption)
2. Adsorbed molecules react (activation energy lowered)
3. Products desorb from catalyst surface
4. Fresh surface available for next cycle

Promoters and Inhibitors

Promoter: Increases catalyst efficiency (not a catalyst itself). Example: K2O, Al2O3 in Haber process. Inhibitor/Poison: Decreases catalyst efficiency. Example: CO poisons Ni catalyst in hydrogenation (CO chemisorbs strongly, blocks surface). Autocatalysis: Product itself acts as catalyst. Example: Mn2+ in KMnO4 + H2C2O4 reaction.

Enzyme Catalysis

Biological catalysts (proteins). Extremely specific (one enzyme, one substrate). Active site: specific region of enzyme where substrate binds.

Lock-and-Key Model: Substrate (key) fits exactly into active site (lock) of enzyme. Induced Fit Model: Active site changes shape to accommodate substrate.

Characteristics:

• Highly efficient (10^3 to 10^8 times faster than uncatalyzed)
• Optimal temperature: 37 C (body temperature); inactivated above 40 C
• Optimal pH: usually 6-8; some exceptions (pepsin: pH 2, arginase: pH 10)
• Inhibited by heavy metal ions (Pb2+, Hg2+)
• Specificity: urease only hydrolyzes urea

Colloids and Emulsions

Colloids

Classification of Dispersions

Colloids

Heterogeneous system with dispersed phase (particle 1-1000 nm) in dispersion medium.

Types of Colloids:

Preparation of Colloids

1. Dispersion methods: Break down bulk matter to colloidal size.

• Mechanical: Colloid mill, ultrasonic vibrations
• Electrical (Bredig's arc): For metal sols (Au, Ag, Pt)
• Peptization: Addition of electrolyte disaggregates precipitate into colloid

2. Condensation methods: Build up from smaller particles.

• Double decomposition: As2O3 + H2S → As2S3 sol
• Oxidation: SO2 + 2H2S → 3S (sulphur sol)
• Reduction: 2AuCl3 + 3SnCl2 → 2Au sol + 3SnCl4

Properties of Colloids

Tyndall Effect: Scattering of light by colloidal particles (visible as blue opalescence). Distinguishes colloids from true solutions.

Brownian Movement: Random zig-zag motion of colloidal particles due to unequal bombardment by dispersion medium molecules. Prevents sedimentation.

Electrophoresis: Migration of colloidal particles under applied electric field. Direction indicates charge on particles.

• Positive sol: Fe(OH)3, Al(OH)3 (migrate to cathode)
• Negative sol: Gold sol, As2S3 (migrate to anode)

Coagulation (Flocculation): Destabilization and settling of colloidal particles. Methods: Adding electrolyte, heating, mixing oppositely charged colloids, electrophoresis.

Hardy-Schulze Rule: Coagulating power increases with valence of coagulating ion. For negative sol: Al3+ > Mg2+ > Na+ (Al3+ most effective) For positive sol: PO4^3- > SO4^2- > Cl-

Emulsions

Colloid of two immiscible liquids.

Types:

• O/W (oil-in-water): Oil drops in water. Examples: milk, vanishing cream. Emulsifier: soap, protein.
• W/O (water-in-oil): Water drops in oil. Examples: butter, cold cream. Emulsifier: long-chain alcohols.

Emulsifier: Reduces surface tension between oil and water. Has hydrophilic and hydrophobic parts. Surrounds oil drop, stabilizes emulsion (e.g., soap acts as emulsifier).

Protective Colloids (Lyophilic)

Lyophilic colloids (macromolecules, gelatin, gum arabic) adsorb on lyophobic colloids and protect them from coagulation. Gold number: Minimum mass (mg) of protective colloid to prevent coagulation of 10 mL gold sol when 1 mL of 10% NaCl is added. Lower gold number = better protecting agent.

Frequently asked questions

Are these Surface Chemistry notes free?

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Do these notes follow CBSE and HBSE?

Yes. The Surface Chemistry notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.

What does the Surface Chemistry chapter cover?

Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Surface Chemistry.