Surface Chemistry — NEET Chemistry Notes

Surface chemistry deals with phenomena occurring at the surface of substances — adsorption, catalysis, colloids, and emulsions. Though a lower-weight topic, it has consistent presence in NEET with conceptual clarity questions.

Quick Revision

• Adsorption: Accumulation of gas/liquid on solid surface (not absorption into bulk)
• Physisorption: Weak van der Waals forces, reversible, low activation energy
• Chemisorption: Chemical bond formation, strong, often irreversible
• Catalyst: Changes rate of reaction without being consumed
• Colloid: Heterogeneous mixture with particle size 1–1000 nm
• Tyndall Effect: Scattering of light by colloidal particles
• Coagulation: Process of settling colloidal particles (precipitation)

Standard Study

Adsorption

Characteristics of Adsorption:

• Exothermic process (ΔH negative) — heat is released
• Occurs spontaneously (ΔG < 0, ΔS < 0)
• Increases with pressure, decreases with temperature
• Surface area of adsorbent directly proportional to extent of adsorption
• Reversible for physical adsorption, irreversible for chemical adsorption

Factors Affecting Adsorption:

1. Surface Area: Larger surface area → greater adsorption
2. Temperature: Lower temperature favours physisorption
3. Pressure: Higher pressure increases gas adsorption
4. Nature of Gas: Easily liquefiable gases (CO₂, NH₃) adsorb more

Freundlich Adsorption Isotherm:

• x/m = K × P^(1/n)
• log(x/m) = log K + (1/n) log P
• x = mass of adsorbate, m = mass of adsorbent, P = pressure

Types:

• Physical Adsorption (Physisorption): Weak forces, low specificity, multilayer formation
• Chemical Adsorption (Chemisorption): Strong forces, high specificity, monolayer formation

Catalysis

Homogeneous Catalysis: Catalyst and reactants in same phase

• Example: SO₂ oxidation in lead chamber process (NO as catalyst)

Heterogeneous Catalysis: Catalyst in different phase from reactants

• Example: Haber process (Fe catalyst for N₂ + H₂ → NH₃)
• Surface phenomena — adsorption of reactants on catalyst surface
• Activated complexes form on catalyst surface

Characteristics of Catalysts:

1. Remain unchanged in amount but may change form
2. Small amount can catalyse large reaction
3. Cannot start a reaction that is not thermodynamically feasible
4. Specificity — different catalysts for different reactions
5. Catalyst does not affect equilibrium — only rate

Enzyme Catalysis:

• Biological catalysts are highly specific
• Active site and lock-and-key model
• Optimum temperature and pH for maximum activity
• Michaelis-Menten kinetics: v = V_max [S]/(K_m + [S])

Colloids

Classification Based on Phase:

• Sols: Solid in liquid (e.g., paint, ink) — Tyndall effect present
• Gels: Liquid in solid (e.g., jelly, cheese)
• Emulsions: Liquid in liquid (e.g., milk, butter)
• Foams: Gas in solid/liquid (e.g., shaving foam)
• Aerosols: Solid/liquid in gas (e.g., smoke, fog)

Classification Based on Nature of Dispersed Phase:

• Lyophilic (solvent-loving): Solvent as dispersion medium — reversible, stable
• Lyophobic (solvent-hating): Solvent as dispersion medium — irreversible, less stable

Properties of Colloids:

1. Tyndall Effect: Scattering of light by colloidal particles — used to distinguish from true solutions
2. Brownian Motion: Zigzag movement of colloidal particles — prevents sedimentation
3. Charge on Colloidal Particles: All particles of same type carry same charge — prevents coagulation
4. Coagulation: Adding electrolytes neutralises charges → particles aggregate and settle
5. Dialysis: Removal of ions from colloidal solution using semi-permeable membrane

Schulze-Hardy Rule:

• Greater the charge on ions used for coagulation, greater their effectiveness
• For As₂S₃ (negative sol): Al³⁺ > Ba²⁺ > Na⁺
• For Fe(OH)₃ (positive sol): [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻

Emulsions

• Type of colloidal system with two immiscible liquids
• Oil in Water: Milk (oil droplets in water)
• Water in Oil: Butter (water droplets in oil)
• Emulsifying agents: soap, detergent, proteins
• They stabilise the emulsion by reducing interfacial tension

Deep Study

Adsorption Isotherms

Langmuir Adsorption Isotherm:

• Assumes monolayer adsorption, identical sites, no interaction between adsorbed molecules
• θ = (KP)/(1 + KP)
• x/m = (axP)/(1 + bP)

BET Theory (for multilayer adsorption):

• Used for physical adsorption at high pressures
• Important for gas adsorption on catalysts

Catalytic Activity at Surfaces

• Rate depends on available surface area
• Steps: Diffusion → Adsorption → Reaction → Desorption → Diffusion
• Activated adsorption intermediate states
• Turnover frequency (TOF): number of reactions per active site per unit time

Colloid Stability (DLVO Theory)

• Balance between van der Waals attraction and electrical repulsion
• Energy barrier must be overcome for coagulation
• Steric stabilisation: adsorbed polymer layers prevent close approach

Exam Tips

1. Adsorption vs Absorption: adsorption is on surface; absorption is into bulk
2. Catalyst doesn’t change ΔG — only lowers activation energy
3. Tyndall effect distinguishes colloids from true solutions
4. Charge on colloid determines which ions are most effective for coagulation
5. Freundlich isotherm: log(x/m) vs log P gives straight line with slope 1/n
6. Enzyme catalysis follows Michaelis-Menten kinetics
7. Emulsions require emulsifying agent — without it, oil and water separate

Common Pitfalls

• Confusing physisorption with chemisorption — energy of adsorption different
• Forgetting that catalyst doesn’t affect equilibrium position
• Mixing up coagulation and dialysis — one settles particles, other removes ions
• Confusing Tyndall effect with scattering by any particle — only colloidal particles
• Forgetting that adsorption is exothermic (ΔH < 0) — Le Chatelier applies

Suggested Study Order

1. Adsorption fundamentals — types and characteristics
2. Freundlich adsorption isotherm
3. Catalysis — homogeneous, heterogeneous, enzyme
4. Colloids — classification and properties
5. Tyndall effect and Brownian motion
6. Coagulation and Schulze-Hardy rule
7. Emulsions and emulsifying agents
8. Applications of surface chemistry