Chapter 5 — Surface Chemistry — explores what happens at the surface of substances. From why charcoal purifies water to how catalysts speed up industrial reactions, surface chemistry is everywhere! This chapter carries 4-5 marks and focuses on adsorption, colloids, and catalysis. It’s highly conceptual with many real-life applications.
Key Concepts
Adsorption
The accumulation of molecules on a surface (not in the bulk). The surface = adsorbent; the molecule adsorbed = adsorbate.
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces | Weak van der Waals forces | Chemical bonds (covalent/ionic) |
| Enthalpy | Low (20-40 kJ/mol) | High (80-240 kJ/mol) |
| Specificity | Non-specific | Highly specific |
| Reversibility | Reversible | Usually irreversible |
| Temperature effect | Decreases with temp | Increases then decreases |
| Layers | Multilayer | Monolayer only |
| Activation energy | Low/none | Sometimes high |
Factors Affecting Adsorption of Gases on Solids
- Nature of gas: Easily liquefiable gases (NH₃, HCl, SO₂) are adsorbed more (higher critical temperature)
- Surface area: More surface area → more adsorption (finely divided metals are best)
- Temperature: Physisorption decreases; chemisorption increases initially then decreases
- Pressure: Adsorption increases with pressure (follows Freundlich isotherm)
Freundlich Adsorption Isotherm
Taking log: log(x/m) = log k + (1/n) log P
At low pressure: x/m ∝ P (1/n = 1)
At high pressure: x/m = constant (independent of P)
Catalysis
Homogeneous vs Heterogeneous Catalysis
| Feature | Homogeneous | Heterogeneous |
|---|---|---|
| Phase | Same phase as reactants | Different phase |
| Example | H₂SO₄ in esterification | Fe in Haber process (N₂ + H₂) |
| Example | NO in lead chamber process | V₂O₅ in contact process (SO₂→SO₃) |
Important Catalytic Concepts
- Activity: Ability of a catalyst to increase reaction rate
- Selectivity: Ability to direct a reaction to yield a particular product
- Promoter: Substance that enhances catalyst activity (e.g., Mo in Haber process with Fe catalyst)
- Poison/Inhibitor: Substance that decreases catalyst activity (e.g., CO poisons Fe catalyst in Haber)
Colloids
Classification by Particle Size
| Type | Particle Size | Example |
|---|---|---|
| True Solution | < 1 nm | NaCl in water, sugar solution |
| Colloidal Solution | 1 nm – 1000 nm | Milk, blood, ink, smoke |
| Suspension | > 1000 nm | Muddy water, chalk in water |
Types of Colloidal Systems
| Dispersed Phase | Dispersion Medium | Type | Example |
|---|---|---|---|
| Solid | Liquid | Sol | Paints, cell fluids |
| Liquid | Liquid | Emulsion | Milk, face cream |
| Gas | Liquid | Foam | Shaving cream, whipped cream |
| Solid | Gas | Aerosol (solid) | Smoke, dust |
| Liquid | Gas | Aerosol (liquid) | Fog, mist, cloud |
| Solid | Solid | Solid sol | Gemstones, coloured glass |
Properties of Colloids
- Tyndall Effect: Scattering of light by colloidal particles (beam visible in colloid but not in true solution)
- Brownian Motion: Zigzag movement of colloidal particles due to bombardment by dispersion medium molecules
- Electrophoresis: Movement of colloidal particles under electric field (shows they carry charge)
- Coagulation/Flocculation: Settling of colloidal particles by adding electrolyte (neutralizes charge)
For negatively charged sol: Al³⁺ > Ba²⁺ > Na⁺ (coagulating power)
For positively charged sol: PO₄³⁻ > SO₄²⁻ > Cl⁻
Emulsions
- Oil-in-water (O/W): Oil droplets in water — milk, vanishing cream. Stabilised by water-soluble emulsifiers.
- Water-in-oil (W/O): Water droplets in oil — butter, cold cream. Stabilised by oil-soluble emulsifiers.
Preparation of Colloids
Chemical methods: Double decomposition (As₂S₃ sol), oxidation, reduction (gold sol by reducing HAuCl₄ with HCHO)
Physical methods: Bredig’s arc method (metal sols), peptization (adding electrolyte to precipitate)
Purification of Colloids
- Dialysis: Removal of dissolved ions through semipermeable membrane
- Electrodialysis: Faster dialysis under electric field
- Ultrafiltration: Using ultrafilter paper to separate colloid from true solution
Important Definitions
| Term | Definition |
|---|---|
| Adsorption | Accumulation of molecules on the surface of a substance |
| Adsorbent | The surface on which adsorption takes place |
| Catalyst | Substance that changes reaction rate without being consumed |
| Colloid | Heterogeneous system with particle size between 1-1000 nm |
| Tyndall Effect | Scattering of light by colloidal particles |
| Coagulation | Settling of colloidal particles by neutralising their charge |
| Peptization | Converting a precipitate into a colloidal sol by adding electrolyte |
| Emulsion | Colloidal system of two immiscible liquids |
Solved Examples — NCERT Based
Example 1: Hardy-Schulze Rule
Q: Arrange the following in increasing order of coagulating power for a negatively charged As₂S₃ sol: NaCl, BaCl₂, AlCl₃
Solution:
For negatively charged sol, the coagulating ion is the cation. By Hardy-Schulze rule, higher charge → greater coagulating power.
Na⁺ (1+) < Ba²⁺ (2+) < Al³⁺ (3+)
Increasing coagulating power: NaCl < BaCl₂ < AlCl₃
Example 2: Identifying Colloid Type
Q: Classify the following colloids: (a) Smoke (b) Milk (c) Fog (d) Butter
Solution:
(a) Smoke — solid in gas → Aerosol (solid)
(b) Milk — liquid in liquid → Emulsion (O/W type)
(c) Fog — liquid in gas → Aerosol (liquid)
(d) Butter — liquid in solid (water droplets in fat) → Gel / W/O emulsion
Example 3: Adsorption Type
Q: At low temperature, N₂ gas shows adsorption on iron surface. At high temperature, the same gas shows adsorption with higher enthalpy. Explain.
Solution:
At low temperature → physisorption (weak van der Waals forces, low enthalpy, reversible)
At high temperature → chemisorption (N₂ molecules gain enough energy to break N≡N bond and form chemical bonds with Fe surface, high enthalpy)
This is exactly what happens in the Haber process — N₂ chemisorbs on Fe catalyst at high temperature.
Example 4: Freundlich Isotherm
Q: For adsorption of a gas on charcoal, log(x/m) vs log P gives a straight line with slope 0.5 and intercept 1.2. Find k and 1/n in the Freundlich equation.
Solution:
log(x/m) = log k + (1/n) log P
Slope = 1/n = 0.5
Intercept = log k = 1.2 → k = 10^1.2 = 15.85
Important Questions for Board Exams
1 Mark Questions
- What is the difference between adsorption and absorption?
- What is Tyndall effect?
- Name the type of colloid formed when a liquid is dispersed in a gas.
- What is peptization?
2 Mark Questions
- Distinguish between physisorption and chemisorption (any 4 points).
- What is an emulsion? Name two types with examples.
- State Hardy-Schulze rule with an example.
- What are lyophilic and lyophobic colloids?
3 Mark Questions
- Write the Freundlich adsorption isotherm. How can it be verified graphically?
- What are the various methods of preparing colloids? Explain any two chemical methods.
- Explain the mechanism of enzyme catalysis.
- What is electrophoresis? How does it show that colloidal particles are charged?
5 Mark Questions
- Discuss the properties of colloidal solutions: Tyndall effect, Brownian motion, electrophoresis, and coagulation.
- What is adsorption? Compare physisorption and chemisorption. Write the Freundlich isotherm and explain the effect of temperature and pressure on adsorption.
Quick Revision Points
- Adsorption: surface phenomenon | Absorption: bulk phenomenon
- Physisorption: weak, reversible, multilayer | Chemisorption: strong, irreversible, monolayer
- Freundlich: x/m = kP^(1/n); log(x/m) vs log P → straight line
- Catalyst: lowers Ea, provides alternative pathway
- Colloids: 1-1000 nm; show Tyndall effect, Brownian motion
- Hardy-Schulze: higher valency ion → greater coagulating power
- Emulsions: O/W (milk) and W/O (butter)
- Purification: dialysis, electrodialysis, ultrafiltration
- Enzymes: biological catalysts, optimum temp 25-37°C, highly specific
Chapter Navigation
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Related Chapters in Class 12 Chemistry
- Chemical Kinetics Class 12 Notes
- Electrochemistry Class 12 Notes
- General Principles and Processes of Isolation of Elements Class 12 Notes
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