MP Board Class 12 Chemistry Chapter 5: Surface Chemistry — Complete Notes for 2027 Board Exam
Chapter 5: Surface Chemistry (पृष्ठ रसायन) is an important chapter in the MP Board Class 12 Chemistry syllabus, typically contributing 10–14 marks in the board exam. This chapter deals with phenomena that occur at the surfaces or interfaces of substances — from adsorption and catalysis to colloids and emulsions. These comprehensive notes cover adsorption, catalysis, colloids, emulsions, and purification methods with clear explanations and MP Board–focused exam tips.
📑 Table of Contents
🧲 Adsorption
Adsorption is the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The substance that gets adsorbed is called the adsorbate, and the substance on whose surface adsorption occurs is called the adsorbent.
🔬 Adsorption vs Absorption
| Property | Adsorption | Absorption |
|---|---|---|
| Definition | Surface phenomenon — substance accumulates on surface | Bulk phenomenon — substance penetrates throughout |
| Concentration | Higher on surface than in bulk | Uniform throughout the bulk |
| Example | Water vapour adsorbed on silica gel | Water absorbed by sponge |
| Rate | Fast initially, then slows down | Uniform rate throughout |
📊 Types of Adsorption
| Property | Physisorption | Chemisorption |
|---|---|---|
| Forces involved | Van der Waals forces (weak) | Chemical bonds (strong) |
| Enthalpy | Low: 20–40 kJ/mol | High: 80–240 kJ/mol |
| Reversibility | Reversible | Irreversible |
| Temperature | Decreases with increase in temperature | Increases initially, then decreases |
| Specificity | Non-specific — any gas adsorbs on any solid | Highly specific — depends on chemical affinity |
| Layers | Multilayer formation | Monolayer formation |
📈 Factors Affecting Adsorption
- Surface area of adsorbent: Greater surface area → more adsorption (porous substances like charcoal are excellent adsorbents)
- Nature of gas: Easily liquefiable gases (NH₃, HCl, CO₂) are adsorbed more than permanent gases (H₂, He, O₂)
- Temperature: Physisorption decreases with temperature; chemisorption increases then decreases
- Pressure: Adsorption increases with pressure (Freundlich and Langmuir isotherms)
📐 Freundlich Adsorption Isotherm
The Freundlich adsorption isotherm gives the relationship between the amount of gas adsorbed (x/m) and pressure (P) at constant temperature:
x/m = k · P^(1/n) (where k and n are constants, n > 1)
In logarithmic form:
log(x/m) = log k + (1/n) log P
Plotting log(x/m) vs log P gives a straight line with slope = 1/n and intercept = log k.
⚗️ Catalysis
Catalysis is the process of increasing the rate of a chemical reaction by adding a substance called a catalyst, which itself remains chemically unchanged at the end of the reaction.
🧪 Types of Catalysis
| Type | Definition | Example |
|---|---|---|
| Homogeneous Catalysis | Catalyst and reactants in same phase | 2SO₂ + O₂ → 2SO₃ catalysed by NO (all gases) |
| Heterogeneous Catalysis | Catalyst in different phase than reactants | Haber process: Fe catalyst, reactants are gases |
🏭 Important Industrial Catalytic Processes
| Process | Catalyst | Product |
|---|---|---|
| Haber’s Process | Fe + Mo as promoter | NH₃ (ammonia) |
| Ostwald’s Process | Platinum gauze | HNO₃ (nitric acid) |
| Contact Process | V₂O₅ | H₂SO₄ (sulphuric acid) |
| Hydrogenation of Oils | Nickel (Ni) | Vanaspati ghee |
🔍 Adsorption Theory of Heterogeneous Catalysis
Heterogeneous catalysis occurs through these steps:
- Diffusion of reactants to the catalyst surface
- Adsorption of reactant molecules on the catalyst surface (active sites)
- Chemical reaction on the surface — formation of intermediate
- Desorption of products from the surface
- Diffusion of products away from the surface
🧬 Enzyme Catalysis
Enzymes are biological catalysts that are proteins with high molecular mass. They are highly specific and efficient.
| Enzyme | Source | Reaction Catalysed |
|---|---|---|
| Zymase | Yeast | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (fermentation) |
| Invertase | Yeast | Sucrose → Glucose + Fructose |
| Pepsin | Stomach | Proteins → Amino acids |
💡 Characteristics of Enzyme Catalysis
- Highly specific: One enzyme catalyses only one reaction (lock and key model)
- Maximum efficiency at optimum temperature: 37°C (body temperature) — denatures at high temperature
- Optimum pH: Each enzyme works best at a specific pH (pepsin at pH ~2, trypsin at pH ~8)
- Activators and inhibitors: Metal ions (activators) increase activity; poisons (inhibitors) decrease it
🧴 Colloids
A colloid is a heterogeneous system in which the particle size ranges from 1 nm to 1000 nm (10⁻⁹ to 10⁻⁶ m). The dispersed phase particles are distributed throughout the dispersion medium.
📋 Classification of Colloids
A) Based on Physical State of Dispersed Phase and Dispersion Medium
| Dispersed Phase | Dispersion Medium | Colloid Type | Example |
|---|---|---|---|
| Solid | Solid | Solid sol | Ruby glass, coloured gemstones |
| Solid | Liquid | Sol | Paint, ink, muddy water |
| Solid | Gas | Aerosol | Smoke, dust particles in air |
| Liquid | Solid | Gel | Cheese, butter, jelly |
| Liquid | Liquid | Emulsion | Milk, mayonnaise |
| Gas | Solid | Solid foam | Pumice stone, foam rubber |
| Gas | Liquid | Foam | Whipped cream, froth |
B) Based on Nature of Interaction — Lyophilic vs Lyophobic
| Property | Lyophilic Colloids | Lyophobic Colloids |
|---|---|---|
| Formation | Easily formed — just mix with solvent | Requires special methods (condensation/dispersion) |
| Reversibility | Reversible — can be regenerated after evaporation | Irreversible — cannot be regenerated |
| Viscosity | Similar to or slightly higher than solvent | Similar to solvent |
| Examples | Starch, gum, gelatin, proteins in water | Metallic sols (Au, Ag), Fe(OH)₃, As₂S₃ |
✨ Tyndall Effect
The Tyndall effect is the scattering of light by colloidal particles. When a beam of light passes through a colloidal solution, its path becomes visible. This happens because the colloidal particles are large enough (1–1000 nm) to scatter light. True solutions (particles < 1 nm) do not show the Tyndall effect.
⚡ Electrophoresis
The movement of colloidal particles under an electric field is called electrophoresis. Positively charged particles move towards the cathode (negative electrode) and negatively charged particles move towards the anode (positive electrode). This principle is used in electrodeposition of paints and rubber coatings.
🔄 Coagulation (Flocculation)
Coagulation is the process of settling colloidal particles by neutralizing the charge on them. Methods of coagulation include:
- Addition of electrolytes: Ions neutralize the charge on colloidal particles — Hardy-Schulze rule states that the ion with opposite charge and higher valency is more effective
- Electrophoresis: Particles migrate to oppositely charged electrode, get neutralized and coagulate
- Boiling: Increases kinetic energy, reducing adsorption of ions
- Mutual precipitation: Mixing oppositely charged sols causes mutual coagulation
📏 Hardy-Schulze Rule
“The ion having the charge opposite to that of the colloidal particles is effective in causing coagulation, and the greater the valency of the ion, the greater is its coagulating power.”
| Colloid | Charge | Coagulating Ion (increasing power →) |
|---|---|---|
| As₂S₃ sol | Negative | Na⁺ < Mg²⁺ < Al³⁺ |
| Fe(OH)₃ sol | Positive | Cl⁻ < SO₄²⁻ < PO₄³⁻ |
🥛 Emulsions
An emulsion is a colloidal system where both the dispersed phase and the dispersion medium are liquids. Examples include milk (fat dispersed in water) and cream.
🔬 Types of Emulsions
| Type | Dispersed Phase | Dispersion Medium | Example |
|---|---|---|---|
| Oil-in-water (O/W) | Oil | Water | Milk, vanishing cream |
| Water-in-oil (W/O) | Water | Oil | Butter, cold cream |
🧪 Emulsifiers and Demulsifiers
- Emulsifiers stabilize emulsions (e.g., soap, detergents, proteins, gums)
- Demulsifiers break emulsions (e.g., heat, centrifugation, chemical addition)
- Emulsions can be broken by electrophoresis, freezing, heating, or adding electrolytes
🧹 Purification of Colloids
🔬 Methods of Purification
| Method | Principle | Use |
|---|---|---|
| Dialysis | Semipermeable membrane allows ions to pass but not colloids | Purification of blood (haemodialysis) |
| Electrodialysis | Electric field speeds up ion migration through membrane | Faster purification of colloids |
| Ultrafiltration | Colloidal particles pass through filter paper but not ultrafilter | Concentration of colloids |
| Ultracentrifugation | High-speed centrifugation separates colloids by density | Separating colloidal particles from impurities |
💡 Applications of Colloids
- 📝 Industrial: Paint, ink, rubber, plastics, lubricants
- 🌾 Agriculture: Soil colloids (clay and humus) retain nutrients
- 💊 Medicine: Colloidal gold for arthritis, silver as antiseptic, iron injections
- 🍞 Food: Milk, butter, ice cream, bread, cheese — all colloidal systems
- 🚰 Water purification: Alum (potassium aluminium sulphate) coagulates colloidal impurities
- 🧪 Cottrell precipitator: Removes smoke particles from chimneys using electrostatic precipitation
📝 Important Questions for MP Board 2027
✏️ Very Short Answer (1 mark)
- What is the difference between adsorption and absorption?
- Define the Tyndall effect.
- What is an emulsifier? Give one example.
- State the Hardy-Schulze rule.
- What is electrophoresis?
📄 Short Answer (3-4 marks)
- Distinguish between physisorption and chemisorption with examples.
- Explain Freundlich adsorption isotherm. Write its mathematical expression.
- What are lyophilic and lyophobic colloids? Differentiate between them.
- Explain the mechanism of enzyme catalysis with the lock and key model.
- Describe different methods of coagulation of colloidal solutions.
📚 Long Answer (5-6 marks)
- Explain the adsorption theory of heterogeneous catalysis with steps. Give four industrial examples.
- What are colloids? Classify them based on the physical state of the dispersed phase and dispersion medium.
- Discuss the various methods of purification of colloidal solutions. Explain the principle of each.
- What are emulsions? Explain the different types with examples and applications in daily life.
📊 Quick Formula & Concept Summary
| # | Concept | Key Point / Formula |
|---|---|---|
| 1 | Adsorption | Surface phenomenon; x/m = k·P^(1/n) |
| 2 | Physisorption | Weak van der Waals forces; ΔH = 20-40 kJ/mol |
| 3 | Chemisorption | Chemical bonds; ΔH = 80-240 kJ/mol |
| 4 | Colloid size | 1 nm to 1000 nm (10⁻⁹ to 10⁻⁶ m) |
| 5 | Hardy-Schulze | Coagulation power ∝ valency of opposing ion |
| 6 | Tyndall Effect | Scattering of light by colloidal particles |
| 7 | Electrophoresis | Motion of colloidal particles under electric field |
| 8 | Dialysis | Separation using semipermeable membrane |
| 9 | Enzyme catalysis | Highly specific; optimum T = 37°C |
| 10 | Emulsions | O/W (milk) and W/O (butter) types |
📖 Exam Tips for MP Board 2027
- Numericals: Freundlich isotherm calculations and coagulation value problems — prepare thoroughly
- Comparisons: Adsorption vs Absorption, Physisorption vs Chemisorption, Lyophilic vs Lyophobic — commonly asked in 3-mark questions
- Diagrams: Freundlich isotherm graph, Tyndall effect setup, dialysis apparatus — draw neatly with labels
- Definitions: Adsorption, desorption, sorption, colloid, emulsion, gel, sol — be precise with size ranges
- Applications: Cottrell precipitator, water purification, blood dialysis — real-world applications are frequently tested
- Important for 2027: Hardy-Schulze rule numericals, enzyme catalysis mechanism, classification of colloids, and purification methods
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