Class 12 Chemistry Chapter 5 Notes: Surface Chemistry | Important Questions & PYQs

 Class 12 Chemistry – Chapter 5: Surface Chemistry (NCERT Notes)

1. Surface Chemistry

Surface chemistry deals with phenomena occurring at surfaces or interfaces, such as adsorption, catalysis and colloids.

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2. Adsorption

Definition

Adsorption is the phenomenon of accumulation of molecules of a substance on the surface of a solid or liquid.

• Substance adsorbed → Adsorbate

• Surface → Adsorbent

✔ It is a surface phenomenon
✖ Different from absorption (bulk phenomenon)

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3. Types of Adsorption

(A) Physical Adsorption (Physisorption)

• Weak van der Waals forces

• Low enthalpy (20–40 kJ mol⁻¹)

• Reversible

• Occurs at low temperature

• Multilayer formation possible

(B) Chemical Adsorption (Chemisorption)

• Strong chemical bonds

• High enthalpy (80–240 kJ mol⁻¹)

• Irreversible

• Occurs at high temperature

• Monolayer formation only

• Highly specific

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4. Why Physisorption Decreases with Temperature

• Adsorption is exothermic

• Increase in temperature shifts equilibrium backward (Le Chatelier’s principle)

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5. Factors Affecting Adsorption

1. Nature of adsorbate
   Easily liquefiable gases are adsorbed more

2. Nature of adsorbent
   Porous and activated solids adsorb more

3. Surface area
   Powdered substances are better adsorbents

4. Pressure
   Adsorption increases with pressure

5. Temperature
   Adsorption decreases with rise in temperature

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6. Adsorption Isotherm

Freundlich Adsorption Isotherm

$$\frac{x}{m}=k{P}^{1\mathrm{/}n}$$

• Empirical relationship

• Valid at low pressure

• Shows saturation at high pressure

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7. Catalysis

Definition

Catalysis is the process of altering the rate of a reaction by adding a substance called catalyst, which remains chemically unchanged.

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Heterogeneous Catalysis

• Catalyst and reactants in different phases

• Reaction occurs on catalyst surface

Steps:

1. Adsorption of reactants

2. Formation of activated complex

3. Desorption of products

✔ Desorption is essential to free catalyst surface

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8. Catalyst Poisoning

• Some substances reduce catalyst activity

• Example: CO poisons iron catalyst in Haber process

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9. Enzyme Catalysis

• Enzymes are biological catalysts

• Highly specific

• Follow lock-and-key or induced-fit model

• Sensitive to temperature and pH

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10. Colloids

Definition

A colloid is a heterogeneous system in which particles of one substance are dispersed in another with particle size 1–1000 nm.

• Dispersed phase

• Dispersion medium

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11. Classification of Colloids

(A) Based on Physical State

Dispersed Phase	Medium	Example
Solid	Liquid	Sol (starch sol)
Liquid	Gas	Aerosol (fog)
Gas	Liquid	Foam
Liquid	Liquid	Emulsion (milk)

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(B) Based on Nature of Particles

1. Multimolecular colloids – sulphur sol, gold sol

2. Macromolecular colloids – proteins, starch

3. Associated colloids (micelles) – soaps, detergents

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(C) Based on Interaction with Medium

• Lyophilic sols → stable, solvent loving (gelatin)

• Lyophobic sols → less stable, solvent hating (gold sol)

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12. Properties of Colloids

Tyndall Effect

• Scattering of light by colloidal particles

• Helps distinguish colloids from true solutions

Electrophoresis

• Movement of colloidal particles under electric field

Coagulation

• Neutralisation of charge causes particles to aggregate

Dialysis

• Removal of ions from colloids using membrane

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13. Hardy–Schulze Law

• Greater the charge on coagulating ion, greater is its coagulating power

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14. Emulsions

Definition

Emulsions are colloids where both phases are liquids

Types

1. Oil in water (O/W) – milk

2. Water in oil (W/O) – butter

Emulsifiers

• Soaps, detergents, proteins, gums

• Reduce interfacial tension

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15. Micelles & Cleansing Action of Soap

• Soap molecules form micelles

• Hydrophobic tail traps grease

• Hydrophilic head interacts with water

• Grease removed by emulsification

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16. Zeolites & Shape-Selective Catalysis

• Zeolites are microporous aluminosilicates

• Act as shape-selective catalysts

• Example: ZSM-5 converts alcohols to hydrocarbons

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17. Important Uses of Colloids

✔ Medicines
✔ Paints
✔ Food products
✔ Photographic films
✔ Soaps & detergents

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18. Key Exam Points

✔ Adsorption is always exothermic
✔ Powdered solids adsorb more
✔ Enzymes are highly specific
✔ Soap works due to micelle formation
✔ Colloid is a state, not a substance

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✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What is surface chemistry?
   Surface chemistry deals with chemical phenomena occurring at surfaces or interfaces.

2. Define adsorption.
   Adsorption is the accumulation of molecules of a substance on the surface of a solid or liquid.

3. What is adsorbate and adsorbent?
   Adsorbate is the substance adsorbed, and adsorbent is the surface on which adsorption occurs.

4. How is adsorption different from absorption?
   Adsorption is a surface phenomenon, while absorption is a bulk phenomenon.

5. What type of forces are involved in physisorption?
   Weak van der Waals forces.

6. Why is chemisorption irreversible?
   Because strong chemical bonds are formed between adsorbate and adsorbent.

7. Why does adsorption decrease with increase in temperature?
   Because adsorption is an exothermic process.

8. What is catalysis?
   Catalysis is the process of changing the rate of a reaction using a catalyst.

9. What is a colloid?
   A colloid is a heterogeneous system with particle size between 1–1000 nm.

10. What is Tyndall effect?
    Scattering of light by colloidal particles.

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📝 Long Answer Questions

1. Explain adsorption and its types.

Adsorption is the accumulation of particles on the surface of a solid or liquid.

Types of Adsorption:

(a) Physical Adsorption (Physisorption)

• Weak van der Waals forces

• Low enthalpy (20–40 kJ mol⁻¹)

• Reversible

• Occurs at low temperature

• Multilayer adsorption possible

(b) Chemical Adsorption (Chemisorption)

• Strong chemical bonds

• High enthalpy (80–240 kJ mol⁻¹)

• Irreversible

• Occurs at high temperature

• Monolayer formation only

• Highly specific

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2. Explain heterogeneous catalysis and catalyst poisoning.

Heterogeneous Catalysis

• Catalyst and reactants are in different phases

• Reaction occurs on catalyst surface

Steps involved:

• Adsorption of reactants

• Formation of activated complex

• Desorption of products

Desorption is essential to keep catalyst surface free.

Catalyst Poisoning

• Certain substances reduce activity of catalyst

• Example: CO poisons iron catalyst in Haber process

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3. Explain properties of colloids.

Important properties of colloids:

• Tyndall effect: Scattering of light by colloidal particles

• Electrophoresis: Movement of colloidal particles in electric field

• Coagulation: Aggregation of particles due to neutralisation of charge

• Dialysis: Removal of ions using semipermeable membrane

📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. Define adsorption.

2. What is meant by adsorbate?

3. Name the forces involved in physisorption.

4. Why is adsorption called a surface phenomenon?

5. What is Tyndall effect?

6. What is the size range of colloidal particles?

7. Name one emulsifying agent.

8. What are enzymes?

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🔹 2 Mark PYQs

1. Differentiate between adsorption and absorption.

2. Write any two differences between physisorption and chemisorption.

3. Why does adsorption decrease with increase in temperature?

4. What is catalysis? Write one characteristic of a catalyst.

5. Define colloids. Name dispersed phase and dispersion medium.

6. What is catalyst poisoning? Give one example.

7. What is electrophoresis?

8. Why are powdered solids better adsorbents?

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🔹 3 Mark PYQs

1. Explain factors affecting adsorption of gases on solids.

2. Explain Freundlich adsorption isotherm.

3. Describe heterogeneous catalysis with steps involved.

4. Explain enzyme catalysis with any two characteristics.

5. Classify colloids on the basis of physical state with examples.

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🔹 5 Mark PYQs

1. (a) What is adsorption?
   (b) Explain physical and chemical adsorption with suitable differences.

2. (a) What is catalysis?
   (b) Explain heterogeneous catalysis.
   (c) What is catalyst poisoning?

3. (a) Define colloids.
   (b) Classify colloids based on interaction with dispersion medium.

4. (a) What are emulsions?
   (b) Explain types of emulsions with examples.
   (c) What is the role of emulsifiers?

5. (a) Explain cleansing action of soap.
   (b) What are micelles?

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Physisorption vs Chemisorption
✔ Freundlich adsorption isotherm
✔ Heterogeneous catalysis & poisoning
✔ Colloids (definition, classification, properties)
✔ Tyndall effect & electrophoresis
✔ Cleansing action of soap

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❓ FAQs (Concept Clarity)

1. Why are powdered substances better adsorbents?
   Because they have larger surface area.

2. Why are enzymes highly specific?
   Because they follow lock-and-key or induced-fit model.

3. Why is desorption important in catalysis?
   To free the catalyst surface for further reaction.

4. Why are lyophilic sols more stable than lyophobic sols?
   Because they have strong attraction with dispersion medium.

5. Why does soap clean grease?
   Because soap molecules form micelles that emulsify grease.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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