Class 12 Surface Chemistry Notes | Complete Guide with Questions

Surface Chemistry is a fascinating branch of chemistry that deals with the study of chemical phenomena occurring at the interface of two phases, such as solid–liquid, solid–gas, or liquid–gas interfaces. It explains how reactions happen on surfaces, how catalysts work, and why processes like adsorption, colloids, and emulsions are important in industries and daily life.

In Class 12 Chemistry, Surface Chemistry is a scoring chapter if you understand its concepts clearly. This blog provides detailed notes on every topic, covering definitions, explanations, equations, examples, and important points for exams.

Class 12 Surface Chemistry Notes | Complete Guide with Questions

Meaning of Surface Chemistry

Surface chemistry involves the study of physical and chemical properties of surfaces and interfaces. The surface of a substance often shows properties different from its bulk because molecules at the surface experience unbalanced forces.

• Interface: A boundary separating two different phases, e.g., solid-liquid, liquid-gas.

• Surface: The outermost layer where interaction occurs.

Example: Adsorption of gases on charcoal, rusting of iron, catalysis reactions on metal surfaces.

Importance of Surface Chemistry

Surface Chemistry plays a crucial role in:

• Industrial processes like Haber’s process for ammonia production.

• Heterogeneous catalysis in chemical industries.

• Purification techniques like adsorption chromatography.

• Biological systems like enzyme catalysis.

• Environmental processes such as pollutant adsorption.

Topics Covered in Surface Chemistry

We will study the following topics:

1. Adsorption

2. Catalysis

3. Colloids

4. Emulsions and Applications

1. Adsorption

Meaning of Adsorption

• Adsorption is the process of accumulation of molecules of a substance (adsorbate) on the surface of another substance (adsorbent) leading to a higher concentration on the surface compared to the bulk.

Example: Adsorption of gases like O₂, H₂, CO on charcoal or silica gel.

• Adsorbate: The substance being adsorbed.

• Adsorbent: The surface on which adsorption takes place.

Types of Adsorption

Adsorption can be classified into two main types:

(a) Physical Adsorption (Physisorption)

• Involves weak van der Waals forces.

• Reversible in nature.

• Low heat of adsorption (20–40 kJ/mol).

• Multilayer adsorption possible.

• Example: Adsorption of CO₂ on charcoal.

(b) Chemical Adsorption (Chemisorption)

• Involves chemical bond formation between adsorbate and adsorbent.

• Irreversible in nature.

• High heat of adsorption (80–240 kJ/mol).

• Usually forms monolayer.

• Example: Adsorption of H₂ on Ni catalyst in hydrogenation.

Factors Affecting Adsorption

1. Surface Area of Adsorbent – Higher surface area → higher adsorption.

2. Nature of Adsorbent/Adsorbate – Specific interactions influence adsorption capacity.

3. Pressure – For gases, adsorption increases with pressure (at constant temperature).

4. Temperature – Adsorption decreases with increase in temperature (exothermic process).

5. Concentration – Higher concentration → more adsorption until equilibrium.

Adsorption Isotherms

Adsorption isotherms describe how adsorption varies with pressure at constant temperature.

(a) Freundlich Adsorption Isotherm

Mathematically expressed as:

x/m = k·p^(1/n)

Where:

• x = mass of adsorbate

• m = mass of adsorbent

• p = pressure

• k, n = constants

Taking logarithms:
log(x/m) = log k + (1/n) log p

Plot: log(x/m) vs log p → straight line with slope 1/n.

(b) Langmuir Adsorption Isotherm

Assumes:

• Adsorption is monolayered.

• Surface has finite identical sites.

Equation: p/xm = 1/(k·b) + p/b

Where b = adsorption constant, k = equilibrium constant.

Plot: p/(x/m) vs p → straight line.

2. Catalysis

Catalysis is the process of altering the rate of a chemical reaction using a substance called a catalyst which remains chemically unchanged at the end of the reaction.

Types of Catalysis

(a) Homogeneous Catalysis

• Reactants and catalyst in the same phase.

• Example: Oxidation of SO₂ to SO₃ using NO gas as catalyst.

(b) Heterogeneous Catalysis

• Reactants and catalyst in different phases.

• Example: Hydrogenation of oils using Ni catalyst (solid) with reactants in liquid/gas phase.

Characteristics of Catalysis

• A catalyst remains unchanged in mass and composition after the reaction.

• It alters the reaction rate (positive catalyst increases rate, negative decreases rate).

• Highly specific in nature: A catalyst for one reaction may not work for another.

• Effective in small amounts.

Promoters and Poisons

• Promoters: Substances that increase catalyst efficiency (e.g., Molybdenum in Haber’s process).

• Poisons: Substances that reduce catalyst efficiency (e.g., As₂O₃ poisoning Pt catalyst).

Modern Theory of Catalysis

• Reactants adsorb on the catalyst surface → weaken bonds → reaction occurs → products desorb from surface → catalyst regenerated.

Example: Hydrogenation of ethene on Ni catalyst.

Enzyme Catalysis

• Enzymes are biological catalysts, e.g., invertase, zymase, amylase.

• Work under mild conditions of temperature and pH.

• Highly specific in action.

Mechanism follows:

1. Lock and Key Model – Substrate fits into enzyme like a key in a lock.

2. Induced Fit Model – Enzyme changes shape slightly to fit substrate.

3. Colloids

Definition

Colloids are heterogeneous systems where one substance (dispersed phase) is uniformly distributed in another (dispersion medium).

Particle size: 1 nm – 1000 nm.

Classification of Colloids

(a) Based on Physical State

• Sol: Solid in liquid (e.g., paint).

• Gel: Liquid in solid (e.g., jelly).

• Emulsion: Liquid in liquid (e.g., milk).

• Aerosol: Liquid or solid in gas (e.g., fog, smoke).

(b) Based on Nature of Interaction

• Lyophilic (solvent-loving): Stable, reversible, e.g., starch, gum.

• Lyophobic (solvent-hating): Unstable, irreversible, e.g., metals, sulphides.

Preparation of Colloids

1. Condensation Methods – Molecules join to form colloidal particles (e.g., chemical reactions, exchange of solvents).

2. Dispersion Methods – Large particles broken into colloidal size by mechanical methods (e.g., peptization, Bredig’s arc method).

Properties of Colloids

1. Tyndall Effect: Scattering of light by colloidal particles.

2. Brownian Motion: Zig-zag motion of particles.

3. Electrophoresis: Movement of colloidal particles in electric field.

4. Charge on Colloids: Due to ion adsorption; helps in stability.

Coagulation of Colloids

• The process of settling down of colloidal particles is called coagulation.

• Methods: By adding electrolytes, mutual coagulation, or electrophoresis.

• Hardy–Schulze Rule: Higher valency of oppositely charged ion → higher coagulating power.

4. Emulsions

• Emulsions are colloidal solutions of two immiscible liquids where one is dispersed in the other.

Types of Emulsions

1. Oil in Water (O/W): e.g., milk, vanishing cream.

2. Water in Oil (W/O): e.g., butter, cold cream.

Emulsifying Agents

Substances that stabilize emulsions, e.g., soaps, detergents, proteins.

Applications of Surface Chemistry

• Industrial Catalysis: Manufacture of NH₃, H₂SO₄, CH₃OH.

• Adsorption: Gas masks, pollution control, chromatography.

• Medicines: Enzyme catalysis in digestion, antibiotics.

• Food Industry: Emulsions in ice creams, butter.

• Rubber Industry: Preparation of latex emulsions.

Objective Questions (MCQs with One-Word Answers)

Q1. Which of the following is a lyophobic colloid?

(a) Starch
(b) Gum
(c) Ferric hydroxide
(d) Gelatin

Answer:
(c) Ferric hydroxide

Q2. Adsorption of gases on solid surfaces is generally:

(a) Endothermic
(b) Exothermic
(c) Reversible and endothermic
(d) None of these

Answer:
(b) Exothermic

Q3. Which enzyme converts glucose to ethanol?

(a) Maltase
(b) Invertase
(c) Zymase
(d) Diastase

Answer:
(c) Zymase

Q4. The process of settling down of colloidal particles is called:

(a) Emulsification
(b) Coagulation
(c) Peptization
(d) None of these

Answer:
(b) Coagulation

Q5. In Freundlich adsorption isotherm, 1/n = 1 indicates:

(a) Chemisorption
(b) Multilayer adsorption
(c) Linear adsorption
(d) Irreversible adsorption

Answer:
(c) Linear adsorption

Short Answer Questions

Q1. Define adsorption and give one example.

Answer:
Adsorption is the process in which molecules of a substance get accumulated on the surface of another substance rather than in its bulk. For example, adsorption of gases like hydrogen or oxygen on the surface of charcoal.

Q2. Differentiate between homogeneous and heterogeneous catalysis.

Answer:

• Homogeneous Catalysis: Catalyst and reactants are in the same phase. Example: Oxidation of SO₂ to SO₃ using NO as catalyst.

• Heterogeneous Catalysis: Catalyst and reactants are in different phases. Example: Hydrogenation of vegetable oils using Ni catalyst.

Q3. What is Tyndall effect?

Answer:
The scattering of light by colloidal particles in all directions is called the Tyndall effect. It proves the heterogeneous nature of colloids.

Q4. What is meant by promoters and poisons in catalysis?

Answer:
Promoters are substances that enhance the activity of a catalyst, while poisons are substances that reduce or destroy its catalytic activity. Example: Molybdenum acts as a promoter in Haber’s process; Arsenic oxide acts as a poison for platinum catalyst.

Q5. What are emulsions? Give two examples.

Answer:
Emulsions are colloidal systems where one liquid is dispersed in another immiscible liquid. Examples: Milk (oil in water), Butter (water in oil).

Long Answer Questions

Q1. Explain Freundlich adsorption isotherm with the help of a graph.

Answer:
Freundlich adsorption isotherm gives a relationship between the amount of gas adsorbed per unit mass of adsorbent (x/m) and the pressure (p) of the gas at constant temperature. The empirical equation is:
x/m=k⋅p1/nx/m = k \cdot p^{1/n}x/m=k⋅p1/n
Taking log on both sides:
log⁡(x/m)=log⁡k+1nlog⁡p\log(x/m) = \log k + \frac{1}{n} \log plog(x/m)=logk+n1​logp
A plot of $\log (x/m)$ vs $\log p$ gives a straight line with slope 1/n. It shows that adsorption increases with pressure but reaches a limit.

Q2. Describe the mechanism of enzyme catalysis.

Answer:
Enzyme catalysis occurs when the substrate (reactant) fits into the active site of the enzyme, forming an enzyme-substrate complex. This lowers the activation energy and speeds up the reaction. There are two models:

1. Lock and Key Model – Substrate fits perfectly into the enzyme’s active site.

2. Induced Fit Model – Enzyme modifies its shape slightly to accommodate the substrate.
   After the reaction, products leave the active site, and the enzyme is regenerated.

Q3. Discuss the different methods of preparation of colloids.

Answer:
Colloids can be prepared by:

1. Condensation Methods: Molecules join to form colloidal particles by chemical reactions (e.g., hydrolysis, oxidation) or change of solvent.

2. Dispersion Methods: Large particles broken into colloidal size using mechanical or electrical methods like Bredig’s arc method or peptization.

Q4. What are emulsions? Explain their types and applications.

Answer:
Emulsions are colloidal systems where one liquid is dispersed in another immiscible liquid. Types:

1. Oil in Water (O/W): Oil droplets dispersed in water; e.g., milk, vanishing cream.

2. Water in Oil (W/O): Water droplets dispersed in oil; e.g., butter, cold cream.

Applications: Used in medicines, cosmetics, food products like mayonnaise, and paints.

Q5. Explain the role of adsorption in heterogeneous catalysis.

Answer:
In heterogeneous catalysis, reactants get adsorbed on the surface of the solid catalyst. The adsorbed molecules are in close proximity, bonds weaken, and reaction occurs rapidly on the surface. The products then desorb, leaving the catalyst unchanged. Example: Hydrogenation of ethene to ethane using Ni catalyst.

Conclusion

Surface Chemistry is an essential branch connecting chemistry to real-world applications in industries, biology, environment, and technology. Understanding adsorption, catalysis, colloids, and emulsions forms the base for advanced studies and competitive exams.

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