4 Most SAQ’s of Surface Chemistry Chapter in Inter 2nd Year Chemistry (TS/AP)

4 Marks

SAQ-1 : Define adsorption. Discuss the differences between Physical adsorption and chemical adsorption.

Introduction

Adsorption is a process where molecules of a gas or liquid accumulate on the surface of a solid or liquid, resulting in a higher concentration at the surface than in the bulk. This phenomenon is fundamental in various natural and industrial processes.

Adsorption Defined

Adsorption refers to the accumulation of gas or liquid molecules on the surface of a solid or liquid.

Examples of Adsorption

1. Adherence of paints to surfaces.
2. Corrosion of metals.
3. Adsorption of hydrogen (H₂) on charcoal.

Physical Adsorption vs. Chemical Adsorption

1. Nature of Forces
   • Physical Adsorption: Involves van der Waals forces – weak physical forces.
   • Chemical Adsorption: Characterized by the formation of strong chemical bonds between the adsorbent and adsorbate, akin to covalent bonds.
2. Specificity
   • Physical Adsorption: Non-specific; any gas can be adsorbed under suitable conditions.
   • Chemical Adsorption: Highly specific, dependent on the nature of the gas and the solid surface.
3. Reversibility
   • Physical Adsorption: Generally reversible.
   • Chemical Adsorption: Often irreversible.
4. Heat of Adsorption
   • Physical Adsorption: Exhibits a low heat of adsorption (20−40 kJ/mol).
   • Chemical Adsorption: Associated with a higher heat of adsorption (40−400 kJ/mol).
5. Temperature Dependence
   • Physical Adsorption: Increases with a decrease in temperature and ceases at very low or very high temperatures.
   • Chemical Adsorption: Exhibits an optimum temperature where adsorption first increases with temperature, then decreases.
6. Speed
   • Physical Adsorption: Fast initially, then slows down.
   • Chemical Adsorption: Slow at the start, speeds up, and eventually becomes constant.
7. Multilayer Adsorption
   • Physical Adsorption: Capable of multilayer adsorption.
   • Chemical Adsorption: Generally forms only a monolayer of gas molecules.

Summary

Adsorption is a key surface phenomenon with wide-ranging implications in industry, research, and environmental science. Distinguishing between physical and chemical adsorption—based on their forces, specificity, reversibility, heat of adsorption, temperature dependence, speed, and layering—provides critical insights into their applications and underlying mechanisms.

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SAQ-2 : What is catalysis? How is catalysis classified? Give two examples for each type of catalysis.

Introduction

Catalysis is a fundamental concept in chemistry where an additional substance, known as a catalyst, is added to a chemical reaction to alter its speed without itself being consumed.

Definition of Catalysis

Catalysis is the process that involves the acceleration or deceleration of the rate of a chemical reaction by a catalyst. The catalyst remains chemically unchanged after the reaction.

Classification of Catalysis

Catalysis is broadly classified into two types based on the phase of the catalyst relative to the reactants:

1. Homogeneous Catalysis: In homogeneous catalysis, the catalyst and the reactants are in the same phase.
   • Example 1: Oxidation of sulfur dioxide to sulfur trioxide with nitrogen oxide as the catalyst. $$2SO_2 (g) + O_2 (g) \overset{NO(g)}{\rightarrow} 2SO_3 (g)$$
   • Example 2: Hydrolysis of methyl acetate in the presence of acid. $$CH_3COOCH_3 (l) + H_2O (l) \overset{H^+}{\rightarrow} CH_3COOH (aq) + CH_3OH (aq)$$
2. Heterogeneous Catalysis: In heterogeneous catalysis, the catalyst and the reactants are in different phases.
   • Example 1: Oxidation of sulfur dioxide to sulfur trioxide using platinum as a catalyst. $$2SO_2 (g) + O_2 (g) \overset{Pt(s)}{\rightarrow} 2SO_3 (g)$$
   • Example 2: Synthesis of ammonia from nitrogen and hydrogen using iron as a catalyst. $$N_2 (g) + 3H_2 (g) \overset{Fe}{\rightarrow} 2NH_3 (g)$$

Summary

Catalysis significantly impacts chemical industries and natural processes by altering reaction rates. It is classified into homogeneous and heterogeneous catalysis, each distinguished by the phase relationship between the catalyst and the reactants. This classification underpins the mechanism of action of catalysts in facilitating various chemical transformations essential in both industrial applications and biological systems.

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SAQ-3 : What are Emulsions? How are they classified? Describe the applications of emulsions.

Introduction

Emulsions are specialized mixtures comprised of two immiscible liquids, one dispersed in the other, typically stabilized by an emulsifying agent.

Definition of Emulsions

Emulsions are colloidal systems where both the dispersed phase and the dispersion medium are liquids, such as oil and water, which ordinarily do not mix.

Classification of Emulsions

Emulsions are mainly classified into two types:

1. Oil in Water (O/W) Type
   • Oil acts as the dispersed phase, and water serves as the dispersion medium.
   • Examples:
     • Milk: Comprises fat droplets dispersed in water.
     • Vanishing cream: A cosmetic cream with oil droplets dispersed in water.
2. Water in Oil (W/O) Type
   • Water is the dispersed phase, and oil functions as the dispersion medium.
   • Examples:
     • Butter: Contains water droplets dispersed in a fat matrix.
     • Cod liver oil: A dietary supplement with water-soluble vitamins in oil.
     • Cold cream: A moisturizer with water droplets dispersed in oil.

Applications of Emulsions

Emulsions are utilized across various domains, including:

1. Medicines: Tonics and supplements, like cod liver oil, often are emulsions.
2. Cleaning with Soaps: Soap cleaning action forms an emulsion, trapping dirt or grease in droplets for easy removal.
3. Dairy Products: Milk is a natural emulsion critical for its nutritional and textural properties.

Summary

Emulsions represent a crucial class of colloidal mixtures, instrumental in diverse sectors like food, cosmetics, and pharmaceuticals. Their understanding, including types and applications, facilitates their effective use in enhancing product performance and consumer experience.

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SAQ-4 : Explain Tyndall Effect and Brownian movement.

Introduction

In the realm of colloids, two pivotal phenomena, the Tyndall effect and Brownian movement, offer deep insights into the characteristics and behavior of colloidal systems.

Tyndall Effect

1. Definition: The Tyndall effect is the scattering of light by colloidal particles, making the light path visible.
2. Observation: Visible when light passes through a colloidal solution, such as sunlight through fog or the beam of a projector in a dusty room.
3. Conditions for Tyndall Effect:
   • The size of dispersed particles should be close to or smaller than the wavelength of light.
   • A notable difference in refractive indices between the dispersed phase and the dispersion medium is necessary.

Brownian Movement

1. Definition: Brownian movement is characterized by the erratic, random motion of colloidal particles within a dispersion medium.
2. Cause: It results from the uneven bombardment of colloidal particles by the molecules of the dispersion medium.
3. Significance: It plays a crucial role in maintaining the stability of colloidal solutions by preventing the particles from settling, thus acting as a natural form of stirring.

Summary

The Tyndall effect and Brownian movement are foundational to understanding colloidal solutions. The Tyndall effect illustrates how light interacts with colloids, while Brownian movement explains the dynamic and stable nature of colloidal systems. Together, these phenomena underscore the unique properties and behaviors of colloids in both natural environments and technological applications.