Class 12th chemistry Chapter 5 Surface Chemistry

Chapter 5 of Class 12 Chemistry, titled “Surface Chemistry,” primarily deals with the study of phenomena that occur at surfaces or interfaces, rather than in bulk. Surface chemistry plays a crucial role in various physical and chemical processes, including catalysis, adsorption, and the formation of colloids.

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Key Concepts of Surface Chemistry

Adsorption: Adsorption is the process by which atoms, ions, or molecules from a substance (like gas, liquid, or dissolved solids) adhere to a surface of another substance. This process creates an interface between the two substances. Adsorption can be of two types: physisorption (physical adsorption) where the interaction is weak, and chemisorption (chemical adsorption) where the interaction involves strong chemical bonds.

Catalysis: Catalysts are substances that increase the rate of a chemical reaction without undergoing any permanent change themselves. In surface chemistry, catalysts often work by providing a surface on which reactants are adsorbed, allowing them to react more easily. Catalysis can be either homogeneous (where the catalyst is in the same phase as the reactants) or heterogeneous (where the catalyst is in a different phase than the reactants).

Colloids: Colloids are mixtures where one substance is dispersed evenly throughout another. The size of the particles in a colloid is between that of true solutions and suspensions, making them stable and preventing them from settling out quickly. Colloids can exhibit various unique properties, such as the Tyndall effect, where light is scattered by colloidal particles.

Emulsions: An emulsion is a type of colloid where both the dispersed phase and the dispersion medium are liquids. Emulsions are typically stabilized by agents known as emulsifiers, which prevent the liquids from separating. They are commonly used in food, cosmetics, and pharmaceuticals.Applications of Surface Chemistry: Surface chemistry has wide applications in industries, including the production of catalysts, detergents, and paints, as well as in environmental protection, where adsorption is used to remove pollutants from water and air.

Importance of Surface Chemistry

Surface chemistry is essential for understanding and designing processes in industries such as pharmaceuticals, agriculture, and materials science. The study of surface phenomena helps in developing better catalysts, more efficient drug delivery systems, and improved methods for pollution control.

This chapter provides a foundation for understanding how reactions at surfaces influence the overall behavior of materials and how these principles can be applied in various real-world applications.

Question & answer

1. What is adsorption?

Adsorption is the phenomenon where molecules from a gas or liquid accumulate on the surface of a solid or liquid, forming a molecular or atomic film. This process occurs because the particles at the surface of a solid or liquid are not completely surrounded by other adsorbate particles, leading to an imbalance of forces, which they attempt to neutralize by attracting adsorbate molecules.

2. Differentiate between physisorption and chemisorption.

Physisorption involves the weak van der Waals forces between adsorbate and adsorbent, making it reversible and occurring at lower temperatures. Chemisorption, on the other hand, involves stronger chemical bonds, making it usually irreversible and occurring at higher temperatures, often with an activation energy requirement.

3. What is the significance of surface area in adsorption?

The extent of adsorption increases with the surface area of the adsorbent. Finely divided metals and porous substances like activated charcoal have high surface areas, making them highly effective adsorbents.

4. Explain the Freundlich adsorption isotherm.

The Freundlich adsorption isotherm gives the relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and the pressure of the gas at a constant temperature. It is expressed as ( \frac{x}{m} = kP^{1/n} ), where ( x ) is the mass of gas adsorbed, ( m ) is the mass of the adsorbent, ( P ) is the pressure, and ( k ) and ( n ) are constants.

5. What are the applications of adsorption in daily life?

Adsorption has various applications such as in the purification of water using activated charcoal, in gas masks to filter out harmful gases, in the adsorption of moisture by silica gel, and in heterogeneous catalysis where the reactants are adsorbed on the catalyst’s surface.

6. Describe the concept of catalysis and its types.

Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst. Catalysts can be heterogeneous (different phase from reactants) or homogeneous (same phase as reactants). Heterogeneous catalysts typically involve adsorption of reactants on the catalyst surface.

7. What is the role of enzymes in biochemical reactions?

Enzymes are biological catalysts that speed up biochemical reactions in living organisms. They work by lowering the activation energy of the reaction, thus increasing the reaction rate. Enzymes are highly specific for their substrates and operate under mild conditions of temperature and pH.

8. Explain the concept of colloids and their classification.

Colloids are mixtures where one substance is dispersed uniformly throughout another. They are classified based on the nature of the dispersed phase and dispersion medium, such as sols, gels, emulsions, and aerosols. Depending on the interaction between the phases, colloids can be lyophilic (solvent-attracting) or lyophobic (solvent-repelling).

9. What is the Tyndall effect?

The Tyndall effect is the scattering of light by colloidal particles in a colloid. This scattering causes the path of the light beam to become visible. The Tyndall effect is used to distinguish between a true solution and a colloid.

10. How do you prepare colloids?

Colloids can be prepared by various methods, including condensation methods (where small molecules combine to form larger colloidal particles) and dispersion methods (where larger particles are broken down into colloidal size). Techniques like Bredig’s arc method or peptization can be used for preparation.

11. Explain the process of coagulation of colloids.

Coagulation or precipitation of colloids refers to the process where colloidal particles aggregate to form larger particles that settle out of the dispersion medium. This can be induced by adding electrolytes, which neutralize the charges on the colloidal particles, leading to their aggregation.

12. What is the Hardy-Schulze rule?

The Hardy -Schulze rule states that the coagulating power of an electrolyte depends on the valency of the ion carrying a charge opposite to that of the colloidal particles. Higher the valency, greater is the coagulating power.

13. Discuss the properties of colloids.

Colloids exhibit properties such as Brownian movement (random motion of particles), Tyndall effect (light scattering), electrophoresis (movement of colloidal particles under electric field), and adsorption. These properties arise due to the large surface area and interaction between dispersed particles and the medium.

14. What are emulsions and their types?

Emulsions are colloidal systems in which both dispersed phase and dispersion medium are liquids. They are classified into oil-in-water (O/W) emulsions, where oil is the dispersed phase, and water-in-oil (W/O) emulsions, where water is the dispersed phase. Milk is an example of an O/W emulsion.

15. Explain the process of dialysis in colloids.

Dialysis is a process used to separate colloidal particles from smaller dissolved ions or molecules in a solution. It involves the diffusion of small molecules or ions through a semipermeable membrane, while colloidal particles are retained.

16. How are colloids used in medicine?

In medicine, colloidal gold and silver are used for therapeutic purposes. Colloidal silver is known for its antibacterial properties. Colloids are also used in drug delivery systems to enhance the absorption and efficacy of drugs.

17. What is electrophoresis and its application in colloids?

Electrophoresis is the movement of colloidal particles in a liquid under the influence of an electric field. It is used to determine the charge on colloidal particles and is also a key technique in DNA and protein analysis in biochemistry.

18. Describe the mechanism of heterogeneous catalysis.

In heterogeneous catalysis, the reactants are adsorbed on the catalyst’s surface. This adsorption increases the concentration of reactants at the surface, weakens their bonds, and lowers the activation energy. After the reaction, the products desorb from the surface, leaving the catalyst unchanged.

19. What are micelles and their formation?

Micelles are aggregates of surfactant molecules formed in a colloidal solution when the concentration of surfactants exceeds a certain threshold, known as the critical micelle concentration (CMC). They have hydrophobic tails inward and hydrophilic heads outward, helping in the emulsification and solubilization of lipophilic substances.

20. Discuss the application of colloids in the industry.

Colloids are widely used in industries such as in the manufacturing of paints, inks, and cosmetics. They are also crucial in the formulation of pharmaceuticals, wastewater treatment, and food processing. The stabilization of colloids is essential in these processes to ensure the quality and effectiveness of the products.

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