Chemistry Class 12 NCERT Solutions Chapter 5 Surface Chemistry – Important Questions

Chemistry, as a subject, plays a pivotal role in the Class 12 curriculum, and Chapter 5 of the NCERT Chemistry textbook is a crucial part of this syllabus. This chapter, titled "Surface Chemistry," delves into the fascinating world of chemical processes occurring at interfaces. It includes fundamental concepts and complex phenomena that are essential for understanding more advanced topics in chemistry. For students preparing for their Class 12 board exams, grasping the important questions from this chapter is crucial for scoring well. This article aims to provide a comprehensive guide to the important questions from Chapter 5, helping students to prepare effectively.

1. Introduction to Surface Chemistry

Surface Chemistry is the study of chemical processes that occur at surfaces or interfaces, including the surface of solids, liquids, and gases. This chapter introduces the concept of surfaces, adsorption, catalysis, and colloids. It provides a detailed understanding of how these processes impact various chemical reactions and applications.

2. Important Topics in Chapter 5

To excel in this chapter, students need to focus on the following key topics:

• Adsorption: The process by which atoms, ions, or molecules from a substance adhere to a surface of another substance. Adsorption is categorized into physical adsorption (physisorption) and chemical adsorption (chemisorption).

• Catalysis: The process of accelerating a chemical reaction using a substance called a catalyst. Catalysts are classified into homogeneous and heterogeneous catalysts.

• Colloids: Mixtures where one substance is dispersed in another. Colloids are classified based on the state of the dispersed phase and dispersion medium, such as sols, gels, and emulsions.

• Applications of Surface Chemistry: Understanding practical applications such as catalysis in industrial processes, environmental chemistry, and the formulation of various products.

3. Key Questions and Solutions

Below are some of the important questions from Chapter 5 along with their solutions:

1. Define adsorption. What are its types?

Solution:

Adsorption is the process by which atoms, ions, or molecules from a substance adhere to a surface of another substance. This process creates a film of adsorbate on the surface of the adsorbent. Adsorption is of two main types:

• Physical Adsorption (Physisorption): This involves weak Van der Waals forces between the adsorbate and adsorbent. It is generally reversible and does not involve any chemical change. For example, the adsorption of gases on charcoal.

• Chemical Adsorption (Chemisorption): This involves strong chemical bonds between the adsorbate and adsorbent. It is often irreversible and involves a chemical reaction. For example, the adsorption of hydrogen on a metal surface.

2. Explain the Freundlich adsorption isotherm.

Solution:

The Freundlich adsorption isotherm describes how the amount of adsorbate adsorbed on the surface of an adsorbent varies with its pressure or concentration at a constant temperature. It is expressed as:

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

Where:

• $x/m$ is the amount of adsorbate adsorbed per unit mass of the adsorbent.
• $P$ is the pressure or concentration of the adsorbate.
• $k$ and $n$ are constants specific to the adsorbent-adsorbate pair.

This equation suggests that adsorption increases with an increase in pressure or concentration, but at a decreasing rate. It is applicable to physisorption and can describe a wide range of adsorption processes.

3. What is the role of catalysts in chemical reactions? Provide examples.

Solution:

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy. Catalysts can be classified into:

• Homogeneous Catalysts: These catalysts are in the same phase as the reactants. For example, sulfuric acid in the esterification reaction.

• Heterogeneous Catalysts: These catalysts are in a different phase than the reactants. For example, platinum or palladium used in hydrogenation reactions.

Examples of catalytic processes include:

• The Haber process for ammonia synthesis, where iron acts as a catalyst.
• The contact process for sulfuric acid production, where vanadium pentoxide is used as a catalyst.

4. Discuss the different types of colloids with examples.

Solution:

Colloids are mixtures where one substance is dispersed in another. They can be classified based on the state of the dispersed phase and the dispersion medium:

• Sols: Solid particles dispersed in a liquid. Example: Paint, blood.

• Gels: A gel is a colloidal system where the dispersed phase forms a network structure throughout the dispersion medium. Example: Gelatin, agar.

• Emulsions: Liquid droplets dispersed in another liquid. Example: Milk, mayonnaise.

• Aerosols: Solid or liquid particles dispersed in a gas. Example: Smoke, fog.

5. Explain the process of adsorption in terms of its applications in environmental chemistry.

Solution:

Adsorption plays a crucial role in environmental chemistry by helping in the removal of pollutants from the environment. Some applications include:

• Water Purification: Activated carbon adsorbents are used to remove contaminants and impurities from water. For example, activated carbon filters are used to purify drinking water by adsorbing organic compounds and chlorine.

• Air Pollution Control: Adsorbents are used to remove volatile organic compounds (VOCs) and other pollutants from the air. For example, activated carbon is used in air filters to adsorb harmful gases.

• Soil Remediation: Adsorption processes are employed to remove contaminants from the soil. For instance, clay minerals are used to adsorb heavy metals from polluted soil.

6. Derive the Langmuir adsorption isotherm equation.

Solution:

The Langmuir adsorption isotherm describes adsorption as a surface reaction where the adsorbate forms a monolayer on the adsorbent surface. The equation is derived as follows:

Assume:

• The surface has a maximum adsorption capacity, $a_m$.
• The adsorption is reversible and occurs on a uniform surface with no interaction between adsorbed molecules.

The fraction of the surface covered by the adsorbate is given by:

$\theta = \frac{P}{P + K}$

Where:

• $\theta$ is the fraction of the surface covered.
• $P$ is the pressure of the adsorbate.
• $K$ is the adsorption constant.

The amount adsorbed per unit mass of adsorbent is:

$x/m = a_m \cdot \frac{P}{P + K}$

This equation represents the Langmuir isotherm, showing that adsorption increases with pressure but tends to level off as the surface becomes saturated.

7. What is the difference between physisorption and chemisorption?

Solution:

The primary differences between physisorption and chemisorption are:

• Nature of Bonding:

  • Physisorption: Involves weak Van der Waals forces. The bonds are non-specific and reversible.
  • Chemisorption: Involves strong chemical bonds and often forms new chemical species. It is usually irreversible.

• Temperature Dependence:

  • Physisorption: It generally occurs at lower temperatures and decreases with increasing temperature.
  • Chemisorption: It occurs at higher temperatures and increases with increasing temperature.

• Surface Coverage:

  • Physisorption: Can cover a large surface area with multiple layers of adsorbate.
  • Chemisorption: Typically forms a monolayer on the surface.

• Examples:

  • Physisorption: Adsorption of gases on activated charcoal.
  • Chemisorption: Adsorption of hydrogen on a metal catalyst.

8. Explain the role of adsorption in the preparation of catalysts.

Solution:

Adsorption plays a vital role in the preparation and function of catalysts. In heterogeneous catalysis, the adsorption of reactants on the catalyst's surface is a crucial step. Here’s how it works:

• Preparation of Catalysts: During catalyst preparation, adsorbates are often used to modify the surface properties of the catalyst. For instance, promoters are adsorbed onto catalysts to enhance their activity.

• Reaction Mechanism: Adsorption of reactants onto the catalyst surface lowers the activation energy by providing an alternate reaction pathway. This enhances the rate of the chemical reaction.

• Selectivity: Adsorption helps in selective reactions by providing specific sites for particular reactions, thus improving the selectivity of the catalyst.

9. Describe the role of colloids in pharmaceutical applications.

Solution:

Colloids are widely used in pharmaceuticals due to their unique properties. Their role includes:

• Drug Delivery: Colloidal carriers such as liposomes and nanoparticles are used to deliver drugs more effectively. They improve the solubility and bioavailability of drugs.

• Formulation: Colloidal systems like emulsions and gels are used in the formulation of various pharmaceutical products, including creams, ointments, and suspensions.

• Stability: Colloids help in stabilizing drug formulations by preventing the aggregation or settling of particles, thus ensuring consistent drug efficacy.

10. What are the applications of surface chemistry in everyday life?

Solution:

Surface chemistry has numerous applications in everyday life, including:

• Detergents: Surface-active agents (surfactants) in detergents reduce the surface tension of water, allowing it to penetrate and clean surfaces more effectively.

• Catalysts in Vehicles: Catalysts in automotive exhaust systems help in reducing harmful emissions by facilitating the breakdown of pollutants into less harmful substances.

• Food Industry: Surface chemistry principles are used in the formulation of food products, including flavor enhancers and preservatives that interact with surfaces to improve food quality.

• Cosmetics: The formulation of cosmetic products such as creams and lotions involves surface chemistry to ensure proper texture, stability, and absorption.

4. Conclusion

Chapter 5 of the Class 12 NCERT Chemistry textbook, "Surface Chemistry," provides a deep dive into essential concepts like adsorption, catalysis, and colloids. Understanding these topics not only helps in mastering the subject but also in appreciating the practical applications of surface chemistry in various fields. By focusing on important questions and practicing their solutions, students can enhance their grasp of the subject and perform better in their exams.