CBSE Class 12 Chemistry NCERT Solutions: Surface Chemistry

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NCERT Solutions PDF Class 12 PDF

This chapter delves into the fascinating world of Surface Chemistry, exploring phenomena that occur at the boundaries between different phases. The NCERT Solutions for Class 12 Chemistry Chapter 5 cover key concepts such as adsorption, including the differences between physisorption and chemisorption, and the factors affecting them. It also explains catalysis, distinguishing between homogeneous and heterogeneous catalysis, and discusses the nature of colloids, their preparation, and properties. The solutions provide clear explanations for various types of colloids, electrophoresis, and coagulation. This chapter is crucial for understanding how surfaces influence chemical processes. These solutions are designed to help students grasp these complex topics thoroughly, aiding in their exam preparation and providing a strong foundation for further studies in chemistry.

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BoardCBSE
ClassClass 12
SubjectChemistry Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 5

Chapter summary

Chapter 5 of the CBSE Class 12 Chemistry syllabus focuses on Surface Chemistry. The NCERT Solutions cover essential topics like adsorption (physisorption and chemisorption), factors influencing adsorption, and the concept of sorption. It also explains catalysis, particularly heterogeneous catalysis, and the role of adsorption in it. The chapter further explores colloids, including their classification, preparation, and properties like the Tyndall effect and Brownian movement. Solutions are provided for understanding electrophoresis and coagulation. This chapter is vital for understanding interfacial phenomena in chemistry.

Learning outcomes

  • Understand the difference between adsorption and absorption.
  • Identify factors affecting physisorption and chemisorption.
  • Differentiate between homogeneous and heterogeneous catalysis.
  • Explain the formation and properties of colloidal solutions.
  • Describe phenomena like electrophoresis and coagulation in colloids.

Topics covered

Paper topics

  • Surface Chemistry
  • Adsorption
  • Absorption
  • Sorption
  • Physisorption
  • Chemisorption
  • Catalysis
  • Homogeneous Catalysis
  • Heterogeneous Catalysis
  • Colloids
  • Colloidal Solutions
  • Interfacial Phenomena

Important topics

  • Adsorption vs. Absorption
  • Factors affecting Physisorption and Chemisorption
  • Homogeneous vs. Heterogeneous Catalysis
  • Colloidal Solutions: Formation and Properties
  • Thermodynamics of Adsorption

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Questions and Solutions

Multiple Choice Questions (MCQs) - Q. 1

Q. 1 Which of the following processes does not occur at the interface of phases?
  1. Crystallisation
  2. Heterogeneous catalysis
  3. Homogeneous catalysis
  4. Corrosion
Solution: Homogeneous catalysis is the correct answer. In homogeneous catalysis, the reactants and the catalyst exist in the same phase (e.g., all are liquids or all are gases). Therefore, the reaction occurs throughout the bulk of the mixture, not specifically at an interface between different phases. Processes like crystallisation, heterogeneous catalysis, and corrosion all involve distinct interfaces where reactions or changes occur.

Multiple Choice Questions (MCQs) - Q. 2

Q. 2 At the equilibrium position in the process of adsorption, what is the relationship between enthalpy change (\Delta H) and the product of temperature (T) and entropy change (\Delta S)?
  1. \Delta H > 0
  2. \Delta H = T\Delta S
  3. \Delta H > T\Delta S
  4. \Delta H < T\Delta S
Solution: The correct option is (b) \Delta H = T\Delta S. Adsorption is a spontaneous process, which means the Gibbs free energy change (\Delta G) must be negative (\Delta G < 0). The relationship between Gibbs free energy, enthalpy, and entropy is given by the equation: \Delta G = \Delta H - T\Delta S. At equilibrium, the system reaches its lowest energy state, and the Gibbs free energy change is zero (\Delta G = 0). Substituting this into the equation gives 0 = \Delta H - T\Delta S, which rearranges to \Delta H = T\Delta S. This indicates that at equilibrium, the enthalpy change is exactly balanced by the temperature-entropy term.

Multiple Choice Questions (MCQs) - Q. 3

Q. 3 Which of the following interfaces cannot be obtained?
  1. Liquid-liquid
  2. Solid-liquid
  3. Liquid-gas
  4. Gas-gas
Solution: The correct option is (d) Gas-gas. Gases are completely miscible with each other, meaning they form a homogeneous mixture. For example, air is a mixture of gases like nitrogen, oxygen, and carbon dioxide, and there are no distinct interfaces between these gases. Interfaces are formed between phases that do not mix completely, such as liquid-liquid (oil and water), solid-liquid (sugar dissolved in water), and liquid-gas (water vapor above liquid water).

Multiple Choice Questions (MCQs) - Q. 4

Q. 4 The term 'sorption' stands for which of the following?
  1. Absorption only
  2. Adsorption only
  3. Both absorption and adsorption
  4. Desorption
Solution: The correct option is (c) Both absorption and adsorption. The term 'sorption' is used as a general term to describe the process where a substance is taken up by another substance, without specifying whether it occurs by absorption (penetration into the bulk) or adsorption (adherence to the surface), or both. The provided diagrams illustrate that adsorption involves molecules adhering to the surface, absorption involves molecules entering the bulk, and sorption encompasses both phenomena.

Multiple Choice Questions (MCQs) - Q. 5

Q. 5 The extent of physisorption of a gas increases with which of the following conditions?
  1. Increase in temperature
  2. Decrease in temperature
  3. Decrease in the surface area of adsorbent
  4. Decrease in the strength of van der Waals' forces
Solution: The correct option is (b) Decrease in temperature. Physisorption is an exothermic process, meaning it releases heat. According to Le Chatelier's principle, lowering the temperature favors exothermic processes. Weak van der Waals' forces hold the gas molecules to the adsorbent surface, and these forces become more effective at lower temperatures, leading to increased adsorption. Conversely, increasing temperature provides kinetic energy to the gas molecules, facilitating their desorption.

Multiple Choice Questions (MCQs) - Q. 6

Q. 6 The extent of adsorption of an adsorbate from a solution phase increases with which of the following?
  1. Increase in the amount of adsorbate in the solution
  2. Decrease in the surface area of the adsorbent
  3. Increase in the temperature of the solution
  4. Decrease in the amount of adsorbate in the solution
Solution: The correct option is (a) Increase in the amount of adsorbate in the solution. When the concentration (amount) of the adsorbate in the solution increases, there are more adsorbate particles available to interact with and adhere to the surface of the adsorbent. This higher concentration gradient drives more adsorbate onto the adsorbent surface, thus increasing the extent of adsorption. Increasing temperature generally decreases adsorption from solutions, and decreasing the surface area of the adsorbent would also decrease the extent of adsorption.

Multiple Choice Questions (MCQs) - Q. 7

Q. 7 Which one of the following is NOT applicable to the phenomenon of adsorption?
  1. \Delta H > 0
  2. \Delta G < 0
  3. \Delta S < 0
  4. \Delta H < 0
Solution: The condition that is NOT applicable is (a) \Delta H > 0. Adsorption is a spontaneous process, so the Gibbs free energy change (\Delta G) must be negative (\Delta G < 0). Adsorption involves the decrease in the randomness of the system as gas molecules become fixed on the surface, leading to a decrease in entropy (\Delta S < 0). For \Delta G to be negative when \Delta S is negative, the enthalpy change (\Delta H) must be negative (\Delta H < 0), indicating that adsorption is an exothermic process. Therefore, \Delta H > 0 is not applicable.

Multiple Choice Questions (MCQs) - Q. 8

Q. 8 Which of the following is NOT a favourable condition for physical adsorption?
  1. High pressure
  2. Negative \Delta H
  3. Higher critical temperature of adsorbate
  4. High temperature
Solution: The correct option is (d) High temperature. Physical adsorption is favored by conditions that increase the interaction between the adsorbate and adsorbent and reduce the kinetic energy of the adsorbate molecules. High pressure increases the concentration of adsorbate gas, favoring adsorption. A negative \Delta H indicates an exothermic process, which is typical for physisorption. A higher critical temperature of the adsorbate means the gas is more easily liquefied, indicating stronger intermolecular forces, which also favors physisorption. High temperature, however, increases the kinetic energy of gas molecules, leading to desorption and thus is unfavorable for physisorption.

Multiple Choice Questions (MCQs) - Q. 9

Q. 9 Physical adsorption of a gaseous species may change to chemical adsorption with which of the following changes?
  1. Decrease in temperature
  2. Increase in temperature
  3. Increase in the surface area of adsorbent
  4. Decrease in the surface area of adsorbent
Solution: The correct option is (b) Increase in temperature. Physisorption is a reversible process driven by weak van der Waals' forces and occurs readily at low temperatures. Chemisorption involves the formation of strong chemical bonds and requires a higher activation energy. As the temperature increases, the molecules gain sufficient energy to overcome the weak forces of physisorption and form chemical bonds with the adsorbent surface, leading to a transition from physisorption to chemisorption. While surface area is important for both, temperature is the key factor driving this transition.

Common mistakes

  • Confusing adsorption with absorption.
  • Not understanding the conditions favoring physisorption vs. chemisorption.
  • Misinterpreting the thermodynamic conditions (ΔH, ΔS, ΔG) for adsorption.
  • Difficulty in classifying different types of colloids.

Revision tips

  • Clearly distinguish between adsorption and absorption with examples.
  • Memorize the factors affecting physisorption and chemisorption.
  • Understand the thermodynamic principles governing adsorption.
  • Draw diagrams to illustrate colloidal phenomena like Brownian movement and Tyndall effect.
  • Practice identifying different types of colloids and their preparation methods.

Practice MCQs

Q1. Which of the following processes does NOT occur at the interface of phases?

Q2. At the equilibrium position in the process of adsorption, what is the relationship between enthalpy change (ΔH) and the product of temperature (T) and entropy change (ΔS)?

Q3. Which of the following interfaces cannot be obtained?

Q4. The term 'sorption' is used to describe which of the following processes?

Q5. The extent of physisorption of a gas typically increases with:

Q6. Which of the following conditions is NOT favorable for physical adsorption?

Q7. Physical adsorption of a gaseous species can change to chemical adsorption under which condition?

Frequently asked questions

What is the main difference between adsorption and absorption?

Adsorption is the adhesion of a substance onto the surface of another, while absorption is the process where a substance permeates or dissolves into the bulk of another.

What are the two main types of adsorption?

The two main types are physisorption (physical adsorption), involving weak van der Waals' forces, and chemisorption (chemical adsorption), involving the formation of chemical bonds.

How does temperature affect physisorption?

Physisorption is generally favored by low temperatures because it is an exothermic process. Increasing temperature usually decreases the extent of physisorption.

What is the significance of the term 'sorption'?

Sorption is a general term used when it is not specified whether the process is adsorption or absorption, or when both occur simultaneously.

Why is homogeneous catalysis not considered an interfacial phenomenon?

In homogeneous catalysis, the reactants and the catalyst are in the same phase, leading to a uniform distribution and reaction throughout the bulk, rather than at a specific interface.

What are colloidal solutions?

Colloidal solutions are heterogeneous mixtures where one substance is dispersed as very fine particles throughout another substance. The dispersed particles are larger than molecules but too small to be seen with the naked eye.

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