Chemistry Class 12 NCERT Solutions Chapter 4 Chemical Thermodynamics

Introduction

Chemistry is an essential subject in the Class 12 curriculum, especially for students preparing for competitive exams and future academic pursuits in science and engineering. Chapter 4 of the Class 12 NCERT Chemistry textbook, titled "Chemical Thermodynamics," delves into the fundamental principles governing energy changes in chemical reactions. Understanding the important questions from this chapter is crucial for mastering the concepts and excelling in exams.

This article aims to provide a detailed exploration of the important questions from Chapter 4, focusing on key topics and concepts. By studying these questions thoroughly, students can gain a comprehensive understanding of the subject and improve their problem-solving skills.

Key Concepts in Chapter 4 – Chemical Thermodynamics

Before diving into the important questions, it's essential to review the key concepts covered in Chapter 4:

Thermodynamics: The branch of chemistry that deals with the study of energy changes in chemical reactions and physical processes.
System and Surroundings: A system refers to the part of the universe under study, while the surroundings include everything else outside the system.
Internal Energy (U): The total energy contained within a system, including kinetic and potential energy at the molecular level.
Enthalpy (H): The heat content of a system at constant pressure. It is defined as $H = U + PV$ , where $P$ is the pressure and $V$ is the volume of the system.
First Law of Thermodynamics: States that energy cannot be created or destroyed, only transferred or converted from one form to another. Mathematically, it is expressed as $\Delta U = Q - W$ , where $Q$ is the heat added to the system, and $W$ is the work done by the system.
Second Law of Thermodynamics: Introduces the concept of entropy, which is a measure of the disorder or randomness in a system. It states that the entropy of the universe always increases in spontaneous processes.
Gibbs Free Energy (G): A thermodynamic potential that measures the maximum reversible work that can be performed by a system at constant temperature and pressure. It is defined as $G = H - TS$ , where $T$ is the temperature and $S$ is the entropy.
Hess's Law: States that the enthalpy change of a chemical reaction is the same regardless of the number of steps in which the reaction occurs.

Important Questions from Chapter 4

1. Define the First Law of Thermodynamics and explain its significance with an example.

Answer: The First Law of Thermodynamics, also known as the Law of Energy Conservation, states that energy can neither be created nor destroyed, but it can be transformed from one form to another. This law can be mathematically expressed as:

$\Delta U = Q - W$

where $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, and $W$ is the work done by the system.

Significance: The First Law of Thermodynamics is fundamental in understanding how energy transfers occur in chemical reactions. For example, in an exothermic reaction, heat is released into the surroundings, which means the internal energy of the system decreases. Conversely, in an endothermic reaction, heat is absorbed from the surroundings, leading to an increase in the system's internal energy.

Example: Consider the combustion of methane ( $CH_4$ ):

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$

In this reaction, the chemical energy stored in methane is converted into heat energy. According to the First Law, the heat released (Q) is equal to the decrease in the system's internal energy (ΔU), and no energy is lost; it is merely transferred.

2. What is enthalpy, and how is it different from internal energy?

Answer: Enthalpy (H) is a thermodynamic property that represents the total heat content of a system at constant pressure. It is defined as:

$H = U + PV$

where $U$ is the internal energy of the system, $P$ is the pressure, and $V$ is the volume of the system.

Difference from Internal Energy:

Internal Energy (U): Represents the total energy contained within the system, including kinetic and potential energies at the molecular level.
Enthalpy (H): Includes the internal energy plus the product of pressure and volume. Enthalpy is particularly useful for studying processes occurring at constant pressure, such as chemical reactions in open systems.

In summary, while internal energy focuses solely on the energy content of the system, enthalpy accounts for both the internal energy and the work done by the system due to pressure-volume changes.

3. Explain Hess's Law with an example.

Answer: Hess's Law states that the total enthalpy change of a chemical reaction is the same regardless of the number of steps or intermediate stages the reaction goes through. This law is based on the principle of conservation of energy and allows us to calculate enthalpy changes for reactions that are difficult to measure directly.

Example: Consider the formation of carbon dioxide from graphite and oxygen:

Formation of carbon dioxide from graphite: $C_{(s)} + O_2_{(g)} \rightarrow CO_2_{(g)}$ Enthalpy change: $\Delta H_1$
Formation of carbon dioxide from carbon monoxide: $CO_{(g)} + \frac{1}{2} O_2_{(g)} \rightarrow CO_2_{(g)}$ Enthalpy change: $\Delta H_2$
Formation of carbon monoxide from graphite: $C_{(s)} + \frac{1}{2} O_2_{(g)} \rightarrow CO_{(g)}$ Enthalpy change: $\Delta H_3$

According to Hess's Law: $\Delta H_1 = \Delta H_2 + \Delta H_3$

This allows us to determine the enthalpy change for a reaction indirectly by combining the enthalpy changes of related reactions.

4. What is Gibbs Free Energy, and how does it determine the spontaneity of a reaction?

Answer: Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work that can be performed by a system at constant temperature and pressure. It is defined as:

$G = H - TS$

where $H$ is the enthalpy, $T$ is the temperature in Kelvin, and $S$ is the entropy.

Spontaneity of a Reaction: A reaction is considered spontaneous if the Gibbs Free Energy change ( $\Delta G$ ) is negative:

$\Delta G = \Delta H - T \Delta S$

If $\Delta G < 0$ , the reaction is spontaneous.
If $\Delta G > 0$ , the reaction is non-spontaneous.
If $\Delta G = 0$ , the system is at equilibrium.

In summary, Gibbs Free Energy combines the effects of enthalpy and entropy to predict whether a reaction will occur under specific conditions.

5. Derive the relationship between enthalpy change and internal energy change for a constant-pressure process.

Answer: For a constant-pressure process, the relationship between enthalpy change ( $\Delta H$ ) and internal energy change ( $\Delta U$ ) is given by:

$\Delta H = \Delta U + P \Delta V$

where $P$ is the pressure and $\Delta V$ is the change in volume of the system.

Derivation: Starting with the definition of enthalpy:

$H = U + PV$

Taking the differential form for a process:

$dH = dU + d(PV)$

Since $P$ is constant:

$d(PV) = P \, dV$

Thus:

$dH = dU + P \, dV$

For a specific process, the change in enthalpy ( $\Delta H$ ) is:

$\Delta H = \Delta U + P \Delta V$

This equation shows that the enthalpy change is the sum of the internal energy change and the work done by or on the system due to volume change.

6. What is the significance of entropy in thermodynamics?

Answer: Entropy (S) is a measure of the disorder or randomness in a system. It quantifies the number of ways in which the components of a system can be arranged while maintaining the same energy level. Entropy is a central concept in the Second Law of Thermodynamics.

Significance:

Predicting Spontaneity: Entropy helps predict the spontaneity of a process. According to the Second Law of Thermodynamics, the entropy of the universe always increases in spontaneous processes.
Direction of Processes: Entropy determines the direction of natural processes. For a spontaneous process, the total entropy of the system and surroundings must increase.
Entropy and Heat: Entropy changes are related to heat transfer. For a reversible process, the change in entropy is given by:

$\Delta S = \frac{Q_{rev}}{T}$

where $Q_{rev}$ is the heat added or removed reversibly and $T$ is the temperature.

Entropy and Energy: Entropy is also related to the distribution of energy within a system. Higher entropy means a higher number of possible energy states.

Conclusion

Chapter 4 of the Class 12 NCERT Chemistry textbook, "Chemical Thermodynamics," covers essential concepts and principles that are foundational for understanding energy changes in chemical reactions. By focusing on important questions such as the First Law of Thermodynamics, enthalpy, Hess's Law, Gibbs Free Energy, and entropy, students can deepen their understanding of these fundamental topics.

Mastering these concepts not only aids in solving exam questions but also provides a solid foundation for advanced studies in chemistry and related fields. Regular practice and a clear grasp of these principles will help students excel in their exams and develop a strong understanding of chemical thermodynamics.

For further study, students are encouraged to solve additional problems and refer to the NCERT textbook for a more detailed exploration of the topics discussed in this article.

Chemistry Class 12 NCERT Solutions Chapter 4 Chemical Thermodynamics – Important Questions