68. When ice absorbs latent heat at a constant pressure to melt from solid to liquid state at
constant temperature, which of the following changes are happening?
A. Entropy of the system is increasing.
B. 𝐶𝑝 (heat capacity at constant pressure) of the system is infinite or undefined.
C. Enthalpy of the system is increasing.
D. 𝐶𝑝 of the system is zero.
Choose the correct answer:
A. A, B, C
B. A, C, D
C. A, B
D. C, D

Introduction:

The process of melting ice involves several thermodynamic changes, particularly when the ice absorbs latent heat at a constant pressure to transition from a solid to a liquid state. This phase change occurs at a constant temperature, which means it has specific implications for the system’s entropy, enthalpy, and heat capacity. Understanding these changes helps in analyzing the behavior of substances during phase transitions.

Thermodynamic Changes When Ice Melts:

When ice melts, it undergoes a phase transition where it absorbs latent heat without changing its temperature. This raises several thermodynamic questions, especially related to entropy, heat capacity, and enthalpy. Let’s explore the changes that happen during this process:

1. A. Entropy of the system is increasing:

   • Correct. When ice melts, the molecules move from a more ordered solid structure to a less ordered liquid state. This increase in disorder leads to an increase in entropy, which is a measure of the randomness or disorder of a system.

2. B. Cp (heat capacity at constant pressure) of the system is infinite or undefined:

   • Correct. During the phase change, the temperature remains constant, but heat is still being absorbed by the system (latent heat). In this case, the heat capacity at constant pressure becomes undefined or theoretically infinite, because the system is not experiencing a temperature change despite the heat absorption.

3. C. Enthalpy of the system is increasing:

   • Correct. Enthalpy is a measure of the total energy in a system. As ice absorbs latent heat to melt, the enthalpy of the system increases, because the energy required for the phase change is being added to the system.

4. D. Cp of the system is zero:

   • Incorrect. The heat capacity at constant pressure (Cp) is not zero during melting. While it becomes infinite or undefined at the melting point, it is not zero. A zero heat capacity would imply that the system requires no heat to change temperature, which is not the case during a phase transition.

Conclusion:

During the melting of ice, the following thermodynamic changes occur:

• Entropy of the system increases because the ice molecules move to a more disordered state as they transition from solid to liquid.

• Heat capacity at constant pressure (Cp) becomes infinite or undefined because the temperature does not change during the phase transition, despite the absorption of heat.

• Enthalpy increases as the system absorbs latent heat.

Thus, the correct answer is: A. A, B, C