ANS:C - entropy vs. enthalpy

The Mollier diagram, also known as the h-s diagram, is a plot of specific enthalpy (h) versus specific entropy (s). So, the correct description is: Entropy vs. Enthalpy. Entropy (𝑆S) and enthalpy (𝐻H) are two important thermodynamic properties that describe the state of a system.

1. Enthalpy (𝐻H): Enthalpy is a measure of the total energy of a system, including its internal energy and the energy associated with the system's pressure and volume. Mathematically, it is defined as: 𝐻=𝑈+𝑃𝑉H=U+PV Where:
   • 𝑈U is the internal energy of the system.
   • 𝑃P is the pressure.
   • 𝑉V is the volume.
   Enthalpy is often useful in analyzing processes where there is heat transfer at constant pressure, such as in chemical reactions and phase changes.
2. Entropy (𝑆S): Entropy is a measure of the disorder or randomness of a system. It represents the amount of energy in a system that is unavailable for doing work. Entropy is a state function, meaning it depends only on the current state of the system and not on the path taken to reach that state. Mathematically, entropy is defined as: 𝑆=𝑑𝑄𝑇S=TdQ​ Where:
   • 𝑑𝑄dQ is the infinitesimal amount of heat added to or removed from the system.
   • 𝑇T is the temperature at which the heat transfer occurs.
   Entropy tends to increase in isolated systems over time, reflecting the tendency of systems to move towards higher disorder.

In the entropy vs. enthalpy (h-s) diagram, entropy is plotted on the vertical axis (usually increasing upwards) and enthalpy is plotted on the horizontal axis. This diagram is particularly useful in analyzing thermodynamic processes, especially for steam and refrigeration cycles. It allows engineers to visualize and analyze processes involving changes in both enthalpy and entropy, such as heat transfer, phase changes, and power generation.