5.1

## Hanyang University

Skip to 0 minutes and 16 secondsThis week of thermodynamics in energy engineering, we are going to be talking about free energy functions. During week 3 and 4, the criteria for reversibility was stated by the 2nd law. According to the 2nd law, the sign of total entropy change of universe is the criteria for the reversibility. However, the total entropy change is not the system property. It is the sum of entropy change of the system and surrounding. So we need information on surrounding. That’s not very convenient thermodynamically. We need some system properties that can determine the reversibility. The free energies are the system property. Under a certain limited condition, their sign can be the criteria for reversibility equivalent to the total entropy change.

Skip to 1 minute and 7 secondsWe are going to be looking at two important free energies, the Helmholtz free energy and the Gibbs free energy. Then we are going to be talking about the physical meaning of free energies to see why they are called the “free energy”. With the introduction of two free energies, we have four thermodynamic potentials. Internal energy, enthalpy, Helmholtz free energy and Gibbs free energy. Mathematical manipulation of these energy equations gives us one of the most essential equations called Maxwell relations or property relations. We are going to derive the Maxwell relations this week.

# The week ahead

Welcome to the 5th week of the course. This week, we focus our attention to the free energy functions.

With free energies, we can determine the reversibility of the process only with the system properties. We will be looking at two important free energies, the Helmholtz free energy and the Gibbs free energy. Then we are going to be talking about the physical meaning of free energies to see why they are called the ‘free energy’.