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Summary of week 5

This week you looked at the free energy functions.

You can determine the reversibility of the process by looking at the total entropy change. However, information on the surrounding is need to estimate the total entropy and that’s not very convenient thermodynamically. The free energy, such as Gibbs free energy and Helmholtz free energy, is the system property and with some restriction, their sign can be the criteria for the reversibility equivalent to the total entropy change.

The free energies are “free energy”. Thermodynamic free energy is the amount of work that a system can perform. They are energies freely available for us to use! The Helmholtz free energy is the maximum work that can be obtained, and the Gibbs free energy is the maximum non-expansion work.

With these two free energies, we have four fundamental functions: internal energy U, enthalpy H, Helmholtz free energy F, and Gibbs free energy G. They are “potential energy” defined as capacity to do work. Starting from the first and second laws of thermodynamics, we derive expressions for the differential form of four thermodynamic potentials. They are called fundamental equations.

Thermodynamic properties such as temperature, pressure, volume and entropy are related with each other. Their mutual relations are called property relations or Maxwell relations, and the equations showing property relations are derived from the differential form of thermodynamic potentials.

In the final week, you will look at the power of property relations in deriving various property changes upon other parameter changes. The property relations can convert non-simple property changes into more familiar ones with some mathematical manipulation. The versatility of property relations is demonstrated in various situations.

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This article is from the free online course:

Thermodynamics in Energy Engineering

Hanyang University

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