# 22: Helmholtz and Gibbs Energies

- Page ID
- 11818

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- 22.1: Helmholtz Energy
- We have answered the question: what is entropy, but we still do not have a general criterion for spontaneity, just one that works in an isolated system. Let's fix that now by showing that Helmholtz energy is the indicator for spontaneity when temperature and volume are held constant.

- 22.2: Gibbs Energy Determines the Direction of Spontaneity at Constant Pressure and Temperature
- Gibbs energy is the maximum amount of non-\(PV\) work that can be extracted from a thermodynamically closed system. At constant temperature and pressure, Gibbs energy determines the direction of spontaneous processes, such as chemical reactions.

- 22.3: The Maxwell Relations
- To fully exploit the power of the state functions we need to develop some mathematical machinery by considering a number of partial derivatives.

- 22.4: The Enthalpy of an Ideal Gas is Independent of Pressure
- Ideal gases do not interact with each other (no intermolecular forces), so the enthalpy of an ideal gas is independent of pressure.

- 22.5: Thermodynamic Functions have Natural Variables
- The fundamental thermodynamic equations follow from five primary thermodynamic definitions and describe internal energy, enthalpy, Helmholtz energy, and Gibbs energy in terms of their natural variables. Here they will be presented in their differential forms.

- 22.6: The Standard State for a Gas is an Ideal Gas at 1 Bar
- Tabulated thermodynamic data are expressed in terms of standard state conditions (SSC), which for a gas, is an ideal gas at 1 bar.

- 22.7: The Gibbs-Helmholtz Equation
- The first order partial on G versus P is the volume V; this allows us to find the dependence of G on P by simply integrating over the volume V from one pressure to the other.

- 22.8: Fugacity Measures Nonideality of a Gas
- Fugacity, \(f\), is the effective pressure of a real gas. It takes into account the non-ideality of a gas in a way that allows us to calculate properties of a real gas using equations derived for an ideal gas.