Entropy and enthalpy9/12/2023 One method involves the use of standard enthalpies and entropies to compute standard free energy changes, Δ G°, according to the following relation. A convenient and common approach to the calculation of free energy changes for physical and chemical reactions is by use of widely available compilations of standard state thermodynamic data. Calculating Free Energy Changeįree energy is a state function, so its value depends only on the conditions of the initial and final states of the system. Similar reasoning may be applied to a nonspontaneous process, for which the free energy change represents the minimum amount of work that must be done on the system to carry out the process. In addition, the technologies used to extract work from a spontaneous process (e.g., batteries) are never 100% efficient, and so the work done by these processes is always less than the theoretical maximum. However, as noted previously in this chapter, such conditions are not realistic. Where w max w max refers to all types of work except expansion (pressure-volume) work. This new property is called the Gibbs free energy ( G) (or simply the free energy), and it is defined in terms of a system’s enthalpy and entropy as the following: An alternative approach involving a new thermodynamic property defined in terms of system properties only was introduced in the late nineteenth century by American mathematician Josiah Willard Gibbs. One of the challenges of using the second law of thermodynamics to determine if a process is spontaneous is that it requires measurements of the entropy change for the system and the entropy change for the surroundings.
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