8: Entropy and Free Energy
Among the many capabilities of chemistry is its ability to predict if a process will occur under specified conditions. Thermodynamics, the study of relationships between the energy and work associated with chemical and physical processes, provides this predictive ability. Previous chapters in this text have described various applications of thermochemistry, an important aspect of thermodynamics concerned with the heat flow accompanying chemical reactions and phase transitions. This chapter will introduce additional thermodynamic concepts, including those that enable the prediction of any chemical or physical changes under a given set of conditions.
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- 8.1: Spontaneity
- Chemical and physical processes have a natural tendency to occur in one direction under certain conditions. A spontaneous process occurs without the need for a continual input of energy from some external source, while a nonspontaneous process requires such. Systems undergoing a spontaneous process may or may not experience a gain or loss of energy, but they will experience a change in the way matter and/or energy is distributed within the system.
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- 8.2: Entropy
- Entropy (S) is a state function that can be related to the number of microstates for a system (the number of ways the system can be arranged) and to the ratio of reversible heat to Kelvin temperature. It may be interpreted as a measure of the dispersal or distribution of matter and/or energy in a system, and it is often described as representing the “disorder” of the system. For a given substance, Ssolid<Sliquid<SgasSsolid<Sliquid<SgasS_{solid} < S_{liquid} < S_{gas} in a given physical state at
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- 8.3: The Second and Third Laws of Thermodynamics
- The second law of thermodynamics states spontaneous processes increases the entropy of the universe. If a process would decrease the entropy of the universe, then the process is nonspontaneous, and if no change occurs, the system is at equilibrium. The third law of thermodynamics establishes the zero for entropy at 0 J/Kelvin for a perfect, pure crystalline solid at 0 K with only one possible microstate.
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- 8.4: Gibbs Free Energy
- Gibbs free energy (G) is a state function defined with regard to system quantities only and may be used to predict the spontaneity of a process. A negative value for ΔG indicates that the reaction will proceed in the forward direction to reach equilibrium; a positive ΔG indicates that the reaction will proceed in the reverse direction to reach equilibrium; and a ΔG of zero indicates that the system is at equilibrium. A number of approaches to the computation of free energy changes are possible.