The recognition of two classes of process, spontaneous and non-spontaneous, is summarized by the Second Law of thermodynamics. Thermodynamics is silent on the rate at which a spontaneous change in fact occurs, and some spontaneous processes (such as the conversion of diamond to graphite) may be so slow that the tendency is never realized in practice whereas others (such as the expansion of a gas into a vacuum) are almost instantaneous. An important point, though, is that throughout this text ‘spontaneous’ must be interpreted as a natural tendency that may or may not be realized in practice. However, none of these processes is spontaneous each one must be brought about by doing work. A gas can be confined to a smaller volume, an object can be cooled by using a refrigerator, and some reactions can be driven in reverse (as in the electrolysis of water). Some aspect of the world determines the spontaneous direction of change, the direction of change that does not require work to be done to bring it about. A gas expands to fill the available volume, a hot body cools to the temperature of its surroundings, and a chemical reaction runs in one direction rather than another. Some things happen naturally some things don’t.
These expressions will prove useful later when we discuss the effect of temperature and pressure on equilibrium constants.Ĭoncentrating on the system 3.5 The Helmholtz and GibbsĮnergies Combining the First and Second Laws 3.7 The fundamental equation 3.8 Properties of the internalĮnergy 3.9 Properties of the Gibbs energyĬhecklist of key ideas Further reading Further information 3.1: The Born equation Further information 3.2: Real gases: the fugacity Discussion questions Exercises Problems
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We also see how to derive expressions for the variation of the Gibbs energy with temperature and pressure and how to formulate expressions that are valid for real gases.
Several relations of this kind can be established by making use of the fact that the Gibbs energy is a state function. As we began to see in Chapter 2, one application of thermodynamics is to find relations between properties that might not be thought to be related. The Gibbs energy also enables us to predict the maximum non-expansion work that a process can do. The chapter also introduces a major subsidiary thermodynamic property, the Gibbs energy, which lets us express the spontaneity of a process in terms of the properties of a system. We examine two simple processes and show how to define, measure, and use a property, the entropy, to discuss spontaneous changes quantitatively. The Second Law The purpose of this chapter is to explain the origin of the spontaneity of physical and chemical change. 3 The direction of spontaneous change 3.1 The dispersal of energy 3.2 Entropy I3.1 Impact on engineering:Īccompanying specific processes 3.4 The Third Law of