Atkins, Cap.2

CHAPTER

The Second Law
ome things happen; some things don’t. A gas expands to fill the vessel it occupies; a gas that already fills a vessel does not suddenly contract into a smaller volume. A hot object cools to the temperature of its surroundings; a cool object does not suddenly become hotter than its surroundings. Hydrogen and oxygen combine explosively (once their ability to do so hasbeen liberated by a spark) and form water; water left standing in oceans and lakes does not gradually decompose into hydrogen and oxygen. These everyday observations suggest that changes can be divided into two classes. A spontaneous change is a change that has a tendency to occur without work having to be done to bring it about. A spontaneous change has a natural tendency to occur. A non-spontaneouschange is a change that can be brought about only by doing work. A non-spontaneous change has no natural tendency to occur. Non-spontaneous changes can be made to occur by doing work: a gas can be compressed into a smaller volume by pushing in a piston, the temperature of a cool object can be raised by forcing an electric current through a heater attached to it, and water can be decomposed by thepassage of an electric current. However, in each case we need to act in some way on the system to bring about the non-spontaneous change. There must be some feature of the world that accounts for the distinction between the two types of change. Throughout the chapter we shall use the terms “spontaneous” and “non-spontaneous” in their thermodynamic sense. That is, we use them to signify that achange does or does not have a natural tendency to occur. In thermodynamics the term spontaneous has nothing to do with speed. Some spontaneous changes are very fast, such as the precipitation reaction that occurs when solutions of sodium chloride and silver nitrate are mixed. However, some spontaneous changes are so slow that there may be no observable change even after millions of years. Forexample, although the decomposition of benzene into carbon and hydrogen is spontaneous, it does not occur at a measurable rate under normal conditions, and benzene is a common laboratory commodity with a shelf life of (in principle) millions of years. Thermodynamics deals with the tendency to change; it is silent on the rate at which that tendency is realized. We shall use the concepts introduced inthis chapter to guide our study of bioenergetics and structure in biological systems. Our discussion of energy conversion in biological cells has focused on the chemical sources of energy that sustain life. We now begin an investigation—to be continued throughout the text—of the mechanisms by which energy in the form of radiation from the Sun or ingested as oxidizable molecules is converted to workof muscle contraction, neuronal activity, biosynthesis of essential molecules, and transport of material into and out of the cell. We shall also explain a remark made in Chapter 1, that only part of the energy of biological fuels leads to work, with the rest being dissipated in the surroundings as heat. Finally, we begin to describe some of the important thermodynamic and chemical factors thatcontribute to the formation and stability of proteins and biological membranes.

2
Entropy 2.1 The direction of spontaneous change 2.2 Entropy and the Second Law 2.3 The entropy change accompanying heating 2.4 The entropy change accompanying a phase transition 2.5 Entropy changes in the surroundings 2.6 Absolute entropies and the Third Law of thermodynamics 2.7 The standard reaction entropy 2.8 Thespontaneity of chemical reactions The Gibbs energy 2.9 Focusing on the system 2.10 Spontaneity and the Gibbs energy
CASE STUDY 2.1: Life and the Second Law of thermodynamics 2.11 The Gibbs energy of assembly of proteins and biological membranes 2.12 Work and the Gibbs energy change CASE STUDY 2.2: The action of adenosine triphosphate

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Exercises

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Entropy

77
Non-spontaneous...