7/23/2019 In One of the Following Sections

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In one of the following sections, a physical interpretation will be given for the thermodynamicalfunction called entropy . When approached from the point of view of statisticalmechanics, this quantity is given a real significance that is not apparent in classicalthermodynamics.It should now be mentioned that thermodynamics and statistical mechanics are onlyapplicable to problems involving equilibrium, and cannot predict the speed of a chemicalor metallurgical reaction. This latter is the special province of the kinetic theory. As a simple example of a system in equilibrium, consider a liquid metal and its equilibriumvapor in which the average number of metal atoms leaving the liquid to join the vaporequals the corresponding number traveling in the opposite direction. The concentration ofatoms in the vapor, and therefore the vapor pressure, is a constant with respect to time.Under conditions such as these, thermodynamics and statistical mechanics are able toproduce much useful information; for example, how equilibrium-vapor pressure changeswith a change in temperature. Suppose, however, that liquid metal is placed inside the bell jar of a vacuum system so that the vapor is swept away as fast as it forms. In this case, therecan be no equilibrium because atoms will leave the liquid at a much faster rate than theyreturn to it. Because the liquid-vapor system is no longer in equilibrium, thermodynamicsand statistical mechanics can no longer be used. Questions relating to how fast the metalatoms evaporate belong in the realm of the kinetic theory. Kinetic theory is thus most useful