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3 The Thermodynamics of Simple Systems
Thermodynamics, literally heat motion, is one of the pillars of physical chemistry
and one of the great achievements of modern science. Classical thermodynamics is
the study of those quantities that are conserved, like energy, enthalpy, and free energy.
Statistical thermodynamics is the link between this great theoretical edifice and that
of quantum mechanics.3.1 CONSERVATION LAWS AND EXACT DIFFERENTIALS
Much of nineteenth-century thermodynamics was devoted to the discovery, definition,
and characterization of physical quantities that are conserved. The simple statement
energy is conservedis one of the ways of stating thefirst law of thermodynamics.
Conservation can be illustrated by carrying a rock up a hill and then bringing it back
down again. The potential energy of the rock in the gravitational field of the earth
increases on the way up and decreases on the way down, but it is the same at the end
of the process as it was at the beginning; it is conserved. By contrast, work is not
conserved. Carrying the rock around the circular path up and down the hill can be
easy or hard, depending on the path. Also, the work done on the way up is not the
same as the work that can be obtained by allowing the rock to drive some kind of
primitive motor as it rolls down the hill.
Work is not the only factor that has to be taken into account. The difference between
an easy path and a hard path up the hill is illustrated by the simplified model of a
mass being pushed up over the same change in height (altitude) but over two differentConcise Physical Chemistry,by Donald W. Rogers
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