Some things happen naturally; some things don’t. 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 aspect of the world determines
the spontaneous direction of change, the direction of change that does not
require work to bring it about. 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). However, none of these processes is spontaneous; each
one must be brought about by doing work. 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. 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.
The recognition of two classes of process, spontaneous and non-spontaneous, is
summarized by the Second Law of thermodynamics. This law may be expressed in
a variety of equivalent ways. One statement was formulated by Kelvin:
No process is possible in which the sole result is the absorption of heat from a reservoir
and its complete conversion into work.
For example, it has proved impossible to construct an engine like that shown in
Fig. 3.1, in which heat is drawn from a hot reservoir and completely converted into
work. All real heat engines have both a hot source and a cold sink; some energy is
always discarded into the cold sink as heat and not converted into work