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Physical Chemistry 36

The heat capacity at constant pressure, C p and at constant
volume, C v , are approximately equal for solids and liquids, but
the difference for gases is given by C p =C v +nR.
Related topics Enthalpy (B2) Entropy and change (B5)
Thermochemistry (B3) Free energy (B6)
Entropy (B4) Statistical thermodynamics (G8)

Thermodynamics

Thermodynamics is a macroscopic science, and at its most fundamental level, is the
study of two physical quantities, energy and entropy. Energy may be regarded as the
capacity to do work, whilst entropy (see Topics B4 and G8) may be regarded as a
measure of the disorder of a system. Thermodynamics is particularly concerned with the
interconversion of energy as heat and work. In the chemical context, the relationships
between these properties may be regarded as the driving forces behind chemical
reactions. Since energy is either released or taken in by all chemical and biochemical
processes, thermodynamics enables the prediction of whether a reaction may occur or not
without need to consider the nature of matter itself. However, there are limitations to the
practical scope of thermodynamics which should be borne in mind. Consideration of the
energetics of a reaction is only one part of the story. Although hydrogen and oxygen will
react to release a great deal of energy under the correct conditions, both gases can coexist
indefinitely without reaction. Thermodynamics determines the potential for chemical
change, not the rate of chemical change—that is the domain of chemical kinetics (see
Topics F1 to F6). Furthermore, because it is such a common (and confusing)
misconception that the potential for change depends upon the release of energy, it should
also be noted that it is not energy, but entropy which is the final arbiter of chemical
change (see Topic B5).
Thermodynamics considers the relationship between the system—the reaction,
process or organism under study—and the surroundings—the rest of the universe. It is
often sufficient to regard the immediate vicinity of the system (such as a water bath, or at
worst, the laboratory) as the surroundings.
Several possible arrangements may exist between the system and the surroundings
(Fig. 1). In an open system, matter and energy may be interchanged between the system
and the surroundings. In a closed system, energy may be exchanged between the
surroundings and the system, but the amount of matter in the system remains constant. In
an isolated system, neither matter nor energy may be exchanged with the surroundings.
A system which is held at constant temperature is referred to as isothermal, whilst an
adiabatic system is one in which energy may be transferred as work, but not as heat, i.e.
it is thermally insulated from its surroundings. Chemical and biological studies are
primarily concerned with closed isothermal systems, since most processes take place at
constant temperature, and it is almost always possible to design experiments which
prevent loss of matter from the system under study.