McKay, Donald. "Front matter" Multimedia Environmental Models ...

likely using an equilibrium criterion and the 10 equations relating it to concentration,
one for each phase.
It is useful to discriminate between partition coefficients and distribution coefficients.
Although usage varies, a partition coefficient is strictly the ratio of the
concentrations of the same chemical species in two phases. A distribution coefficient
is a ratio of total concentrations of all species. Thus, if a chemical ionizes, the
partition coefficient may apply to the unionized species, while the distribution
coefficient applies to ionized and nonionized species in total.
5.1.2 Some Thermodynamic Fundamentals
There are four laws of thermodynamics. They are numbered 0, 1, 2, and 3,
because the need for the zeroth was not realized until after the first was postulated.
Although these laws cannot be proved mathematically, they are now universally
accepted as true, or axiomatic, because they are supported by all available experimental
evidence. On consideration, they are intuitively reasonable, and it now seems
inconceivable that they are ever disobeyed.
The zeroth law introduces the concept of temperature as a criterion of thermal
equilibrium by stating that, when bodies are at thermal equilibrium, i.e., there is no
net heat flow in either direction, their temperatures are equal.
The first law was discovered largely as a result of careful experiments by Joule,
and it establishes the concept of energy and its conservation. Energy takes several
forms—potential, kinetic, heat, chemical, electrical, nuclear, and electromagnetic.
There are fixed conversion rates among these forms. Furthermore, energy can neither
be formed nor destroyed; it merely changes its form. Of particular importance are
conversions between thermal energy (heat) and mechanical energy (work).
The second law is intellectually more demanding and introduces the concept of
entropy and a series of useful related properties, including chemical potential and
fugacity. It is observed that, whereas there are fixed exchange rates between heat and
work energy, it is not always possible to effect the change. The conversion of mechanical
energy to heat (as in an automobile brake) is always easy, but the reverse process
of converting heat to mechanical energy (as in a thermal power station) proves to be
more difficult. If a quantity of heat is available at high temperature, then only a fraction
of it, perhaps one third, can be converted into mechanical energy. The remainder is
rejected as heat, but at a lower temperature. Most thermodynamics texts introduce
hypothetical processes such as the Carnot cycle at this stage to illustrate these conversions.
After some manipulation, it can be shown that there is a property of a system,
called its entropy, that controls these conversions. Apparently, regardless of how it is
arranged to convert heat to work, the overall entropy of the system cannot decrease.
It must increase by what is termed an irreversible process, or in the limit, it could
remain constant by what is called a reversible process. Although there may be a local
entropy decrease, this must be offset by another and greater entropy increase elsewhere.
Clausius summarized this law in the statement that the “entropy of the universe
increases.” It can be shown that entropy is related to randomness or probability. An
increase in entropy corresponds to a change to a more random or disordered or
probable condition. The third law is not important for our immediate purposes.
©2001 CRC Press LLC