BioPHYSICAL chemistry

equivalent molecular picture is to replace the letters with gas molecules.
Initially, the gas molecules are confined in a balloon and then released
by puncturing the balloon. The distribution of the gas molecules is over
a larger area and more random than the original pattern, corresponding
to an increase in the entropy of the gas molecules. It is interesting to note
that living organisms are generally highly organized, with well-defined
cellular organizations corresponding to entropy-poor conditions. However,
the tendency for entropy to always increase can be seen in considering
how living organisms perform basic cellular reactions. For example, the
oxidation of glucose with six oxygen molecules leads to the production of
six molecules of carbon dioxide and six molecules of water. The increase
in the number of molecules, from seven to 12 in this case, allows more
molecular movement and disorder and hence a larger entropy.

Third law of thermodynamics

In addition to the two laws that have already been presented, there is a
third law of thermodynamics that deals with the question of what hap-
pens to objects as the temperature approaches absolute zero. In general,
as temperature is decreased, random motion due to thermal motion
is quenched. For a crystal, all of the atoms or molecules are located in
well-defined, regular arrays and hence spatial disorder is absent. The
lack of any thermal motion or spatial disorder suggests that the entropy
is zero. From a molecular viewpoint, the entropy can also be viewed as
being zero as the arrangement of molecules is uniquely defined. Thus,
for any object, the entropy approaches zero as the temperature approaches
absolute zero provided that the objects are ordered.

CHAPTER 3 SECOND LAW OF THERMODYNAMICS 53

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Entropy

increases

Boiling water Cooled water

Figure 3.5Examples of entropy changes: a cup of boiling water that cools and letters from a
sentence in the text positioned randomly.