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Unformatted text preview: energy). Therefore, the entropy of a pure crystalline substance
at absolute zero temperature is zero since there is no uncertainty about the
state of the molecules at that instant (Fig. 7–25). This statement is known as
the third law of thermodynamics. The third law of thermodynamics provides an absolute reference point for the determination of entropy. The entropy determined relative to this point is called absolute entropy, and it is
extremely useful in the thermodynamic analysis of chemical reactions. Notice
that the entropy of a substance that is not pure crystalline (such as a solid solution) is not zero at absolute zero temperature. This is because more than one
molecular configuration exists for such substances, which introduces some
uncertainty about the microscopic state of the substance.
The concept of entropy as a measure of disorganized energy can also be applied to other areas. Iron molecules, for example, create a magnetic field
around themselves. In ordinary iron, molecules are randomly aligned, and
they cancel each other ’s magnetic effect. When iro...
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- Spring '09