Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 3

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CHAPTE R 14? Entropy and Free Energy

Based on the first two examples, and many others like them, we might conclude that exothermic processes tend to be spontaneous and, indeed, a negative ?H does favor spontaneity. The last
two examples, however, make it clear that the sign of ?H alone is insufficient to predict spontaneity in every circumstance. For the remainder of this chapter, we will examine the two factors that
determine whether or not a process is spontaneous under a given set of conditions.

14.2 ?Entropy
To predict the spontaneity of a chemical or physical process, we need to know both the change in
enthalpy [9 Section 10.3] and the change in entropy associated with the process. We first encountered the concept of entropy in our discussion of solution formation [9 Section 13.2]. We will now
look in more detail at what entropy is, and why it matters.

A Qualitative Description of Entropy

Student Annotation: Motional energy
includes translational energy, in which
the entire molecule moves through space
[9 Section 3.1]; rotational energy, in
which the molecule spins about an axis
running through its center of mass; and
vibrational energy, in which atoms of a
molecule move relative to one another.

Qualitatively, the entropy (S) of a system is a measure of how spread out or how dispersed
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