Because changes in the thermodynamic quantity entropymay be understood in terms
of changes in molecular disorder,we can often predict the sign of Ssys. The following illus-
trations emphasize several common types of processes that result in predictable entropy
changes for the system.1.Phase changes.When melting occurs, the molecules or ions are taken from their quite
ordered crystalline arrangement to a more disordered one in which they are able to
move past one another in the liquid. Thus, a melting process is always accompa-
nied by an entropy increase (Ssys 0). Likewise, vaporization and sublimation both
take place with large increases in disorder, and hence with increases in entropy. For
the reverse processes of freezing, condensation, and deposition, entropy decreases
because order increases.
2.Temperature changes— for example, warming a gas from 25°C to 50°C. As any sample
is warmed, the molecules undergo more (random) motion; hence entropy increases
(Ssys 0) as temperature increases. Likewise, as we raise the temperature of a solid,
the particles vibrate more vigorously about their positions in the crystal, so that at
any instant there is a larger average displacement from their mean positions; this
results in an increase in entropy.
3.Volume changes.When the volume of a sample of gas increases, the molecules can
occupy more positions, and hence are more randomly arranged than when they are
closer together in a smaller volume. Hence, an expansion is accompanied by an
increase in entropy (Ssys 0). Conversely, as a sample is compressed, the mole-
cules are more restricted in their locations, and a situation of greater order (lower
entropy) results.
4.Mixing of substances,even without chemical reaction. Situations in which the mole-
cules are more “mixed up” are more disordered, and hence are at higher entropy.
We pointed out that the mixed gases of Figure 15-12b were more disordered than
the separated gases of Figure 15-12a, and that the former was a situation of higher
entropy. We see that mixing of gases by diffusion is a process for which Ssys 0;
we know from experience that it is always spontaneous. We have already pointedThe vaporization of bromine,
Br 2 () nBr 2 (g) (left) and the
sublimation of iodine, I 2 (s) nI 2 (g)
(right) both lead to an increase in
disorder, so Ssys 0 for each
process. Which do you think results
in the more positive S? Carry out
the calculation using values from
Appendix K to check whether your
prediction was correct.
626 CHAPTER 15: Chemical Thermodynamics