Burdge/Overby, Chemistry: Atoms First, 2e Ch14 | Page 18
SEC TION 14.4 ? Entropy Changes in the Universe
587
Figure 14.5 Entropy increases in a
substance as temperature increases from
absolute zero.
S (J/K ? mol)
Boiling
(?Svap )
Gas
Liquid
Melting
(?Sfus )
Solid
Temperature (K)
The entropy values arrived at in this way are called absolute entropies because they are true
v
alues—unlike standard enthalpies of formation, which are derived using an arbitrary reference.
Because the tabulated values are determined at 1 atm, we usually refer to absolute entropies as
standard entropies, S°. Figure 14.5 shows the increase in entropy of a substance as temperature
increases from absolute zero. At 0 K, it has a zero entropy value (assuming that it is a perfect
crystalline substance). As it is heated, its entropy increases gradually at first because of greater
molecular motion within the crystal. At the melting point, there is a large increase in entropy as
the solid is transformed into the liquid. Further heating increases the entropy of the liquid again
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