The Foundations of Chemistry

(decrease in disorder), then
Sunivmay still be positive (overall increase in disorder) if

Ssurris more positive than
Ssysis negative. A refrigerator provides an illustration. It
removes heat from inside the box (the system) and ejects that heat, plusthe heat gener-
ated by the compressor, into the room (the surroundings). The entropy of the system
decreases because the air molecules inside the box move more slowly. The increase in the
entropy of the surroundings more than makes up for that, however, so the entropy of the
universe (refrigeratorroom) increases.
Similarly, if
Ssysis positive but
Ssurris even more negative, then
Sunivis still nega-
tive. Such a process will be nonspontaneous.
Let’s consider the entropy changes that occur when a liquid solidifies at a temperature
belowits freezing (melting) point (Figure 15-13a).
Ssysis negative because a solid forms
from its liquid, yet we know that this is a spontaneous process. A liquid releases heat to
its surroundings (atmosphere) as it crystallizes. The released heat increases the motion
(disorder) of the molecules of the surroundings, so
Ssurris positive. As the temperature
decreases, the
Ssurrcontribution becomes more important. When the temperature is low
enough (below the freezing point), the positive
Ssurroutweighs the negative
Ssys. Then

Sunivbecomes positive, and the freezing process becomes spontaneous.
The situation is reversed when a liquid is boiled or a solid is melted (Figure 15-13b).
For example, at temperatures above its melting point, a solid spontaneously melts, and

Ssysis positive. The heat absorbed when the solid (system) melts comes from its surround-
ings. This decreases the motion of the molecules of the surroundings. Thus,
Ssurris
negative (the surroundings become less disordered). The positive
Ssysis greater in magni-
tude than the negative
Ssurr, however, so
Sunivis positive and the process is spontaneous.
Above the melting point,
Sunivis positive for melting. Below the melting point,
Suniv
is positive for freezing. At the melting point,
Ssurris equal in magnitude and opposite in
sign to
Ssys. Then
Sunivis zero for both melting and freezing; the system is at equilib-
rium.Table 15-4 lists the entropy effects for these changes of physical state.
We have said that
Sunivis positive for all spontaneous (product-favored) processes.
Unfortunately, it is not possible to make direct measurements of
Suniv. Consequently,
entropy changes accompanying physical and chemical changes are reported in terms of

Ssys. The subscript “sys” for system is usually omitted. The symbol
Srefers to the
change in enthalpy of the reacting system, just as
Hrefers to the change in enthalpy of
the reacting system.

Can you develop a comparable table

for boiling (liquid n gas) and

condensation (gas n liquid)?

(Study Table 15-4 carefully.)

15-14 Entropy, S 623

We abbreviate these subscripts

as follows: systemsys,

surroundingssurr, and

universeuniv.

Figure 15-13 A schematic representation of heat flow and entropy changes for (a) freezing
and (b) melting of a pure substance.

Heat

SURROUNDINGS

SYSTEM

Ssys ↓ so

∆Ssys < 0

Heat

Ssurr ↑ so ∆Ssurr > 0

Heat

SURROUNDINGS

SYSTEM

Ssys ↑ so

∆Ssys > 0

Heat

Ssurr ↓ so ∆Ssurr < 0

(a) Freezing below mp (b) Melting above mp