Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Entropy is an extensive property, and thus the total entropy of a system is
equal to the sum of the entropies of the parts of the system. An isolated sys-
tem may consist of any number of subsystems (Fig. 7–6). A system and its
surroundings, for example, constitute an isolated system since both can be
enclosed by a sufficiently large arbitrary boundary across which there is no
heat, work, or mass transfer (Fig. 7–7). Therefore, a system and its sur-
roundings can be viewed as the two subsystems of an isolated system, and
the entropy change of this isolated system during a process is the sum of the
entropy changes of the system and its surroundings, which is equal to the
entropy generation since an isolated system involves no entropy transfer.
That is,

(7–11)

where the equality holds for reversible processes and the inequality for irre-
versible ones. Note that Ssurrrefers to the change in the entropy of the sur-
roundings as a result of the occurrence of the process under consideration.
Since no actual process is truly reversible, we can conclude that some
entropy is generated during a process, and therefore the entropy of the uni-
verse, which can be considered to be an isolated system, is continuously
increasing. The more irreversible a process, the larger the entropy generated
during that process. No entropy is generated during reversible processes
(Sgen0).
Entropy increase of the universe is a major concern not only to engineers
but also to philosophers, theologians, economists, and environmentalists
since entropy is viewed as a measure of the disorder (or “mixed-up-ness”)
in the universe.
The increase of entropy principle does not imply that the entropy of a sys-
tem cannot decrease. The entropy change of a system canbe negative dur-
ing a process (Fig. 7–8), but entropy generation cannot. The increase of
entropy principle can be summarized as follows:

This relation serves as a criterion in determining whether a process is
reversible, irreversible, or impossible.
Things in nature have a tendency to change until they attain a state of equi-
librium. The increase of entropy principle dictates that the entropy of an iso-
lated system increases until the entropy of the system reaches amaximum
value. At that point, the system is said to have reached an equilibrium state
since the increase of entropy principle prohibits the system from undergoing
any change of state that results in a decrease in entropy.

Some Remarks about Entropy

In light of the preceding discussions, we draw the following conclusions:
1.Processes can occur in a certaindirection only, not in anydirection.
A process must proceed in the direction that complies with the increase
of entropy principle, that is,Sgen0. A process that violates this princi-
ple is impossible. This principle often forces chemical reactions to
come to a halt before reaching completion.

Sgen •

7 0 Irreversible process

 0 Reversible process

6 0 Impossible process

Sgen¢Stotal¢Ssys
¢Ssurr 0

Chapter 7 | 337

Subsystem

1

Subsystem

3

Subsystem

2

Subsystem

N

(Isolated)

∆Stotal = ∆Si > 0

i=1

N

Σ

FIGURE 7–6

The entropy change of an isolated

system is the sum of the entropy

changes of its components, and is

never less than zero.

Surroundings

System

Q, W

Isolated system

boundary mQ = 0 = 0

W = 0

m

FIGURE 7–7

A system and its surroundings form an

isolated system.