BioPHYSICAL chemistry

Reversible process: (3.12)

Irreversible process:

Thus, the entropy change for the irreversible process is found to be larger
than that of the reversible process.

The second law of thermodynamics

In general terms, the second law of thermodynamics deals with the con-
cept that energy and matter tend to be dispersed. With the introduction
of the concept of entropy, which provides a measure of the order of a
system, the second law can be expressed in the following way.

The second law of thermodynamics states that the entropy of an isolated

system tends to increase with time.

This general expression for the second law of thermodynamics can now be
stated quantitatively using the expressions for entropy in terms of revers-
ible and irreversible processes. For a reversible process, the entropy change
is equal to the heat divided by the temperature:

(3.13)

The entropic changes for a spontaneous, or irreversible, process can be
stated by the Clausius inequalityto be always greater than the heat divided
by the temperature:

(3.14)

Thus, the second law of thermodynamics can be expressed more precisely
as follows.

The second law states that the entropy of an isolated system increases

in the course of a spontaneous change.

Thermodynamically, irreversible processes, such as the free expansion of
a gas, will always be accompanied by an increase in the entropy. Thus,
the total entropy change can be used to determine whether a process is
spontaneous or not. A positive value for the entropy change ΔSmeans

ΔS

q

T

>

ΔS

q

T

=

ΔS

q

T

w

T

==−= 05. nRT

ΔS

q

T

w

T

==−=0 693. nRT

CHAPTER 3 SECOND LAW OF THERMODYNAMICS 51