Page 36 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch03 Elementary Physical Chemistry

The Second Law of Thermodynamics 21

Apparently, the First Law does not account for a large number of
observations. Evidently another Law is needed and that other Law is the
Second Law of Thermodynamics.
Actually, the Second Law is not directly concerned with questions
of spontaneity. Rather, a new concept (the entropy) is introduced, which
as a consequence of the second Law, behaves in a characteristic manner,
depending on whether the change is reversible or irreversible. This new
function, the entropy, S, is defined as
dS =dq rev /T (3.1)

or if T is constant,
S = q rev/T (3.2)

It is important to keep in mind that the simple relation between S and q is
valid only under reversible conditions. This does not mean that when the
process is not reversible, entropy does not exist but rather that under those
circumstances entropy is not simply related to q.
The entropy functions have the following characteristics:

1) S is a state function, i.e. the integral dS is independent of path. This
means that the integral of dq rev /T is also independent of path in spite
of the fact that dq rev is path-dependent.
2) In an isolated system, any transformation will result in
∆S> 0 if change is irreversible or spontaneous (3.3)
∆S = 0 if change is reversible or in equilibrium (3.4)
Since all naturally occurring phenomena are irreversible changes, the
system, if isolated, experiences an increase in entropy.
3) For a non-isolated system at constant temperature,
∆S = q rev/T if the process proceeds reversibly (3.5)
∆S> q irr /T if the process proceeds irreversibly (3.6)

3.1. Statements of the Second Law
There are several ways to express the Second Law (Laws are often
called Principles), which are based on the observation that heat cannot
spontaneously flow from a colder to a hotter body. In more fancy language,