Page 38 - Basic physical chemistry for the atmospheric sciences

24 Basic physical chemistry

where a is the specific volume of the system. On the other hand, for
s
an irreversible expansion between the same two state , Psurroundings <
Psystem. Therefore,

Hence,
(2. 1 8 )

Applying the first law of thermodynamics in the form of Eq. (2. 1 ) to
the reversible transformation

Similarly, for the irreversible transformation

q irrcv - Wirrev = U2 - U I
Therefore,

or,

Since, from Eq. (2. 1 8), wrev - wirrev > 0, it follows that

or,
(2. 1 9 )

If a system receives heat dqirre v at temperature T during an irrevers­
ible transformation, the change in the entropy of the system is not
equal to dqirreJT. Also, for an irreversible transformation there is no
simple relationship between the change in the entropy of the system
and the change in the entropy of its surroundings. H o wever, the
remaining part of the second law of thermodynamics states that the
entropy o f the universe increases as a result o f irreversible transf o r­
mations.
The two parts of the second law of thermodynamics stated above
w
can be summarized as follo s :