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this chapter. Engineers are usually concerned with the changes in entropy.
Therefore, the entropy of a substance can be assigned a zero value at some arbitrarily selected reference state, and the entropy values at other states can be
determined from Eq. 7–5 by choosing state 1 to be the reference state (S 0)
and state 2 to be the state at which entropy is to be determined.
To perform the integration in Eq. 7–5, one needs to know the relation between Q and T during a process. This relation is often not available, and the
integral in Eq. 7–5 can be performed for a few cases only. For the majority of
cases we have to rely on tabulated data for entropy.
Note that entropy is a property, and like all other properties, it has fixed
values at fixed states. Therefore, the entropy change S between two specified
states is the same no matter what path, reversible or irreversible, is followed
during a process (Fig. 7–3).
Also note that the integral of dQ/T will give us the value of entropy change
only if the integration is carried out along an internally reversible...
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 Spring '09

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