We have seen that the First Law of thermodynamics may be written dU= dq + dw. For
a reversible change in a closed system of constant composition, and in the absence of
any additional (non-expansion) work, we may set dwrev = −pdV and (from the definition
of entropy) dqrev = TdS, where p is the pressure of the system and T its temperature.
Therefore, for a reversible change in a closed system