Now let's consider the two reversible paths showed in figure 0.
2.
From thermodynamics it is well known that the energy of such a state can be determined if a reversible path to that state is found.
3.
To find the entropy difference between any two states of a system, the integral must be evaluated for some reversible path between the initial and final states.
4.
This derivation is not entirely rigorous, since it requires a reversible path through a region of thermodynamic instability, while " b " is unstable.
5.
Since entropy is a state function, the entropy change of the system for an irreversible path is the same as for a reversible path between the same two states.