If Z is a state function then the balance of Z remains unchanged during a cyclic process:
Entropy is a state function and is defined in an absolute sense through the Third Law of Thermodynamics as
where a reversible path is chosen from absolute zero to the final state, so that for an isothermal reversible process
In general, for any cyclic process the state points can be connected by reversible paths, so that
meaning that the net entropy change of the working fluid over a cycle is zero.