Thermodynamics - reversibility and heat addition

[Haftred]

I understand entropy is a state function, insofar as we deny the existence of irreversible cycles. However, for a said change of state, the heat transferred as a result of a reversible change is greater than that for an irreversible change. This is simply a reiteration of the Clausius inequality, as because entropy is a state function, a change dS is greater for a dq/T if dq is irreversible. However, it seems to me that it does not logically make sense for more heat having to be added for a reversible change. What is it about reversibility that requires more heat to be added to change states? I understand that entropy works out if the former statement is true; however, I guess I just don't understand how logically more heat is needed to effect a reversible change of state rather than an irreversible one. Where does the extra heat go if the final states are the same?

- Thanks

 

[Chestermiller]

You pretty much have it backwards. During an irreversible change, there is entropy generated within the system so, to get between the same two ends states with a reversible process, you need to compensate for the entropy which is not generated within the system during the reversible process by transferring additional heat into the system.