I know that to calculate the entropy change in a process, I just need to calculate the entropy change in a process that has the initial and final states of the process and is reversible. I just don't understand what the actual difference between the irreversible versus the reversible process is! To clarify: I'm thinking about an isothermal reversible versus an isothermal irreversible process with the same initial and final states. Aren't both of them basically the same if I draw them on a PV diagram because then I'd just use the equation PV=constant? In that case, why would the entropy change be any different between the two? Thanks.