I am trying to grasp the concepts of reversible work, irreversible work and irreversibility.(Last one is the difference between them if i am not mistaken.) Let us consider a rigid and evacuated container at volume V. Then, a valve opens and athmospheric air (P0, T0 is filling the tank. The wall of container is thin enough(allowing heat transfer), so eventually the trapped air in the container reaches thermal and mechanical equilibrium. That means air in the container also finally reaches to P0, T0. Now, is there any work? I know that gas expansion against vacuum doesn't create work, because there is nothing for you to do work on. Is that the case in here? If atmospheric air fills an evacuated container, it is also gas expansion against vacuum in my opinion. Therefore no work? But, there is entropy generation in this case. That means there is exergy destruction and hence, irreversibility. Irreversibility is the difference between reversible and irreversible work. But again, we said there is no work. Was that reversible work, or irreversible? If there is irreversibility can we say that there is no reversible work but some irreversible work. I didn't get this concept.