Efficiency of reversible, equilibrium expansion

[Danielle_Cho_3A]

Why is a system whose pressure changes in an infinite amount of small steps (reversible, equilibrium expansion) more efficient than an irreversible expansion?

[Chem_Mod]

Hi Danielle,

I'm not sure I understand your question. Could you please specify?

[Vishal Reddy Kunam]

Hi, Danielle. I believe that a system whose pressure changes in an infinite amount of small steps-- a reversible, equilibrium expansion-- is more efficient than an irreversible expansion because of the behavior of temperature and pressure. In a reversible system, isothermality is retained; this means that the temperature of the system and the temperature of the surroundings are equal. If you pair this with the fact that the external pressure of a reversible pathway is NOT constant, it's natural to deduce that the gas of the system is constantly putting out work against the piston pushing against. As work is done, heat is lost by the system, but it is continually renewed as heat from the surroundings enters again and allows more work to be done. So, while this is a much SLOWER process, it is more efficient in the amount of work that can be done. On the other hand, in an irreversible pathway, the external pressure IS constant and the piston is hypothetically held in place by "imaginary" pins." The second these pins are removed, the gas will cause the piston to quickly rise as a burst of work is used and heat is lost; this causes the system to temporarily cool until heat from the surroundings enter in a delayed fashion and allows work to be done again. So, an irreversible pathway is technically faster, but much more inefficient in the supply of heat and thus work.