Reversibility by Kent W. Mayhew
As previously stated: A reversible process can equally proceed forwards, or backwards. Another way of viewing this: Reversibility is an idealistic concept for a systemís state, wherein it can be changed, and then readily return to its original state. Conversely, an irreversible process is a process that cannot be readily returned to its original state without an input of resources, such as energy.
Illusional Reversibility in Boiling
Consider a heated vaporizing liquid below a piston-cylinder as shown in Fig 1. Herein the expanding gas in the piston-cylinders does irreversible lost work onto its surrounding atmosphere i.e. (PdV)atm . The energy in (Qin) is again defined by the following first law based equation:
Qin =dEsys+(PdV)atm (1)
As more molecules enter the elevated vaporous state from the liquid state, the system expands isothermally and isobarically (dP=dT=0. This is a case of expansion due to mass transfer from the liquid into the vaporous state! Note: In order for the system to expand it was actually infinitesimally hotter with an infinitesimal higher pressure than its surroundings. Also note: Eqn 1 assumes that the expanded system is insulated so that there is no heat lost into the surroundings during expansion. And the system must expand quasi-statically in order for dP=dT=0, to hold at all times.
Next consider that the heat into the system is turned off and that the insulation is now removed as shown in Fig 2. Heat now slowly (infinitesimally) leaves the expanded system through its walls as vaporous molecules start condensing. Since the work done onto the atmosphere [(PdV)atm] was lost, it cannot return. Hence, the process is irreversible!
I.e. the system cools down infinitesimally as the heat that leaves the system, and dissipates into the surrounding atmosphere. This dissipating heat is defined by
Qout = -dEsys (2)
Even if we could magically collect and then measure the thermal energy leaving the system, one would still determine that the energy out in Fig 2 is less than the energy was in the boiling process i.e. in Fig 1. I.e. the magnitude of eqn (2) is less than eqn (1) by the amount of lost work [(PdV)atm]. It now becomes obvious that the heating process resulting in expansion was an irreversible process! This fact may elude the casual observer who just witnessed the expanded systemís return to its pre heating state. Note: Herein the casual observer has not measured the heat in (Qin) vs heat out (Qout), i.e. in other words he/she simply wrongly assumed that the process was reversible without actually making complete proper and full measurements.
It should be emphasized that the above mistake also occurs when we traditionally think that the magnitude of the latent heat of condensation equals the magnitude of the latent heat of vaporization. This is wrong! Of course the latent heat of vaporization can readily be measured in an isobaric calorimeter however measurteing the latent heat of condensation is not so easily done!
See Latent Heat