Reversible and Irreversible Reactions

[Jessica Patzlaff 1A]

Can anyone explain further how the work done during a reversible expansion of a gas is the maximum expansion work possible?

[David Minasyan 1C]

Reversible processes are incredibly slow whereas irreversible processes are done quickly and inefficiently. If there was all the time in the world where our car engine could turn 100% of the fuel it was given into the energy that's needed to drive the car then it would be reversible and it would take more energy to do so. But in reality, most cars guzzle fuel inefficiently and only turn about 25% of it to the energy needed to run out car - which is super inefficient but much faster.

[Michelle Nguyen 2L]

Think of a piston compressing a gas inside a container; the atmosphere is providing the external pressure whereas the pressure of the system is the pressure of the contained gas. In a reversible reaction, the external pressure and pressure of the system are equal such that the slightest change in either value (an infinitesimal change) results in the piston moving and work being done, one way or another. When a gas expands and increases in volume, its pressure decreases; therefore, for expansion of a gas to be reversible, the external pressure must change along with the pressure of the system. This means that as a gas expands reversibly, it is always pushing against the maximum external pressure which it can actually push against (versus the external pressure being the same and the piston not moving or the external pressure being greater and the gas getting compressed), and so the gas is doing the maximum amount of work possible for it as it expands. In irreversible expansion, the gas is not expanding against the maximum amount of pressure it can push against; therefore, expansion to the same volume as in the reversible counterpart requires less work from the gas.