## Irreversible vs reversible

Rebecca Doan 2L
Posts: 51
Joined: Thu Jul 27, 2017 3:01 am

### Irreversible vs reversible

Can someone please explain to me the difference between irreversible and reversible processes? I am still a little confused on the concept part of it.

Ethan-Van To Dis2L
Posts: 50
Joined: Fri Sep 29, 2017 7:05 am

### Re: Irreversible vs reversible

I like to think of it like an equilibrium problem. A reversible reaction is kinda like a reaction that is at equilibrium. By adding/removing a reactant it will shift the reaction. By adding/removing pressure, the piston will shift. An irreversible reaction is one where the K value is really high so the reaction never really shifts. By adding/removing pressure, the piston will not shift.

Jessica Patzlaff 1A
Posts: 28
Joined: Fri Sep 29, 2017 7:05 am

### Re: Irreversible vs reversible

A reversible reaction is one that can take place in either direction, meaning that it is a process that can be reversed by an infinitely small, or "infinitesimal" change in a variable. If the external or internal pressure increases infinitesimally, then the piston will move in or out respectively. However, if the difference in external and internal pressures is a finite and relatively large amount, then it becomes an irreversible process because an infinitesimal change will not change whether the system compresses or expands. For example, if the pressure of a system is 3 atm and the external pressure is 1 atm, then the gas will expand regardless of infinitesimal increases in the external pressure, making it an irreversible expansion.

Michele 2C
Posts: 23
Joined: Fri Sep 29, 2017 7:06 am

### Re: Irreversible vs reversible

Consider a gas expanding inside a piston. Assume the external pressure is equal to the internal pressure. Let's say the goal is to expand the volume of the gas by 1 L. Theoretically, in a reversible process, you would reduce the external pressure by an infinitesimally small amount. The internal pressure would then also decrease by that infinitesimally small amount and since pressure and volume are inversely related, the volume would increase by an infinitesimally small amount. Theoretically, you would repeat this process over and over again until the volume of the gas inside the piston increased by 1 L. That sounds like it would take forever, which it would. The significance of reversible processes is that the maximum amount of work that the system could have exerted to expand by 1 L, it exerts through the reversible process (as opposed to an irreversible one). I expand on this idea in my answer to a similar question here: viewtopic.php?f=129&t=25926&sid=7fa473b6ed6500e207e6f9847c382fa2#p78232

Reversible processes are called 'reversible' because if instead of reducing external pressure by an extremely small amount, let's say you increased external pressure by an extremely small amount, the volume of gas inside the piston would decrease. You have simply undone what you were trying to do above. Due to the fact that you're able to "undo" the process, it is called reversible.

Irreversible processes do not have the luxury of being able to "undo" the process. Let's say the pressure of the gas inside the piston is 1 atm and the external pressure is 3 atm. Since the external pressure is greater than the internal pressure, the volume will decrease. Now, say you want to undo that. If you decreased the external pressure by an extremely small amount (infinitely smaller than 1 atm), you would NOT be able to undo the process. Ultimately, if you change the external pressure by a measurable amount, the process is irreversible and a small pressure change will have no effect on getting the volume to be what it was originally. Most real life processes are irreversible.

I hope this helps!