# Difference between reversible and irreversible processes

On a quiz, I was asked for the difference between reversible and irreversible processes.

I said:

1. For a reversible process, you can change direction at any time without disturbing equilibrium.

2. For an irreversible process, the macroscopic variables may not always be defined.

I was given 1/4 point for this answer. I asked the professor what the issue is with the answer, and she could not articulate a clear reason, and eventually said she'll re-think it.

Do you see a problem with my answer? If so, please explain.

• Regarding item 2 in Answer 1, in most reversible processes (except those that are adiabatic) there will also be an entropy change. (I couldn't comment because I didn't have enough reputation points.) Sep 30, 2015 at 0:53
• It may be that the professor just asked a sloppy question. She likely expected some answer regarding equilibrium and a second regarding entropy. Especially since you never mentioned entropy, you didn't get the points. This is just a guess, of course, and I'll freely admit that I could be way off base. Sep 30, 2015 at 1:28
• Try to uncook an egg.
– MaxW
Nov 11, 2015 at 3:13

In a reversible process, at each point along the process path, the system is only slightly removed from being in thermodynamic equilibrium with its surroundings. So the path can be considered as a continuous sequence of thermodynamic equilibrium states. For an irreversible process, the system is not close to thermodynamic equilibrium with its surroundings at each point along the path.

At any point along the path of a reversible process, both the system and the surroundings can be returned to their original states without significantly affecting anything else. This cannot be accomplished if the process is irreversible.

I would say that there is no direct problem with your answer. Maybe that you need state it more like definition. What will always be true or not true.

I think I would have answered the following:

1. For a reversible process, the system is ALWAYS in equilibrium with its surroundings.

2. For an irreversible process, there will ALWAYS be an entropy change.

Well, If the question was a four marker then you should have added some points like

• External pressure is infinitesimal less than external pressure at each state in reversible process While Internal pressure is much different from external pressure.

• As W= -P(ext) ∆V, therefore for a given ∆V the W of expansion is maximum in reversible process while this is not so for irreversible process.

A reversible process is a quasi static process in which the system goes through a series of equilibrium states and the system is always with its surrounding.

An irreversible process is a non-quasi static process in which there is always an entropy change.