## irreversible vs reversible expansions

Caitlin Dillon 3G
Posts: 26
Joined: Wed Sep 21, 2016 2:56 pm

### irreversible vs reversible expansions

Can someone please explain the difference between these two? I am having a hard time visualizing it. Irreversible has a constant pressure, and reversible does not? Correct? I also never know what formula to use for work.

Vivian Wang 3J
Posts: 29
Joined: Wed Sep 21, 2016 2:57 pm
Been upvoted: 1 time

### Re: irreversible vs reversible expansions

A reversible process can be reversed because the conditions under which it is carried out are very controlled. For the piston example in the textbook (page 265), the pressure of the confined gas and the external pressure are matched to keep the system in thermodynamic equilibrium. At equilibrium, the system can go either way, so it is considered reversible. The external pressure and the pressure of the confined gas are constantly being matched so that the system can remain in equilibrium. At any point, the piston can move out or in because the external pressure and the pressure of the confined gas are equal.

In an irreversible process, using the same piston example, the external pressure is held constant, which means that the system is not in equilibrium and the piston can only move in one direction.

Use this work formula for reversible process:
$w = -nRT\textup{ln}\frac{V_{final}}{V_{initial}}$

and this one for irreversible process:
$w = -P_{ex}\Delta V$
Last edited by Vivian Wang 3J on Sat Jan 21, 2017 11:17 am, edited 2 times in total.

Chem_Mod
Posts: 18895
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 717 times

### Re: irreversible vs reversible expansions

A reversible process in thermodynamics is one in which the process can be repeated/reversed via. an infinitesmally small change. In the case of gas expansion, this means that the external pressure change is infinitesmally small (changing very, very little at once) and the pressure of the system reacts correspondingly. The defining characteristic is that, in a reversible system, the final and initial states of the system are always at equilibrium since the slight change in external pressure is immediately "felt" by the system, which reacts immediately to reach equilibrium. This is repeated in infinitely small units an infinitely large amount of times until the true final and initial states are reached. By doing so, work is maximized. Thus, the initial state can be restored from the final state without any losing any work - it can be reversed and repeatedly indefinitely. This is an ideal process.

In an irreversible process, which all natural processes are, the initial state cannot be restored without any work done. For example, imagine a piston system in equilibrium with the external pressure i.e it's not moving. If the external pressure suddenly dropped to half of what it was, the piston system would have to respond to the drop in external pressure by expanding. Since the external pressure drop was not in an infinitesmally small amount, the process is irreversible and loses some measure of efficiency.