## Irreversible vs reversible pathway

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Brenton Hwee 2J
Posts: 15
Joined: Wed Sep 21, 2016 3:00 pm

### Irreversible vs reversible pathway

If we know that all real processes are irreversible, then how do we know when to use w = -nRT ln V1/V2 instead of w = -P∆V and vice versa? For calculating entropy, do we never use w = -P∆V because it has to be a reversible expansion and isothermal?

Milan Hirpara 3K
Posts: 20
Joined: Sat Jul 09, 2016 3:00 am

### Re: Irreversible vs reversible pathway

You will know when to use which because the problem will say give the "irreversible" work value or "reversible" work value. Also, in some instances the problem will only give you enough information to use one of the two formulas to solve for work.

Desiree Martin 2A
Posts: 25
Joined: Fri Jul 22, 2016 3:00 am

### Re: Irreversible vs reversible pathway

Brenton Hwee 2J wrote:If we know that all real processes are irreversible, then how do we know when to use w = -nRT ln V1/V2 instead of w = -P∆V and vice versa? For calculating entropy, do we never use w = -P∆V because it has to be a reversible expansion and isothermal?

The equations given will usually specify whether to use the equation for reversible, isothermal processes vs. irreversible processes. The reversible, isothermal process is slower, yet requires more work. The equation for this is w = -nRT ln V1/V2 because you must derive the integral from w=-P∆V. The latter equation is the one used for irreversible processes. This means that the process is faster overall, and less work is used for it. I hope this helps.