### Work Equations

Posted:

**Wed Jan 31, 2018 8:38 pm**How do you know whether to use the work equation -p deltaV or -nRTln(v2/V1)?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=129&t=26659

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Posted: **Wed Jan 31, 2018 8:38 pm**

How do you know whether to use the work equation -p deltaV or -nRTln(v2/V1)?

Posted: **Wed Jan 31, 2018 8:46 pm**

It depends in the situation. If the volume is constant, ΔV = 0 so w = 0. If the pressure is constant, w = -P(ext)ΔV. If you have an isothermal reversible expansion (both pressure and volume change by infinitely small amount), w = -nRTln(V2/V1). I hope that helps?

Posted: **Wed Jan 31, 2018 8:49 pm**

Generally it is based off the information given within the problem. For instance if the temperature and amount of moles or mass of substance is given with aa changing volume, it is probably best to use the 2nd equation. If you are given a constant value of pressure then it would be safe to use the first equation. But I have a feeling that we will be using the 2nd one more often in this chapter.

Posted: **Thu Feb 01, 2018 10:18 am**

If the problem states it's irreversible, then we use . If it's reversible, we use .

Posted: **Thu Feb 01, 2018 12:12 pm**

As mentioned, if the work done is reversible, then you would use w = -nRTln(V2/V1). If the work done is irreversible, then you would use w = -P(ext)ΔV.

Posted: **Fri Feb 02, 2018 3:15 pm**

Do we need to know how to derive these equations/the reasoning that's involved to come up with the equations ourselves?

Posted: **Wed Feb 07, 2018 5:31 pm**

You can use the equation w= -PdeltaV when pressure is constant

You can use the equation w= -nRt lnV2/V1 when temperature is constant

You can use the equation w= -nRt lnV2/V1 when temperature is constant

Posted: **Wed Feb 07, 2018 5:36 pm**

It's easiest to choose equations based on the problem. You can tell by what they give what equation to use. Also, as mentioned, it depends on whether it is reversible or irreversible.

Posted: **Sat Feb 10, 2018 1:19 pm**

Will the problem always say that the reaction is isothermal? Or will it not always be the case and how does this change your way of approaching the problem?

Posted: **Sat Feb 10, 2018 2:30 pm**

Problem could state the reactions occurs at 25 C.

But these type of questions involving work of expansion for a gas typically apply to simple systems (not reactions).

The only reaction they can apply to are gas phase reactions that result in different moles of gas (reactant) and gas (product).

But these type of questions involving work of expansion for a gas typically apply to simple systems (not reactions).

The only reaction they can apply to are gas phase reactions that result in different moles of gas (reactant) and gas (product).