Page 1 of 1

### Entropy Equation

Posted: Mon Feb 05, 2018 2:38 pm
In my notes from the lecture a few weeks ago on wednesday 1/24 I have written that ∆S=-nRTln(V2/V1) but the solutions manual and the book both use that equation without the negative sign. Can anyone explain why that is?

### Re: ∆S=nRTln(Vf/Vi)

Posted: Mon Feb 05, 2018 2:41 pm
The equation you wrote down is actually the equation for work. Please see Dr. Lavelle's constants and equations sheet on his website for clarification. https://lavelle.chem.ucla.edu/wp-conten ... ations.pdf

Also note that there is no T variable in the equation for when there is a volume change.

### Re: ∆S=nRTln(Vf/Vi)

Posted: Mon Feb 05, 2018 2:44 pm
Your expression is indeed that for work, not change in entropy. But this is a matter of going through the derivation.

Under an isothermal process,

, so that

Work under an isothermal reaction is

And change in entropy, with constant temperature, is

Substitute w for q in the above equation

Substitute the expression of W under an isothermal process for

The final equation is how you would calculate the change in entropy for an isothermal process.