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Entropy Equation

Posted: Mon Feb 05, 2018 2:38 pm
by Lindsay H 2B
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
by Chem_Mod
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. ... 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
by Chem_Mod
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.