Entropy Equation


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Lindsay H 2B
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Entropy Equation

Postby Lindsay H 2B » 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?

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Re: ∆S=nRTln(Vf/Vi)

Postby Chem_Mod » 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.

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Re: ∆S=nRTln(Vf/Vi)

Postby Chem_Mod » 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.


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