### Entropy equations

Posted:

**Mon Feb 11, 2019 7:38 pm**In what circumstances would you use the enthalpy equation with the log of volume and which for temperature? I'm just unclear about how the equations are specific to certain situations.

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=131&t=41977

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Posted: **Mon Feb 11, 2019 7:38 pm**

In what circumstances would you use the enthalpy equation with the log of volume and which for temperature? I'm just unclear about how the equations are specific to certain situations.

Posted: **Tue Feb 12, 2019 11:56 am**

Some situations you have to use both and then add them together to find the total entropy change of the system- it all just depends on what is given. If the problem gives you 2 temperatures and 2 volumes and then asks for entropy change, chances are you'll be using both. However if there is just a temperature change, then you would just use the temperature change equation only.

Posted: **Fri Feb 15, 2019 9:39 am**

Can someone please explain the derivation of deltaS = nRlnV2/V1 ? I do not understand why this can be used.

Posted: **Fri Feb 15, 2019 1:47 pm**

you use the different equations depending on which initial conditions you are given and what variables are changing/remain constant in the system

Posted: **Fri Feb 15, 2019 2:14 pm**

I believe when doing these types of problems, the situations will be described and we can do it from there.

Posted: **Sat Feb 16, 2019 5:09 pm**

Danny Elias Dis 1E wrote:Can someone please explain the derivation of deltaS = nRlnV2/V1 ? I do not understand why this can be used.

This equation is used in an isothermal reaction so temperature is constant which means that deltaU is 0, this allows q=-w, because w=-nRTln(v2/v1) then q=nRTln(v2/v1), then, since deltaS=q/T, you can substitute the q that you just found into this equation making deltaS=nRTln(v2/v1)/T, then you can cancel out the T’s making the equation: deltaS=nRln(v2/v1)

Posted: **Sat Feb 16, 2019 5:36 pm**

The one using V2/V1 is when volume is changing and you have a constant temperature. You would use the one with the T2/T1 when temperature is changing. In this case, you would use Cv or Cp as the constant values.

Posted: **Mon Feb 25, 2019 11:03 pm**

You can also use the equation

delta S= nR ln (P2/P1) to calculate the entropy change for an isothermal increase in pressure.

delta S= nR ln (P2/P1) to calculate the entropy change for an isothermal increase in pressure.

Posted: **Sun Mar 10, 2019 8:13 pm**

If you have an initial temp and final temp (that is temp is changing and volume is not), then you would use the equation with ln(T2/T1). Versus if its the opposite (temp is constant and volume is changing), the you would use the equation with ln(V2/V1).