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### 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.

### Re: Entropy equations

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.

### Re: Entropy equations

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.

### Re: Entropy equations

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

### Re: Entropy equations

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.

### Re: Entropy equations

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)

### Re: Entropy equations

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.

### Re: Entropy equations

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.

### Re: Entropy equations

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).