## Entropy equations

$\Delta S = \frac{q_{rev}}{T}$

Fiona Jackson 1D
Posts: 30
Joined: Fri Sep 28, 2018 12:22 am

### Entropy equations

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.

Camille Marangi 2E
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

### Re: Entropy equations

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.

Danny Elias Dis 1E
Posts: 60
Joined: Fri Sep 28, 2018 12:19 am

### Re: Entropy equations

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

LaurenJuul_1B
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

### Re: Entropy equations

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

Bruce Chen 2H
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

### Re: Entropy equations

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

Riley Dean 2D
Posts: 60
Joined: Fri Sep 28, 2018 12:15 am

### Re: Entropy equations

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)

Posts: 57
Joined: Fri Sep 28, 2018 12:27 am

### Re: Entropy equations

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.

MackenziePerillo-1L
Posts: 31
Joined: Fri Sep 28, 2018 12:19 am

### Re: Entropy equations

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

Abby-Hile-1F
Posts: 46
Joined: Fri Sep 28, 2018 12:16 am

### Re: Entropy equations

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).