### Change in vol and temp

Posted:

**Fri Feb 02, 2018 3:06 pm**What formula (for entropy) do we use if there is a change in both volume and temperature?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=133&t=26751

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Posted: **Fri Feb 02, 2018 3:06 pm**

What formula (for entropy) do we use if there is a change in both volume and temperature?

Posted: **Fri Feb 02, 2018 4:21 pm**

If there's a change in volume and temperature, you use two different equations, which you can do because Entropy is a state function, so there's the same change whether both volume and temperature change at the same time, or whether volume changes, then temperature. First you can calculate the entropy change at the initial temperature with the change in volume using Delta S =nR (V2/V1). Then you can assume that the temperature changes while the volume remains at the final volume, b using Delta S= Cv(T2/T1).

Then you add the entropy changes from these two steps together to get the total entropy change. Example 9.5 on page 325 in our textbook has an example problem for this.

Then you add the entropy changes from these two steps together to get the total entropy change. Example 9.5 on page 325 in our textbook has an example problem for this.

Posted: **Fri Feb 02, 2018 4:36 pm**

You can calculate the entropy change resulting from temp change and volume change together separately and then add them up. Use delta S=nRln(v2/v1) to calculate the detla S for volume change and then use detlaS=nCln(T2/T1) to get the entropy change for temp change.