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Finding entropy of vaporization from temperatures NOT = to boiling point

Posted: Thu Feb 08, 2018 9:59 pm
by Tim Foster 2A
When finding the standard entropy of vaporization of water at 85 C, why do we cool the vapor back down from 100 C? I get that entropy is a state function, but enthalpy is also a state function and when finding the enthalpy of vaporization of water at 85 C, we would simply add the Hvap of water to 4.184(100-85), and not include any sort of subtraction. Why do we have the additional step of subtraction for calculating entropy of vaporization?

Re: Finding entropy of vaporization from temperatures NOT = to boiling point

Posted: Fri Feb 09, 2018 1:28 am
by Sara Sasaki 1K
Is this for problem 9.19? I believe you do so because the problem is asking for the entropy when the vaporized water is 85 degrees celsius (as the final temperature), therefore you have to cool the water after you change its state by vaporization. Hope that makes sense!

Re: Finding entropy of vaporization from temperatures NOT = to boiling point

Posted: Fri Feb 09, 2018 2:10 pm
by Jenny Cheng 2K
To find entropy of vaporization at temperatures NOT = to boiling point: example of finding entropy of vaporization of water at 85 degrees C (9.19)
1) increase T of water (from 85 degrees C to 100 degrees C)
2) find entropy of vaporization at boiling point (100 degrees C for water)
3) cool/decrease T of water (from 100 degrees C to 85 degrees C)
4) sum the entropies of the above 3 steps

You need to cool the water vapor from 100 degrees C to 85 degrees C because the problem asks for the entropy at 85 degrees C, and this decrease in T happens after the phase change.

Re: Finding entropy of vaporization from temperatures NOT = to boiling point  [ENDORSED]

Posted: Sat Feb 10, 2018 10:16 am
by Chem_Mod
Excellent answer.