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the idea behind finding entropy of vaporization

Posted: Mon Jan 23, 2017 12:07 am
by Yiling Liu 1N
I'm a bit confused by the textbook's explanation on how one finds the entropy of vaporization of water at 25 deg C and 1 bar (on page 328).

The textbook divides the process into three steps:
1. heat the liquid to its normal boiling point, 100 deg C
2. allow it to vaporize
3. then cool the vapor back to 25 deg C

Why is it necessary to cool the vapor back to 25 deg C? I thought entropy of vaporization was the change in entropy when a substance goes from being a liquid to a vapor.

Re: the idea behind finding entropy of vaporization

Posted: Mon Jan 23, 2017 1:52 pm
by Chem_Mod
It is important to cool the vapor back down to 25 degrees Celsius because you have to consider the difference in entropy that would exist between 25 and 100 degrees Celsius. Water does spontaneously evaporate at temperatures below its boiling point (e.g. water vapor in a water bottle left in the sun). At its boiling point, water and its vapor have much more energy, so the entropy is higher at 100 degrees. Thus, by cooling the vapor down to 25 degrees, you are able to determine the actual change in entropy at that temperature from its initial state. If you simply took the entropy of vaporization at the boiling temperature, you would be overshooting the change in entropy.