HW 9.19

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Leah Savage 2F
Posts: 74
Joined: Fri Sep 29, 2017 7:06 am

HW 9.19

Postby Leah Savage 2F » Tue Feb 06, 2018 5:11 pm

9.19 says: Calculate the standard entropy of vaporization of water at 85 C, given that its standard entropy of vaporization at 100. C is 109.0 J K 1 mol 1 and the molar heat capacities at constant pressure of liquid water and water vapor are 75.3 J K 1 mol 1 and 33.6 J K 1 mol 1, respectively, in this range.
Can someone explain why you can cool water vapor to 85 degrees celsius and still have it as water vapor?

Dylan Davisson 2B
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Joined: Thu Jul 27, 2017 3:00 am
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Re: HW 9.19

Postby Dylan Davisson 2B » Wed Feb 07, 2018 3:32 pm

The only way this problem can be solved in a way that has been taught to us is if we add the sum of the change in entropy for the reversible reactions of heating the water, vaporizing the water, and cooling the water to the desired temperature. The water vapor can exist at lower temperatures as long as it does not undergo a phase change that condense the vapor into a liquid. In regards to this problem, a phase change back to liquid water would not allow us to find the total change in entropy for the heated vapor.

Juanyi Tan 2K
Posts: 33
Joined: Fri Sep 29, 2017 7:05 am

Re: HW 9.19

Postby Juanyi Tan 2K » Sun Feb 11, 2018 3:44 am

This phenomenon is called supercooling, which means lowering the temperature of the gas below its boiling point without it becoming liquid


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