Problem 9.19 about Standard Entropy

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Sophia Bozone 2G
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Problem 9.19 about Standard Entropy

Postby Sophia Bozone 2G » Fri Feb 02, 2018 8:28 pm

For problem 9.19, It asks us to calculate the standard entropy of vaporization of water at 85 degrees. The solution of the problem shows that you have to calculate S of heating the reactants, and then S of vaporization, but then for some reason it says you have to calculate the entropy change as the products cool back down to 85 degrees. Why do you have to calculate is as it cools back down?

Jenny Cheng 2K
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Re: Problem 9.19 about Standard Entropy

Postby Jenny Cheng 2K » Fri Feb 02, 2018 8:58 pm

You should break this problem into 4 steps:
1) heat liquid water (ΔS=C * ln(T2/T1))
2) vaporize water at boiling point (ΔS(vap) is given in the problem)
3) cool water vapor back to 85 degrees C (ΔS=C * ln(T2/T1))
4) sum all the entropies from the above steps to find ΔS(total)

You need to calculate entropy of the water vapor cooling down because the problem asks you to find the standard entropy of water vaporization at a temperature below the boiling temperature of water.

andrewr2H
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Joined: Fri Sep 29, 2017 7:05 am

Re: Problem 9.19 about Standard Entropy

Postby andrewr2H » Fri Feb 02, 2018 11:11 pm

The boiling point of water is 100 degrees C. So to find the entropy of the vaporization of water at 85 degrees you have to account for the cooling down of water. I'm not sure how it is that water is able to be vaporized at 85 degrees though.

zanekoch1A
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Joined: Thu Jul 13, 2017 3:00 am

Re: Problem 9.19 about Standard Entropy

Postby zanekoch1A » Sun Feb 04, 2018 12:19 pm

How is it possible for the water vapor to cool back down to 85 degrees without undergoing another phase change back to a liquid?

Gevork 2E
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Re: Problem 9.19 about Standard Entropy

Postby Gevork 2E » Sun Feb 04, 2018 3:13 pm

Hi, So I was also really confused about this, and I found an article on it.

https://chemistry.stackexchange.com/que ... t-of-water

I found this to be really useful.

Hope this is helpful.

Evelyn L 1H
Posts: 67
Joined: Fri Sep 29, 2017 7:05 am

Re: Problem 9.19 about Standard Entropy

Postby Evelyn L 1H » Sun Feb 04, 2018 3:57 pm

How are we supposed to know to calculate the change in entropy back down to 85 degrees from the way the problem is worded?

Kayla Ikemiya 1E
Posts: 56
Joined: Fri Sep 29, 2017 7:04 am

Re: Problem 9.19 about Standard Entropy

Postby Kayla Ikemiya 1E » Sun Feb 04, 2018 8:33 pm

You have to cool it down to find the standard entropy of vaporization for 85 degrees not 100.


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