Homework Question 8.47


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Lily Guo 1D
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Joined: Fri Sep 29, 2017 7:03 am

Homework Question 8.47

Postby Lily Guo 1D » Thu Jan 18, 2018 4:42 pm

For 8.47, why does the solutions manual use the equation (delta)H = U + P(delta)V instead of just U = q+ w? I'm also confused as to why work would be positive instead of negative because I thought that expansion work done ON a system is negative.

Felicia Fong 2G
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Re: Homework Question 8.47

Postby Felicia Fong 2G » Thu Jan 18, 2018 5:11 pm

I think expansion work done on a system is positive. Work done by the system would be negative.

Sarah_Stay_1D
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Re: Homework Question 8.47

Postby Sarah_Stay_1D » Thu Jan 18, 2018 5:44 pm

Lily Guo 1D wrote:For 8.47, why does the solutions manual use the equation (delta)H = U + P(delta)V instead of just U = q+ w? I'm also confused as to why work would be positive instead of negative because I thought that expansion work done ON a system is negative.


You are in a sense actually using ΔU= q + w. First you can rearrange the equation ΔH = ΔU + PΔV to be ΔU = ΔH - PΔV. Sense we know that at a constant pressure ΔH=q, we can substitute q for ΔH. We also know that w = -PΔV , so we can substitute w for -PΔV. So, know you have the familiar equation ΔU = q + w. Then you just substitute the values given in the problem.

Angela 1K
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Re: Homework Question 8.47

Postby Angela 1K » Thu Jan 18, 2018 9:15 pm

But that ultimately give you a negative work, whereas work done ON a system should be positive???

Lily Guo 1D
Posts: 64
Joined: Fri Sep 29, 2017 7:03 am

Re: Homework Question 8.47

Postby Lily Guo 1D » Sun Jan 21, 2018 4:04 pm

Angela 2I wrote:But that ultimately give you a negative work, whereas work done ON a system should be positive???


I just realized my mistake. Work done ON a system is positive, but EXPANSION work means that work is done on the surroundings/BY the system because the system is expanding. Therefore, work would be negative. That's my bad for not catching that, sorry!


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