8.47

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Jessica Jones 2B
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Joined: Fri Sep 29, 2017 7:04 am

8.47

Postby Jessica Jones 2B » Fri Jan 19, 2018 3:02 pm

I am very confused as to why the solutions manual has it as -15kJ + 22 kJ when I would expect it to be -15kJ - 22 kJ?
wouldn't it be (delta U)= (delta H)-(P delta V)?

Deap Bhandal L1 S1J
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Re: 8.47

Postby Deap Bhandal L1 S1J » Fri Jan 19, 2018 3:09 pm

Actually delta u equals q + w, so delta u = delta h +pv. I think you are confusing it with the equation delta h = delta u -pv which is a derivation of the first one.

Kaylin Krahn 1I
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Re: 8.47

Postby Kaylin Krahn 1I » Fri Jan 19, 2018 3:10 pm

The work is positive because the work was done onto the system instead of by the system.

Brigitte Phung 1F
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Re: 8.47

Postby Brigitte Phung 1F » Fri Jan 19, 2018 3:17 pm

I think that because w = -P∆V for expansion problems, the 22 kJ in expansion work becomes equal to w = -P∆V = -(22 kJ). Therefore, when plugging it back into the equation, ∆U = ∆H - P∆V = -15 kJ - (-22 kJ) = - 15 kJ + 22 kJ = +7 kJ. Hope this helps!


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