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Postby 705121606 » Tue Feb 04, 2020 11:45 pm

"For a certain reaction at constant pressure, ΔH=-15kj and 22kj of expansion work is done on the system by compressing it into a smaller volume. What is ΔU?"

I understand that ΔU=ΔH+w is the general equation. Here we know H is -15
I am confused because if it is expanding, I know an expansion would make the equation -w (same as -pΔv)
so what I originally got was ΔU= -15 - 22 but it should be ΔU=-15+22, is it positive because it is compressing?
can someone explain

Jason Wu 1E
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Re: 4D5

Postby Jason Wu 1E » Wed Feb 05, 2020 12:47 am

W is positive because in the question it states that "work is done on the system". Furthermore, if you do use W = -P delta V, the final volume minus the initial volume will be a negative number since it is compressed into a smaller volume. Thus delta V will be negative and that cancels out with the negative sign in the front to create a positive W.

Justin Sarquiz 2F
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Joined: Fri Aug 30, 2019 12:15 am

Re: 4D5

Postby Justin Sarquiz 2F » Wed Feb 05, 2020 8:19 am

Work done on a system means that work is positive. Work done by the system on the surroundings means that work is negative because some energy is lost to the surroundings through work.

Connor Chappell 2B
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Joined: Wed Feb 20, 2019 12:16 am

Re: 4D5

Postby Connor Chappell 2B » Wed Feb 05, 2020 8:50 am

Work done on a system by its surroundings is positive. Work done by the system on its surroundings is negative. This has to do with the fact that when a system does work, it loses energy (negative), and when work is done on a system, its energy increases (positive).

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