H and Q

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H and Q

Postby Julie_Reyes1B » Sun Feb 09, 2020 10:35 pm

Why does delta H always equal Q, but Q does not always equal delta H? Lyndon was mentioning something about this in the review session and I didn't really understand it. Thank you!

Niharika 1H
Posts: 50
Joined: Thu Jul 25, 2019 12:16 am

Re: H and Q

Postby Niharika 1H » Sun Feb 09, 2020 10:49 pm

delta H is equal to Q when the system has a constant pressure, and no nonexpansion work.

Eesha Sohail 1D
Posts: 100
Joined: Sat Aug 17, 2019 12:16 am

Re: H and Q

Postby Eesha Sohail 1D » Sun Feb 09, 2020 11:11 pm

What would nonexpansion work entail? Is that essentially just heat transfer?

William Francis 2E
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Joined: Wed Sep 18, 2019 12:20 am

Re: H and Q

Postby William Francis 2E » Sun Feb 09, 2020 11:26 pm

ΔH is equal to q when pressure is constant. This can be derived using the equation ΔH=ΔU+PΔV. (q+w) can be substituted for ΔU. (-PexΔV) can then be substituted for w. This results in ΔH=q since -PexΔV cancels out with PΔV as long as P=Pex. I think this condition (P=Pex) may be what was being referred to as nonexpansion in the first response.

Yiyang Jen Wang 4G
Posts: 76
Joined: Wed Nov 21, 2018 12:18 am

Re: H and Q

Postby Yiyang Jen Wang 4G » Mon Feb 10, 2020 1:59 pm

At constant pressure w=-p delta V. Since delta H=delta U + p delta V=q+w+delta V=q+(-p delta V)+p delta V. So delta H=q.

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