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### H and Q

Posted: 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!

### Re: H and Q

Posted: Sun Feb 09, 2020 10:49 pm
delta H is equal to Q when the system has a constant pressure, and no nonexpansion work.

### Re: H and Q

Posted: Sun Feb 09, 2020 11:11 pm
What would nonexpansion work entail? Is that essentially just heat transfer?

### Re: H and Q

Posted: 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.

### Re: H and Q

Posted: 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.