8.49

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Gabriela Carrillo 1B
Posts: 53
Joined: Fri Sep 29, 2017 7:04 am

8.49

Postby Gabriela Carrillo 1B » Sun Feb 11, 2018 2:59 pm

In the solution manual, why are delta H and q shown to be the same when solving for internal energy?

Kyle Sheu 1C
Posts: 87
Joined: Fri Jun 23, 2017 11:39 am

Re: 8.49

Postby Kyle Sheu 1C » Sun Feb 11, 2018 3:04 pm

Under conditions of constant pressure, deltaH = q because the work of expansion terms cancel out:

H = U + PV
deltaH = deltaU + PdeltaV
deltaH = q + w +PdeltaV
w = -PexdeltaV
deltaH = q - PdeltaV + PdeltaV

deltaH = q

Jessica Lutz 2E
Posts: 56
Joined: Fri Sep 29, 2017 7:04 am

Re: 8.49

Postby Jessica Lutz 2E » Sun Feb 11, 2018 6:54 pm

Kyle Sheu 1C wrote:Under conditions of constant pressure, deltaH = q because the work of expansion terms cancel out:

H = U + PV
deltaH = deltaU + PdeltaV
deltaH = q + w +PdeltaV
w = -PexdeltaV
deltaH = q - PdeltaV + PdeltaV

deltaH = q


Is there a situation where deltaH does not equal q that we need to know?

Gevork 2E
Posts: 32
Joined: Fri Sep 29, 2017 7:05 am
Been upvoted: 1 time

Re: 8.49

Postby Gevork 2E » Sun Feb 11, 2018 7:19 pm

Hi,

So usually we are told that a reaction is occurring under constant pressure, which makes it viable to have delta H = q. However, if the reaction is not occurring at a constant pressure, then you need to account for this and hence your delta H is not going to equal q. I have not seen a question like this personally, but I guess it is a concept we have covered and therefore possibly on the midterm.


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