# Internal energy

Discussion in 'MCAT: Medical College Admissions Test' started by nimish, Apr 13, 2007.

1. ### nimish 10+ Year Member

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Can someone please explain how and why Delta H can equal q at constant pressure?
The forumlas are as follow:
#1 U=q+w
#2 Delta H=Delta U+ p(Delta V)

If in #2 you hold pressure constant, wouldn't H=U+pV?
But on page 56 of EK Chemistry, they say "For reactions involving no change in pressure, the change in enthalpy is equal to the heat"

I understand that if the volume is held constant, then there is no PV work done, so U=q (Forumla 1 above). But it doesn't make sense that if pressure is held constant then H=q.
How is this possible?

A brief explanation would be greatly appreciated!
God, I hate thermodynamics!

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3. ### mc4435 7+ Year Member

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If enthalpy is the heat content of the system, then whatever comes out of the system must be ALL heat (if no work is done).

4. ### arsenewenger 2+ Year Member

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The correct equation to #2 ...
Delta H = Delta U + Delta (PV)
which translates into the eqn below:
Delta H = Delta U + Pdelta V + V Delta P
Then :
Delta H = Delta U +P Delta V + 0 ....constatnt pressure
Also:
Delta H = q + W + P delta V + 0 ....constatnt pressure
work get replaced:
Delta H = q + (-P delta V) + P Delta V + 0 ....constant pressure
Delta H = q - P Delta V + P Delta V + 0 .... constant pressure
and then :
Delta H = q... constant presure.

5. ### Symphony101 2+ Year Member

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take a physical chemistry class, and you'll find everything you'll ever need to know about internal energy.

6. ### xucardsfan08 2+ Year Member

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God I loved physical chemistry..........that aside, this might help:
http://en.wikipedia.org/wiki/Thermodynamics

Just go back to your genchem book and review thermodynamics. It seems every question asked here is about thermodynamics.....

7. ### nimish 10+ Year Member

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Well I've taken 2 gen chem classes, but Thermodynamics was my weakest area......which is why I am struggling. I've been trying to figure the answer to this question out for the past couple of days, but I finally gave up and came here.
My biggest concern was that I have never seen the formula written as
Delta H = Delta U + Delta (PV)
I've always seen it written as
Delta H = Delta U + P(Delta V)
which is where the confusion was coming from.
Thanks for the explanation guys, its starting to make sense now.

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