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Internal energy

Discussion in 'MCAT Discussions' started by nimish, Apr 13, 2007.

  1. nimish

    nimish 2+ Year Member

    Jan 13, 2007
    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! :confused:
    God, I hate thermodynamics!
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  3. mc4435

    mc4435 7+ Year Member

    Oct 7, 2006
    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

    arsenewenger 2+ Year Member

    Dec 30, 2006
    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
    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
    grade 2 addtion skills:
    Delta H = q - P Delta V + P Delta V + 0 .... constant pressure
    and then :
    Delta H = q... constant presure.
  5. Symphony101

    Symphony101 2+ Year Member

    Apr 7, 2007
    take a physical chemistry class, and you'll find everything you'll ever need to know about internal energy.
  6. xucardsfan08

    xucardsfan08 2+ Year Member

    Jun 8, 2006
    God I loved physical chemistry..........that aside, this might help:

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

    nimish 2+ Year Member

    Jan 13, 2007
    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. :thumbup:
  8. arsenewenger

    arsenewenger 2+ Year Member

    Dec 30, 2006
    That's how it's written in my physical chemistry text book.
    Gen chem is crap in terms of thermodynamics and those prep companies' explanations are a little bit shallow in thermodynamics.

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