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Internal energy

Discussion in 'MCAT Discussions' 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! :confused:
    God, I hate thermodynamics!
     
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  3. mc4435

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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

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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
    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

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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

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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

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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. :thumbup:
     
  8. arsenewenger

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    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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