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Topic: Novice question about enthalpy  (Read 2272 times)

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Offline ASmith

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Novice question about enthalpy
« on: June 16, 2021, 04:09:35 PM »
Enthalpy is H = E + PV.  In other words something is added to a system's energy E (pressure P and volume V being positive).  So if some energy ΔE is lost through expansion it is simultaneously added back again as PΔV.  (This seems to be like a form of electronic money - Bitconalpy.  If for example you spend $100 then $100 is simultaneously added to your Bitconalpy account.  This of course avoids the problem of running out of money.)  But how can it make sense to treat PV as both positive and negative at the same time?

ΔH is defined at constant pressure so the net force on the system's boundary remains zero.  As energy is force times distance, which here is zero, a volume change wouldn't change the energy.  So why is it necessary to add and subtract PV when it would be ignored anyway?


Offline Corribus

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Re: Novice question about enthalpy
« Reply #1 on: June 16, 2021, 06:51:20 PM »
I think you have some confusion about what enthalpy is and where that equation comes from. Rather than reinventing the wheel, maybe this will help:

https://www.chemicalforums.com/index.php?topic=81681.msg297434#msg297434
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Offline ASmith

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Re: Novice question about enthalpy
« Reply #2 on: June 17, 2021, 06:06:58 AM »
Your earlier post that you reference says “Enthalpy of a system is essentially a measure of how much energy it takes to create the system from nothing”.  The energy you are describing is E=mc^2 not U + PV.  However, according to my textbook enthalpy is defined from the point of view of the system.  If the system creates the space needed for its existence (as you imply) then it would lose energy and PV would be negative, not positive as when PV is added to the positive kinetic energy.  So this doesn’t explain why enthalpy seems to involve conflicting definitions of the sign of PV, nor why PV is relevant if P is constant.

Your reference repeats the idea that ideal gas molecules don’t interact (rather than involving the approximations that their volume is negligible and their interactions are simple elastic collisions).  If gasses at different average temperatures are allowed to mix, how does their kinetic energy tend toward Boltzmann’s distribution if they can’t interact?  In what sense is it ideal to defy Boltzmann’s equation?

Implying that I’m confused about some of the traditional ideas that you and others keep repeating isn’t helpful.  Adding more such ideas doesn’t help either.  I’m seeking logical explanations, not repetitions that ignore the points I'm making about the repetitions.

Offline Corribus

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Re: Novice question about enthalpy
« Reply #3 on: June 17, 2021, 11:05:31 PM »
Your earlier post that you reference says “Enthalpy of a system is essentially a measure of how much energy it takes to create the system from nothing”.  The energy you are describing is E=mc^2 not U + PV. 
No. It has absolutely nothing to do with E=mc^2.

What I wrote so long ago is very similar to what now appears on Wikipedia's Enthalpy page today:

https://en.wikipedia.org/wiki/Enthalpy

See under section "Applications".
"In thermodynamics, one can calculate enthalpy by determining the requirements for creating a system from "nothingness"; the mechanical work required, pV, differs based upon the conditions that obtain during the creation of the thermodynamic system.

Energy must be supplied to remove particles from the surroundings to make space for the creation of the system, assuming that the pressure p remains constant; this is the pV term. The supplied energy must also provide the change in internal energy, U, which includes activation energies, ionization energies, mixing energies, vaporization energies, chemical bond energies, and so forth. Together, these constitute the change in the enthalpy U + pV. For systems at constant pressure, with no external work done other than the pV work, the change in enthalpy is the heat received by the system. "


Like I said, and emphasized by you drawing mass-energy equivalence and electronic currency into it for some reason, I don't think you have a firm understanding of what enthalpy is. I've tried to steer you in the right direction, but if grounding you in working definitions of terms you aren't using properly doesn't constitute "logical explanations" to you, then I don't know that I can offer further help. Maybe someone else can give you what you're looking for.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline ASmith

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Re: Novice question about enthalpy
« Reply #4 on: June 18, 2021, 04:00:59 AM »
Like I said, and emphasized by you drawing mass-energy equivalence and electronic currency into it for some reason, I don't think you have a firm understanding of what enthalpy is.  .   Maybe someone else can give you what you're looking for.

You were the one that said enthalpy is the energy needed to create a system from nothing, which is clearly incorrect.  There is no point trying to blame your inadequate description on my lack of understanding.

I would though welcome comments from someone able to understand that criticisms of traditional ideas are not refuted by repeating the traditional ideas, even if they are to be found in Wikipedia.

Offline ASmith

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Re: Novice question about enthalpy
« Reply #5 on: July 29, 2021, 05:36:16 AM »
I see I won the argument about entropy (as distinct from simply heat) in my question about the Gibbs equation and I’ll assume the same goes for my question about enthalpy.

I don’t intend asking any more questions on this forum so there’s no further need for censorship or personal insults in lieu of rational discussion.

Offline Borek

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Re: Novice question about enthalpy
« Reply #6 on: July 29, 2021, 06:30:09 PM »
Lol, no, you haven't won any argument. You just got to the point were everyone ignored your posts.
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