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Enthalpy change with standard conditions

by Godwin Kessy
Tags: conditions, enthalpy, standard
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Zeppos10
#19
May10-10, 10:37 AM
P: 82
"I have checked many textbooks and websites and it seems that the constant pressure associated with the expression, delta H=q, is the external pressure in which we carried out our experiments and not the pressure of the system". (Chemical Forum,Eutectic6002, 31/8 2006)
This implies that most thermodynamic data available are compatible with H=U+p(env)V.
The throttle-example may not be as bad as it first appeared to me: the problem there is that it assumes the existance of two large 'pressure-bath'. If you assume that the high-pressure side has to be maintained by the operator who runs the throttle, then the proces is not isenthalpic, but H increases, in line with the point of view on enthalpy defended by me.
It is best to compare the throttle with an (irreversible) isothermal expansion. If you use H=U+p(in)V, H=constant, but if you use dH=Q it increases. It seems that not both position can be true.
Studiot
#20
May10-10, 01:20 PM
P: 5,462
This implies that most thermodynamic data available are compatible with H=U+p(env)V.
Any reputable table of data states the conditions, which as you point out is often STP.
DrDu
#21
May11-10, 05:45 AM
Sci Advisor
P: 3,593
Zeppos10, at the beginning of this thread you were claiming that enthalpy, in its full generality as a function of state, has always to be defined with external pressure and not internal pressure and you even denied the existence of internal pressure for a solid body.

Now methinks you boiled down your claim to the much specialized question as to whether it is sufficient to keep the external pressure constant when measuring the heat of a reaction to be able to identify the heat with the reaction enthalpy. The latter point is nearly trivial, as long as your reaction (and the design of the calorimeter) is not so vigorous that the rapid pressure changes lead to the emission of sound instead of heat as e.g. in an explosion.
Zeppos10
#22
May12-10, 05:09 AM
P: 82
DrDu: my thesis here was and is that in the definition of enthalpy H=U+pV, p must be understood as external pressure. I consider that position controversial.
In defending this thesis it is important to see if this is not at odds with the enthalpy data: for this reason I quoted Eutectic6002. And yes: formerly enthalpy was called heat-content. As far as solids is concerned I have not seen a text where p is defined as a stress or strain. (I did not see Buchdahl).
The throttling valve is a challenge which I am currently working out.
DrDu
#23
May12-10, 07:23 AM
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P: 3,593
do you have a link on that eutectic6002? Is this a journal article?
Studiot
#24
May12-10, 10:54 AM
P: 5,462
I think I am beginning to understand Zeppos confusion over the definition of pressure.

Backalong two important statements were made.

Zeppos:

the proper definition of the system, ie system-environment interface.
I agreee that inadequate definition/understanding of the system boundary can lead to confusion.

DrDu:

"p in (pV=nRT) is an internal pressure, but the p in (H=U+pV) is the external pressure." That is not true. The variables in the definition of the thermodynamical potentials are always the internal variables of the system.
Is quite correct, but a bit convoluted with the double negative.

The pressure in the first law is always the system pressure.
When we want to do calculations we obviously have to supply a value or formula for this pressure.
In many circumstances we can equate the system pressure to the external pressure, which does not alter. Lots of texts rush over this important fact and just perform substitution without justification.
In the event that the external pressure does alter during the process this is accounted for in the first law, not by the pdV component but by a separate work term.
Some versions of the first law, eg the flow version, add several more terms as a result changes in internal pressure.

Since there is a preponderance of mechanical scientists/engineers in this forum I usually find myself defending the chemical/chemeng approach.
However in this case I would urge Zeppos to look at mechanical engineering steam tables.

Have a look at this thread and inparticular post#4

What would be the effect of drawing the system boundary to include or exclude the ballon skin if the experiment was carried out in a vacuum?
Zeppos10
#25
May12-10, 02:44 PM
P: 82
Quote Quote by DrDu View Post
do you have a link on that eutectic6002? Is this a journal article?

see Chemical Forum, Eutectic6002, 31/8 2006. (no notr a publication)
Zeppos10
#26
May12-10, 03:00 PM
P: 82
Quote Quote by Studiot View Post
The pressure in the first law is always the system pressure.
//
In the event that the external pressure does alter during the process this is accounted for in the first law, not by the pdV component but by a separate work term.
The first law says nothing about pressure at all: when we do have work terms one in general takes care of the work against external pressure due to volume change: in this work term the p is the external pressure.

What happens to/in the first law if the external pressure changes: that is a very good question. I love to see that worked out.
Studiot
#27
May12-10, 03:05 PM
P: 5,462
The first law says nothing about pressure at all:
That depends upon your defoinition of the first law.
Look at the reference I gave for steam flowing in a pipe.
Zeppos10
#28
May13-10, 02:34 PM
P: 82
Quote Quote by Studiot View Post
That depends upon your defoinition of the first law.
if there is more then one definition of the first law, we have a serious problem.
The (my??) definition of the first law is; "energy is a conserved quantity". For closed systems where internal energy is the only form of energy to take into account, it can be formulated as; delta U = Q+W, where Q is heat exchanged and W ias work exchanged.
For other system the energy ballance equation will look different, but the first law stands.
Borek
#29
May13-10, 03:20 PM
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Quote Quote by Zeppos10 View Post
see Chemical Forum, Eutectic6002, 31/8 2006. (no notr a publication)
http://www.chemicalforums.com/index.php?topic=6997
Studiot
#30
May13-10, 03:46 PM
P: 5,462
The (my??) definition of the first law is; "energy is a conserved quantity". For closed systems where internal energy is the only form of energy to take into account, it can be formulated as; delta U = Q+W, where Q is heat exchanged and W ias work exchanged.
For other system the energy ballance equation will look different, but the first law stands.
Exactly so, as long as you have a suitable sign convention.

Now how would you calculate the W term if it involved a change of pressure and/or a change of volume?

And how would you calculate it if it involved thermally heated fluid moving internal paddles around?
DrDu
#31
May14-10, 02:43 AM
Sci Advisor
P: 3,593
Thank you Borek for giving the link! All I can see is that Eutectics6002 states that in a (quasi-) reversible process p_in =p_ext if the system is in mechanical equilibrium with its surrounding. This I never doubted! However, enthalpy is also defined for systems not in mechanical equilibrium with the surrounding.
Zeppos10
#32
Nov15-10, 11:43 AM
P: 82
Quote Quote by DrDu View Post
How do you describe then the Joule-Thompson experiment? I.e. the expansion of a gas from (internal) pressure p1 to pressure p2 trough a throttle?
According to e.g. the book of Max Planck "Thermodynamik" // The process is called "isenthalpic" as H1=H2.
The answer to this question can be found in a 2010 publication in the journal of the Dutch Process Engineers:
'Energy-balance of the Joule-Thomson experiment: Enthalpy change at decompression.
(npt-procestechnologie 17(4)18-22.)

The throttle process is not isenthalpic except in one (theoretical) case.
Zeppos10
#33
Nov24-10, 07:44 AM
P: 82
Quote Quote by Zeppos10 View Post
in my opinion the proper handling of the joule-kelvin experiment depends on the proper definition of the system, ie system-environment interface. It is not a system in equilibrium however and it brings us far from the problem that started this discussion. To me it seems rather important to the whole of science what the physical meaning is of p in (H=U+pV): I wonder if there are only 2 or 3 people in this forum that are interested in the subject ??
for the throttle-aspect see file attached in thread on nov 24 2010:
Enthalpy, thermal energy, and kinetic vs. potential energy


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