Enthalpy change with standard conditions

DrDu

Dear studiot,
it is nice to have someone else in this discussion. You are certainly right that, once one has several mechanical degrees of freedom, there may be several ways to define enthalpy (or enthalpies). I was bringing up the stress and strain mainly to show that also in solid bodies, internal pressure is a well defined quantity, while Zeppos10 denied that internal pressure can be defined for a solid body and one has to use external pressure.

I do not quite agree with your last statement about bond energies vs. stress energies.
Obviously the latter are much smaller, but in chemical reactions bonds are not only broken but also formed, and the difference of bond energies compares usually well with the mechanical energy, be it pressure or stress.

Zeppos10

"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

Any reputable table of data states the conditions, which as you point out is often STP.

DrDu

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

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

do you have a link on that eutectic6002? Is this a journal article?

Studiot

I think I am beginning to understand Zeppos confusion over the definition of pressure.

Backalong two important statements were made.

Zeppos:

I agreee that inadequate definition/understanding of the system boundary can lead to confusion.

DrDu:

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

see Chemical Forum, Eutectic6002, 31/8 2006. (no notr a publication)

Zeppos10

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

That depends upon your defoinition of the first law.
Look at the reference I gave for steam flowing in a pipe.

Zeppos10

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

http://www.chemicalforums.com/index.php?topic=6997

Studiot

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

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

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

for the throttle-aspect see file attached in thread on nov 24 2010:
Enthalpy, thermal energy, and kinetic vs. potential energy