Enthelpy change with standard conditions

[DrDu]

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

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]

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]

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