# Enthalpy vs. Internal Energy (Thermodynamics)

You are right Zeppos10, yet I preffer the full story and dislike fuzzy definitions.

For me it makes no sense to talk about "a form of energy".
For me, this just means one item in an energy balance, as I said earlier.
In your example, 370 kcal/100g of corn flakes is indeed just a term in an energy balance.
Moreover, you should justify why this is called "enthalpy of combustion" and not "energy of combustion" or even "free energy of combustion", and why this makes sense.

At my work, I use thermodynamics everyday, but mostly at constant pressure.
Therefore, I am using the enthalpy everyday without really going back to its meaning everyday.
I am doing this since twenty years.

Nevertheless, I was interrested by this thread just because of a recent discussion at work.
The question was about CO2 inertisation of a coal grinding mill: how long could we use CO2 from a high-pressure tank before the tank would become too cold.
I realised that in this case, expansion leaves the internal energy constant.
(although this has no big practical impact)

In addition to that I like to make a connection to the microscopic point of view.
That was my initial motivation to post here.

I still do not see how enthalpy can be seen as a sum of microscopic bits of energy,
kinetic and potential term, quantum mechanical version if needed.
Yet, as far as pop corn combustion is involved, this might be related to some molecular binding energy.
But was is the link from the microscopic to the macroscopic and the definition H=E+PV ?

Here is an extract from wikipedia:

"Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure."

I do not feel very satisfied with that definition.
It is much better to write a balance equation and stick to it.
I never feel uncomfortable with balance equations.

This discussion is really a pedagogical issue.
How is it best to teach, or to talk, about enthalpy?

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You are right Zeppos10, yet I preffer the full story and dislike fuzzy definitions.
Although I am said to be right, yet (!) I also prefer (and give) the full story (enthalpy is a form of energy) and I also dislike fuzzy definitions. (Hence: H=U+pV= sum of 2 forms of energy: nothing fuzzy about it.)
Although you state:
For me it makes no sense to talk about "a form of energy".
Yet you do (for 'term' read 'form'):
I still do not see how enthalpy can be seen as a sum of microscopic bits of energy,
kinetic and potential term, quantum mechanical version if needed.
The problem is that the definition of 'potential energy' is somewhat fuzzy. It is believed that " Energy forms are either potential or kinetic." [http://www.eia.doe.gov/kids/energy.cfm?page=about_forms_of_energy-forms].
If this holds then the pV-term in enthalpy will have to be classified as potential energy, since it is obviously not a kinetic term. However now you still have to distinguish different forms of potential energy.
For me, this just means one item in an energy balance, as I said earlier.
In your example, 370 kcal/100g of corn flakes is indeed just a term in an energy balance.
Moreover, you should justify why this is called "enthalpy of combustion" and not "energy of combustion" or even "free energy of combustion", and why this makes sense.
In many instances 'enthalpy' can be replaced by 'energy', because it is a form of energy. In the same way you can replace 'kinetic energy' by 'energy', but in all cases you get less specific, hence more fuzzy, and nothing is gained by that substitution.
The way the measurement is made for the 'caloric value' is the way the enthalpy of combustion is made.
(nb: I am not going to expand this discussion to 'free energy': I consider that as an attempt to sidetrack this discussion.)
At my work, I use thermodynamics everyday, but mostly at constant pressure.
Therefore, I am using the enthalpy everyday without really going back to its meaning everyday. I am doing this since twenty years.
It might be a good idea to reconsider.
I still do not see how enthalpy can be seen as a sum of microscopic bits of energy,
kinetic and potential term, quantum mechanical version if needed.
Yet, as far as pop corn combustion is involved, this might be related to some molecular binding energy. But what is the link from the microscopic to the macroscopic and the definition H=E+PV ?
If you know what 'volume' is on the microscopic level, you have your anwer.
Here is an extract from wikipedia:
"Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure."
I do not feel very satisfied with that definition.
I don't feel satisfied either: enthalpy is not 'total energy' because it does not include macroscopic mechanical energy.
It is much better to write a balance equation and stick to it.
Much better then what ?
This discussion is really a pedagogical issue.
How is it best to teach, or to talk, about enthalpy?
No: this discussion is about terms that have to be taken into account in a proper energy balance, which applies to all systems at bwork. But you are right: teaching thermodynamics becomes much more abstract if enthalpy is considered a 'Legendre Transform' (see elsewere in this forum) or 'An accounting artefact'.