I am reading many online references, but I cannot understand where the concept enthalpy comes from and what it represents.
The Spanish Wikipedia article states it is the Legendre transformation of internal energy.
The English Wikipedia article says it is internal energy plus pV.
I remember a teacher talking about how it was just defined, because the internal energy + pV product was found in many chemical reactions, and someone decided to just give it a name, so it would simplify calculations.
I understand it is something that makes sense, being it a state function.
Also, I can see a utility in it, since it will be equal to a heat transfer during a constant pressure process. And that helps the calculations.
Also, it is stated (in different sources, including the English Wikipedia article for enthalpy) that the internal energy term can be interpreted as the energy required to “create” the system, and the pV term as the work that would be required to “make room” for the system.
My two questions about enthalpy are:
- How was it defined? As the Legendre transformation of U, or directly as U + pV? They may happen to be the same mathematical thing, but... what is the real origin (chemistry, thermodynamics, ... how and why)?
- If it somehow represents the energy to “create” a system (what does that mean: put a bunch of separated atoms together by giving them some linking energy?) and the work required to “make room” for that system (what does that mean: it's not the same to put a system into an iron block than putting it into an air block, it would require a different amount of work to push those different things away. And for the void/emptiness... well, would that take energy at all, if it is already void?)