[pUpcOrn]

Question on Enthalpy
Enthalpy of a system is defined as H = U + pV Where U is the internal energy and pV is the energy of work done from the system to the surroundings?
On wikipedia they only define Enthalpy as a measure of total energy in a thermodynamic system. Therefore according to the part of the first law of thermodynamics: energy can be transferred between systems as either heat or work. Where would the heat component be?
wouldn't it be something like
H = U + pV + q  ?

[tamim83]

Since H = U + PV and U = w + q, H = w + q + PV. 

[olive_oyl]

Hi tamim83,

Was wondering if you can help me clear this up:

So U = q + w and H = U + PV, therefore H = q + w + PV.

But what I'm confused about is the w term in U = q + w, isn't w = -PdV?

So in other words, what exactly is the difference between the PV work term in the definition of U = q + w and the PV work term in H = U + PV?

Hope you know what I mean? If you can help, that would be most appreciated, thanks! 

[rabolisk]

P_extdV is not the same thing as PV. If you take the derivative, dH = dq - P_extdV + d(PV). Only in certain cases would these two cancel out to give dH = dq.

[olive_oyl]

Hi rabolisk,

Thanks for the reply. But what does it mean physically? Is it correct that both "PV terms" refer to the work done on/by the system?

I read in a book the definition of H = U + PV is:
     H = a measure of the total energy of the system
     U = energy to create the system (i.e., the particles)
    PV = energy to 'make room' for that system

Reading these definitions, I understand it as U does not take into account the 'space requirement' of the system, therefore you have to add the PV separately. But then how come the U term itself already contains a "PV term"? 

I'm confused, hope you understand what I mean? 

[rabolisk]

Well, I would say that U does not contain PV term. It's the change in internal energy that contains PdV. There are two ways to alter internal energy, or, equivalently stated, two ways energy can flow, heat and work. PdV is just the mechanical work.