What are the pressure and volume components of enthalpy?

[syang9]

okay, i get that enthaply is a measure of the thermodynamic potential of a system. but what exactly are the 'pressure' and 'volume' components of this? (I'm referring to the definition H = U + pV). I've read that it's "motivated" by the isobaric expansion of a gas inside a cylinder, but how does that describe the thermodynamic potential of any arbitrary system (let's say for example, a slab of metal that has been heated to some temperature)-- is the 'pressure' simply zero? is the pV term only there to describe gases?

 

[AEM]

I will summarize some comments that I found in an old Thermodynamics book I had laying around: "Enthalpy is always defined as a property equal to the sum of internal energy and the pressure-volume product (H = U +PV). Only in the case of a fluid entering or leaving an open system, however, does the PV term represent energy..." According to the author, the main reason for introducing the property enthalpy is to effect simplifications that might arise in problems involving open systems.

This doesn't directly answer your question, but maybe it will help.