Zeppos10

first,I think it is better to define H=U+pV, where U is the internal energy. second, for reasons I cannot explain, there is a habit in thermodynamics to NOT distinguish the inside pressure (notably of gases) from the pressure of the environment (notably the atmosphere). It can be argued that the p in (U+pV) must be the pressure of the environment, in line with the idea of the displaced surroundings mentioned above. The whole thing is worked out in: Mannaerts SH, Energy-balance of the Joule-Thomson experiment: Enthalpy change at decompression. npt-procestechnologie. 2010; 17(4)18-22. (This paper can be retrieved through researchgate.)

If there are no changes in kinetic and gravitational energy, energy balance=enthalpy balance. Hess'Law is a good example of its application to reactive (isothermal) systems: dH=Q.

In general use enthalpy H: since H=U+pV, enthalpy includes internal energy (never mind the conditions) consider the following, symbols: U=internal energy, p=atmospheric pressure, V=system volume, H=enthalpy, Q=heat-transfer, W=work, W'=work done by atm, W"=technical/usefull work (eg work pistonrod transfers to the system. First law: dU=Q+W=Q+W'+W" W'=-pdV, dU+pdV=dH=Q+"W", hence if W"=0 then dH=Q. (this is a conditional equality, not an identity) enthalpy is best understood as a form of energy, ie the sum of internal energy and energy of displacement pV. Heat Q is NOT a form of energy although this is what they tell us in highschool. reference: Mannaerts SHWM, Energy-balance of the Joule-Thomson experiment: Enthalpy change at decompression. npt-procestechnologie. 2010; 17(4)18-22. (can be retrieved at Reseachgate).

symbols: U=internal energy, p=atmospheric pressure, V=system volume, H=enthalpy, Q=heat-transfer, W=work, W'=work done by atm, W"=technical/usefull work (eg work pistonrod transfers to the system. first law: dU=Q+W=Q+W'+W" W'=-pdV, dU+pdV=dH=Q+"W", hence if W"=0 then dH=Q. (this is a conditional equality, not an identity) enthalpy is best understood as a form of energy, ie the sum of internal energy and energy of displacement pV. Heat Q is NOT a form of energy although this is what they tell us in highschool. reference: Mannaerts SHWM, Energy-balance of the Joule-Thomson experiment: Enthalpy change at decompression. npt-procestechnologie. 2010; 17(4)18-22. (can be retrieved at Reseachgate).