Enthalpy

[jmanna88]

I was wondering of someone can clear up the difference between heat and enthalpy?

[pwagen]

Basically, enthalpy is the energy (heat) in a system and heat is energy transfer between bodies.

[Enthalpy]

Heat can be internal to a system or exchanged between several ones.

 

Enthalpy doesn't consist only of heat. Depending on the transformation of a gas, the enthalpy change can give the heat or the work exchanged with the rest of the world, or a mix of both. If the system is not just a gas, the exchange can be something more general.

 

Enthalpy is not fully contained in the system considered. In H = E + PV, only E is stored in the system. For instance if water accelerates in a nozzle from a pressure drop, its E does not change, its PV does and is converted into kinetic energy, but the body that provides this "pressure energy" is not the accelerated water, it's somewhere else - for instance a compressed gas that pushes on water.

 

Though, and that's something very useful from thermodynamics, enthalpy can be computed just from the state of the system (for instance water's pressure), despite not being stored fully in the system. To compute water's speed, we don't even have to know if a gas or a piston and spring pushes on the water.

 

And if the system is a gas, or a vapour-liquid equilibirium... E changes together with PV, but the exchanged heat and work can still be computed, by making some assumptions - for instance that little heat is exchanged and losses are small, and then T and W can be computed from the pressure before and after.

[Zeppos10]

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).