Latent heat of Vaporization vs Condensation by Kent W Mayhew
The latent heat of vaporization can be considered as
an isothermal and isobaric process. Herein “Lvap” shall be used to signify the latent heat. For non-ideal substances, the energy required
for boiling, must also consider any changes to bonding potentials (dU). Therefore:
Lvap=dU+PatmdV (1a)
Or if you prefer
Lvap= dU +PdV (1b)
The subscript “atm” is removed in eqn (1b) with the understanding that the work is done onto the surrounding atmosphere and that this
is lost work. There is no real difference between what is stated here and what is traditionally accepted. To be understand expansion
see expanding piston-cylinder.
Instead of latent heat, often the term “Enthalpy of vaporization” is used.Enthalpy in terms of thermodynamic parameters is written: H=E+PV. Accordingly, enthalpy change is written:
dH=dE+PdV (2a)
Sorry for the repetition but dE is the system’s internal energy change and (PdV)atm is the work done onto the system’s surroundings that being our atmosphere. This is the only instance where d(E+PV) makes any sense Note: this traditionally is not always understood because eqn (2a) lacks clarity. I.e. eqn (2a) should be rewritten: dHsys=dEsys+(PdV)atm
And for the case of vaporization the only internal energy change (dE) is considered to be the change in chemical bonding potential
(dU), thus eqn (2a) becomes:
dHsys = dUsys + (PdV)atm = dUsys + PatmdVsys (2b)
Whether we use eqn (1b) or (2b) for vaporization is splitting
hairs. However consider that enthalpy change in any other form than (2b) is illogical. For example consider condensation. The work
done onto the atmosphere in vaporization cannot be retrieved, which is to say when a gas condenses into a liquid then no work is done!see lost work
Specifically, the latent heat of condensation (Lcond) is simply the change in chemical bonding potential (dU).
Accordingly, we now write:
Lcond = -dU (3)
Eqn 3 separates our understanding of the latent heat of condensation from the traditional understanding
that being latent heat of condensation equals the negative of the latent heat of vaporization.
One might ask how can this be?
It is this author’s understanding that an isobaric colorimeter which is used to measure the latent heat of vaporization, is not well
suited for measuring the latent heat of condensation. Accordingly the (wrong?) traditional assertion has never been experimentally
proven, rather it simply accepted based upon poor traditional understanding. In order to better understand see: Illusional Reversibility
