Re: latent heat of evaporation

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

On Sunday, Sep 26, 2004, John Denker wrote:

> Ludwik Kowalski wrote:
> > As far as i know no bubbles are formed when evaporation takes place
> > without boiling, for example at 90 C. Molecules
> > escape from a flat surface overcoming surface tension forces. That is
> > when, I suppose, the value of 540 cal/g is valid.  This topic is new
> > to
> > me. I do not know what to expect when bubbles are formed. Should L
> > become larger or should it become smaller than 540? Perhaps someone
> > will supply the answer based on thermodynamics. Or on laboratory data.
>
> How about an answer based on conservation of energy?
>
> Suppose you evaporate 50g of water from a vessel at 100C,
> and recondense it in another vessel at 98C.
>
> What happens to the energy?
> What does this tell you about the latent heat?

Are you saying, John, that to accept two different values of L (one for
evaporation without boiling and another with boiling) would
automatically lead to the violation of conservation of energy? I am not
convinced that this is true, unless additional restrictions are
imposed. The energy conservation makes the magnitude of the latent heat
of evaporation equal to the magnitude of the latent heat of
condensation, assuming, for example, that both take place without
forming bubbles. The same would be true for evaporation and
condensation involving bubbling (if condensation of that kind could be
imagined).

Evaporation is usually accompanied by losses of heat via conduction,
convection and radiation. Therefore, it is not difficult to imagine
situations in which evaporation via bubbling and condensation from the
flat surface can possibly have different magnitudes of L without
violating the first law. Suppose, for example, that the magnitude of L
during boiling is 580 cal/g while the magnitude of L during ordinary
condensation is 540 cal/g. The energy would still be conserved if a
change in what goes into evaporation were somehow compensated by what
goes into other forms of thermal losses. The apparently "missing  40
cal/g," in this  illustration, might be responsible for a slightly
different room temperature (in comparison with what it would be for
evaporation without boiling). Yes, I am aware that this argument is not
based on experimental evidence.
Ludwik Kowalski