Re: latent heat of evaporation

[Bob LaMontagne <rlamont@POSTOFFICE.PROVIDENCE.EDU>]

There is a temeprature dependence to L, namely,

dL/dT = C_pv - C_pw

(refer to Bohren: Atmospheric Thermodynamics, p197)

This is not related to bubbles, just the temperature dependence of the
enthalpies of the liquid and vapor.

Bob at PC

*********** REPLY SEPARATOR  ***********

On 9/27/2004 at 8:58 AM Ludwik Kowalski wrote:

> 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).