Re: latent heat of evaporation

[John Denker <jsd@AV8N.COM>]

Ludwik Kowalski wrote:
> 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?

To zeroth order, yes.
To first order in small quantities, there is *some*
dependence of L on temperature and pressure.  BC
suggested it would depend mightily, but I say it
depends only slightily.  The molar entropy and molar
enthalpy of the vapor are nice smooth continuous
indeed differentiable functions in this regime.

> 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.

Please explain why you think bubbling violates conservation of
energy.  The bubbles form, then pop, returning the system to
its previous state w.r.t number of bubbles.  Whatever energy
was stored in the surface tension in the bubble wall is released
when the bubble pops.

> The same would be true for evaporation and
> condensation involving bubbling (if condensation of that kind could be
> imagined).

That doesn't change my understanding of the situation.

> Evaporation is usually accompanied by losses of heat via conduction,
> convection and radiation.

Bah!  Red herrings.

By way of analogy, do you think conservation of momentum is
violated if you measure velocity inaccurately and measure
mass inaccurately when computing p=mv?

When you do an experiment to measure latent heat or anything
else, it is your responsibility to engineer the apparatus to
ensure that the thing of interest is the dominant contribution,
i.e. that junk effects are kept small and/or are accurately
accounted for.

The main energy-difference between vaporization with bubbling
and without is the energy carried by the *sound* of bubbling.
This is negligible in any case, and could be made yet smaller
if you really cared by trivial engineering tricks (boiling
chips, acoustic insulation, et cetera).