Rate of Evaporation and Forces
Name: Kevin M.
How does the rate of evaporation relate to temperature
and attractive forces?
Attractive forces between atoms, ions, and molecules weaken as temperature
is increased because heating makes those particles vibrate and move faster.
The greater their relative motion, the harder it is for them to hang on to
each other. That's why water evaporates faster if you heat it.
RATES of evaporation are difficult to treat quantitatively, because the
rate depends on many "outside" factors like: Is the evaporation by
diffusion, by convection, or by transpiration? Qualitatively, one can say
that the rate of evaporation would be proportional to the vapor pressure of
the substance, which depends upon the temperature like: ln(p) = -(H/RT) +
constant, where p is the vapor pressure, H is the heat of vaporization, R is
the gas constant and T is the absolute temperature in kelvins. In general,
the stronger the attractive forces between molecules, the greater H will be,
but it is not possible to be very precise about the details.
Evaporation has to do with liquid molecules (i.e., molecules that are
jostling against each other) breaking away from a surface and going into the
gas phase where they rarely hit another molecule.
The rate of evaporation is how fast molecules go from the liquid phase to
the gas phase. Anything that makes it easier for the molecules to break
away from each other will increase the rate of evaporation.
Does heating the liquid make it easier or harder for the molecules to break
away from each other?
Do strong attractive forces make it easier or harder for the molecules to
break away from each other?
Click here to return to the Weather Archives
Update: June 2012