The Compleat Distiller

THE COMPLEAT DISTILLER 95
Raoult's Law for Liquids (boiling of mixtures)
Later, Raoult extended this thinking to liquids. Raoult's Law states the vapor pressure contributed by
each component of a mixture of liquids is the vapor pressure of that component multiplied by its mol
fraction of the mixture.
This is because the mol fraction of a component governs the proportion of the liquid surface the
component occupies, and the “share” of the surface occupied by a component controls its ability to
escape and make pressure.
So, if
X 1 = mols of substance 1 in liquid/ (mols of substance 1 + mols of substance 2)
X 2 = mols of substance 2 in liquid/ (mols of substance 1 + mols of substance 2)
P 1 * = vapor pressure of substance 1 on its own
P 2 * = vapor pressure of substance 2 on its own
P 1 = X 1 P 1 * = vapor pressure from substance 1 in the mix
P 2 = X 2 P 2 * = vapor pressure from substance 2 in the mix
P TOTAL = (P 1 + P 2 )= (X 1 P 1 * + X 2 P 2 *)
Saturated Vapor Pressures
If you raise the pressure or lower the temperature of a gas or vapor, the molecules become more
crowded. As they move closer and closer together, the mutual attraction of the molecules eventually
will overcome the thermal energy keeping them in a gaseous state, and droplets of liquid will begin to
form in the vapor. A vapor that has reached this point is called saturated. A very common example of
this process is fog – a suspension of tiny water droplets in air, which is a mixture of gases and water
vapor.
A substance that is normally liquid or solid still has a vapor pressure, and for any specific temperature,
there is a specific pressure at which it begins to condense into liquid – the saturated vapor pressure.
The saturated vapor pressure is the maximum pressure that a substance’s can exert at a given
temperature. The experiment with the mercury barometer in chapter 2 was an illustration of saturated
vapor pressure. The important point to remember about saturated vapors is that they contain the
maximum possible number of molecules possible at that temperature. Whenever you work with
vapors, you need know if they are saturated or not. Their behavior depends upon the answer to that
question.
This can easily be demonstrated with the same barometer
experiment. Simply introduce a few drops of liquid, but not
enough to saturate the space above the mercury, then slowly tilt
the barometer so that the space gets smaller (slowly, to allow the
heat generated by compression to escape and keep the
temperature constant). At some point the liquid will start to
condense out of the vapor, indicating that you have reached the
saturated vapor pressure
Fig. 8-4
In a distillation column, all the vapors are saturated throughout the system, and the temperature of the
column at any point in the column is the boiling point of the mixture at that point. The boiling point of
a substance is the point at which its saturated vapor pressure equals the pressure around it – in this case,
atmospheric pressure. Just as saturated vapor forms above a boiling liquid, saturated vapor can
condense back to a liquid at its boiling point This is the reason that reflux is so valuable.