EXPERIMENT 4

DISTILLATION

One of the most convenient methods used to separate a miscible mixture (a solution) of volatile liquids which have different boiling points is termed DISTILLATION. Distillation involves boiling the volatile mixture and condensing the vapors produced. Simple distillation can effectively separate liquids whose boiling points differ by 80 degrees or more, while Fractional distillation can be adapted to separate liquids whose boiling point difference may be as small as 0.10°C.
When a solution is composed of two or more volatile liquids, the boiling point of that solution lies somewhere between the highest boiling and the lowest boiling points of constituents. Recall that boiling occurs when the vapor pressure of the mixture equals external pressure. Consider a solution composed of two mole of liquid A, boiling at 60°C, and one mole of liquid B, boiling at 160°C. For example: At 60oC, PAo=760 torr, PBo= 256 torr,
The vapor pressure of the solution at any specific temperature is the sum of the vapor pressure of liquid A and that of liquid B,

Ptotal = PA +PB
Partial pressures of liquid A and B can be calculated by Raoult’s equation:

PA = XAPAo
(PA: partial vapor pressure of liquid A, XA: molar fraction, PAo: vapor pressure of pure liquid A)
PB = XBPBo

(PB: partial vapor pressure of liquid B, XB: molar fraction, PBo: vapor pressure of pure liquid B)

XA = 2.0mol A / (2.0 mol +1.0 mol) = 0.67
XB = 1.0mol B / (2.0 mol +1.0 mol)=0.33
PA = XAPAo = 0.67 x 760 torr = 509 torr
PB = XBPBo = 0.33 x 256 torr = 84 torr
Ptotal = 509 torr + 84 torr =593 torr When the sum of these vapor pressures equals 760mm, (or whatever the external pressure may be), the solution boils. This occurs at a temperature which depends upon the composition of the solution and the boiling points of the individual liquids.
When the mixture boils, the vapor produced has a different composition than that of the boiling liquid. The vapor contains a higher concentration of the more volatile component. Since liquid A has a lower boiling point and a higher vapor pressure than liquid B at any given temperature, the vapor will contain a greater fraction of A molecules and a lesser fraction of B molecules. Cooling the vapor to the liquid state produces a liquid which has a higher mole fraction of A and a lower mole fraction of B. This new liquid boils at a lower temperature because of its new composition. If this process of heating and condensing is repeated many times, the vapor resulting from the final heating will be essentially pure A, the more volatile, lower-boiling component of the original solution.
Fractional distillation enables a large number of vapor-liquid equilibria to occur. Each of the many vaporization-condensation processes serves to enrich the distillate (substance condensed in the final phase) in the more volatile component. In practice, the mixture to be distilled is placed in a boiling flask along with several boiling chips. A distilling column, placed directly above the boiling flask, gives a large surface area in which the vapor-liquid equilibria may occur. Collection of the distillate and monitoring of the temperature involves use of the distilling head, thermometer, adapter, condenser and receiver.
Heat for the distillation is usually supplied by an electric heating mantle or by an electric sand bath. The bulb of the thermometer is placed just below the side-arm leading to the condenser. This insures correct temperature readings of the distillate. The condenser, usually supplied with running water, cools the vapors that reach the top of the distilling column and allows them to be collected in a collecting flask.
As the mixture in the distilling flask boils, a vapor forms directly above the liquid which is in equilibrium with the liquid. The vapor cools and condenses, dripping back toward the boiling liquid. As this condensed liquid drips back toward the boiling liquid, it is struck by fresh hot ascending vapor. The