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FRACTIONAL DISTILLATION

THE BOILING POINT• The Boiling Point is the temperature at

which internal vapor pressure of the

liquid is equal to the pressure exerted

by its surroundings

• If the liquid is open to the atmosphere,

the boiling point is the temperature at

which the internal vapor pressure of the

liquid becomes equal to atmospheric

pressure (~760 mm Hg).

• The internal vapor pressure of a pure

liquid rises steadily as the temperature

is increased until the boiling point is

reached.

• P e -C/T

• The temperature remains constant

throughout the boiling process of a pure

liquid. At the boiling point, the liquid

and vapor are in equilibrium...if the

composition of each phase remains

constant, the temperature will remain

constant

In a Distillation Process a liquid is

heated to its boiling point, the vapors

expand out of the container and are

then cooled below the boiling point

temperature, where they recondense

as a liquid

THE TEMP/TIME RELATIONSHIP����

• A thermometer placed in the

vapor of a boiling pure liquid

registers the liquids boiling

point.

• The temperature remains

constant throughout the boiling

process of a pure liquid. At the

boiling point, the liquid and

vapor are in equilibrium...if the

composition of each phase

remains constant, the

temperature will remain

constant

• The temperature of a liquid

mixture AB, where BPA<BPB

will rise steadily over time. The

composition of the liquid and

vapor in equilibrium changes

constantly over time. At the

beginning the vapor contains

more A, at the end more B.

T

E

M

P

TIME or VOLUME

Boiling point of pure Liquid

Boiling Temperature Behavior of Pure Liquid

Boiling Temperature Behavior of Mixture A

and B where BP of A < BP of B

Boiling point of pure A

Boiling Point of Pure B

Raoult’s Law

• For a mixture of two miscible liquids (A and B), the total vapor pressure is the

sum of the individual vapor pressures:

Ptotal = PA + PB

where

PA = NAliquid x P˚A And PB = NB

liquid x P˚B

where

P˚A is the vapor pressure of pure A and P˚B is the vapor pressure of pure B

and

NAliquid is the mole fraction of A and NB

liquid is the mole fraction of B

where

NAliquid = moles A/moles A + B and NB

liquid = moles B/moles A +B

Vapor Enrichment

• From Raoult’s Law we can obtain the following relationships:

NAvapor = P˚A/PT

And

NBvapor = P˚B/PT

• If A is more volatile than B, BPA < BPB and P˚A > P˚B

Then

NAvapor > NA

liquid

• The result of this process is that when a mixture of two miscible liquids with

different boiling points is heated,the vapor will have a different composition than

the liquid. THE VAPOR IS ENRICHED IN THE MORE VOLATILE (LOWER

BOILING) COMPONENT.

Distillation Process

• When a mixture AB of a specific

composition is heated, the total

vapor pressure (composed of the

contributions of PA and PB) will rise

until it is equal to the external vapor

pressure. The mixture will begin to

boil.

• The vapor which first forms is

enriched in the more volatile

component. This behavior is shown

at right,

•Assume a two component mixture with a composition of 30%A:70%B (point W).

The boiling point of this mixture is found by drawing a vertical line from W to where

it intersects the lower curve (point X). A horizontal line drawn from X to where it

intersects the vertical axis (the temperature) gives the bp of composition W. From

the point (Y) where this horizontal line intersects the upper curve (vapor) drop a

vertical line to intersect the lower axis (the composition). Point Z gives the

composition of the vapor which is in equilibrium with a liquid of composition W at its

boiling point.

Liquid-Vapor Composition Diagram

Fractional DistillationAB at composition of 5% A boils at temperature L1 and the vapors with composition V1 enter the column at

that temperature. The vapor will condense to a liquid with composition V1. The condensate L2 has a

lower boiling point (because it has more of the lower boiling liquid A) and will thus vaporize at a lower

temperature (warmed up by coming in contact with the additional vapors from below) to give vapors of

composition V2. These vapors will condense somewhat farther up the column to give a condensate L3.

If the column is long enough or contains sufficient surface area that many successive vaporization-

condensation steps (theoretical plates) can occur, the distillate that comes over the top is nearly pure A.

Distillation yielding pure A continues until all of A is removed, after which the temperature at the

thermometer rises to the boiling point of B.

Distillation Setups

Fractional Distillation Set-up

Proper Thermometer Depth

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