Distillation

... A Separation Method

Background Concepts - Definitions

• Vapor Pressure – Gas pressure created by the molecules

of a liquid, which have acquired sufficient Kinetic Energy to escape to the vapor phase.

• As Temperature increases, the average Kinetic Energy and the Vapor Pressure increase until the boiling point is reached.

Background Concepts (definitions)

• Boiling Point – The temperature at which the vapor pressure of a liquid equals the pressure applied by the surroundings to the liquid.

• Boiling points for ethanol ...

Pressure = 760 torr B Pt = 78 oC

Pressure = 600 torr B Pt = 73 oC

Pressure = 10 torr B Pt = -2 oC

Background Concepts (Relationships)

• For two pure liquids, which have different boiling points…

.... the vapor pressure at a given temperature will be higher for the liquid of lower boiling point.

Background Concepts

• The temperature of the vapors above a boiling liquid will remain constant at the boiling point.

• At the boiling point for a pure liquid, the liquid and vapor (gas) phases are in equilibrium.

• The boiling point (at a specified pressure) is a characteristic property of a pure liquid.

Background – Raoult’s Law

• For an ideal solution with a volatile solvent and a non-volatile solute (e.g. water and salt) …

VPsolution = Xsolvent x VPsolvent

As you increase the concentration of the non-volatile solute, you decrease the vapor pressure of the solution and increase the boiling point.

… hence “Boiling Point Elevation”

So What ??

For an ideal solution with a volatile solvent and a non-volatile solute (e.g. water and salt) …

…separation of the two components is as simple as heating the mixture and collecting and condensing the vapor.

Background – Dalton’s Law

• For an ideal solution made of two volatile solvents, the total vapor pressure is the sum of the vapor pressures of the two solvents.

• For a solution of liquid ‘a’ and liquid ‘b’

VPtotal = Xa (VPa) + Xb (VPb)

Deviations from Dalton’s Law

• For non-ideal solutions, the combined vapor pressure may be either higher (positive deviation) or lower (negative deviation) than predicted by Dalton’s Law.

• When VP is higher, means BP is lower…

• When VP is lower, means BP is higher…

…than predicted by Dalton’s Law.

Deviations from Dalton’s Law• For non-ideal solutions, there is either strong

forces of attraction (negative deviation) or repulsion (positive deviation) between the molecules of the different components of the solution.– Attractive forces = components held together more strongly =

lower VP = requires more heat = higher BP

– Repulsive forces = components not held together = higher VP = requires less heat = lower BP

• A non-ideal solution may distill with a constant boiling point as if it were one pure substance = Azeotrope.

Positive Deviation

• Classic example: ethanol and water

Pure ethanol – B. Pt of 78 oC

Pure water – B. Pt of 100 oC

Mixture of ethanol (95 %) and water (5 %) distills with a constant B. Pt. of about 75 oC.

Background Concepts

• For an ideal solution of two miscible liquids of different boiling points, the composition of the liquid and vapor phases is not the same.

• The vapor will contain more of the liquid with the higher vapor pressure or lower boiling point.

(X’s and O’s represent percentage of each component at each level.)

XX

X XX

OO

OO

O

Po = 0.5 X 912 mmHg = 456 mmHgPx = 0.5 X 608 mmHg = 304 mmHg

temp 100

temp 90

760 mmHg

456/760 X 100 = 60% O304/760 X 100 = 40% X

O

OO

O

O

OXX

XX

X

X X

O OO

OO

OO

Po = 0.6 X 887 mmHg = 532 mmHgPx = 0.4 X 570 mmHg = 228 mmHg

760 mmHg

532/760 X 100 = 70% O228/760 X 100 = 30% X

O

O

O

O

O

O

OO X

X

Po = 0.7 X 867 mmHg = 608 mmHgPx = 0.3 X 507 mmHg = 152 mmHg

760 mmHg

608/760 X 100 = 80% O152/760 X 100 = 20% X

temp 80

Po = 0.8 X 855 mmHg = 684 mmHgPx = 0.2 X 380 mmHg = 76 mmHg

760 mmHg

684/760 X 100 = 90% O76/760 X 100 = 10% X

temp 70O

O

O

O

O

O

O

OO

X

Po = 0.9 X 833 mmHg = 750 mmHgPx = 0.1 X 100 mmHg = 10mmHg

760 mmHg

750/760 X 100 = 99% O10/760 X 100 = 1% X

temp 60O

O

O

O

O

O

O

OO

temp 50OO

essentially pure "O"

O

X

O

X

O X

Phase Diagram (Liquids A and B)

http://www.uwlas.edu/faculty/koster/Distillation305.htm

Terms and Definitions

• Theoretical Plate – one of the horizontal lines in the previous graph or 1 simple equilibration between the liquid and vapor phase.

Simple Distillation

http://www.uwlas.edu/faculty/koster/Distillation305.htm

(Use a 10-mL graduated cylinder to collect fractions)

Simple Distillation

http://www.uwlas.edu/faculty/koster/Distillation305.htm

Fractional Distillation

http://www.uwlas.edu/faculty/koster/Distillation305.htm

• Increasing the surface area that the vapors contact between the liquid and the condenser, increases the number of theoretical plates.

Phase Diagram (Liquids A and B)

http://www.uwlas.edu/faculty/koster/Distillation305.htm

Fractional Distillation

• Greater plates = greater purity of distillate = sharper transition in distillation plot

• The number of Theoretical Plates or the height equivalent to a theoretical plate (HETP) is a measure of the efficiency of a column to separate components.

Preparing your NB…

• It would be helpful to read pgs. 55-57 (“Simple Distillation, Miniscale Apparatus”) and pgs. 131-132 (in addition to Sec. 4.1 and 4.2)

• Figure 2.37‒(a) on pg. 56!• 2 chemicals to include in Table of Properties: ethyl

acetate & butyl acetate• No chemical reactions• Procedure – you can print the procedural steps that

are in the handout (on Moodle).

Results

After collecting the temperature data, prepare a graph of “Temperature (y-axis) vs. Volume (mL) of Distillate Collected (x-axis)” and put it in the Calculations section of your NB. (see slide #17 for a sample of what a good simple distillation looks like)

Remember – a graph must be drawn using either graph paper or a computer – It can not simply be “guesstimated” on the notebook page.

Discussion

• Results = Graph of “Temperature vs. Volume (mL) of Distillate collected”

– How similar does your graph look to the one that’s on slide #17?

– Ideally, you’d want the temperature transition (from low temp. to higher temp.) to be as sharp as possible in order to minimize collection of a mixture of ethyl and butyl acetates.

• Continue writing discussion as outlined in the syllabus.