Download - Experiment 6: Fractional Distillation Reading Assignment –Experiment 6 (pp. 58 -64) –Operation 29
Key Point!
• When conducting a distillation, the vapor should be richer in the lower boiling component than what you started with.
Temperature Behavior During Distillation
A. Single pure componentB. Two components of similar boiling pointsC. Two components with widely different boiling points
Questions based upon the previous slide:
a) What is the bp of pure A?b) What is the bp of pure B?c) What is the bp of a solution with the composition of 30 % B, assuming a simple distilllation apparatus?d) What is the composition of the vapor assuming a
simple distillation apparatus?e) What is the composition of the distillate collected
assuming a simple distillation apparatus?f) What does the “tie-line,” x-y represent? Hint: the upper curve is the vapor curve and the lower curve is the
liquid curve. “Composition of the vapor and liquid that are in
equilibriuim with each other at 130 oC.”
Questions based upon the previous slide:
a) What is the bp of pure toluene?b) What is the bp of pure benzene?c) What is the bp of a solution with the composition of 50 % benzene, assuming a simple distilllation
apparatus?d) What is the composition of the distillate assuming a simple distillation apparatus?e) How many theoretical plates would be necessary for a fractional distillation starting with a 50 % benzene
solution?
When will simple distillation do a reasonable job of separating a mixture?
1) When the difference in boiling points is over 100o
2) When the there is a fairly small amount of impurity, say less than 10 %.3) When one of the components will not distil because of a lack of volatility (i.e. sugar dissolved in water).
How many theoretical plates are need to separate a mixture
starting at L?
• Looks like about 5 plates are needed to separate the mixture on the previous slide!
• Count the “tie-lines” (horizontal lines) to come up with the 5 plates (labelled with arrows on the next slide)!
Theoretical Plates Required to Separate Mixtures based on
BPBoiling Point Difference Theoretical Plates
108 172 254 343 436 520 1010 207 304 502 100
Azeotrope
• Some mixtures of liquids, because of attractions or repulsions between the molecules, do not behave ideally
• These mixtures do not obey Raoult’s Law• An azeotrope is a mixture with a fixed
composition that cannot be altered by either simple or fractional distillation
• An azeotrope behaves as if it were a pure compound, and it distills from beginning to end at a constant temperature.
Types of Azeotropes
• There are two types of non-ideal behavior:– Minimum-boiling-point
• Boiling point of the mixture is lower than the boiling point of either pure component
– Maximum-boiling-point• Boiling point of the mixture is higher than
the boiling point of either pure component
Observations with maximum boiling azeotrope
On the right side of the diagram:Compound B will distill (lowest bp). Once B has been removed, the azeotrope will distill (highest bp).
On the left side of the diagram: Compound A will distill (lowest bp) Once A has been removed, the azeotrope will distill. (highest bp)
The azeotrope acts like a pure “compound”
Observations with minimum boiling azeotrope
On the right side of the diagram:The azeotrope is the lower boiling “compound,” and it will be removed first. Pure ethanol will distill oncethe azeotrope has distilled.
On the left side of the diagram: the azeotrope is the lower boiling “compound,” and it will distill first. Once the azeotrope has been removed,then pure water will distill.
The azeotrope acts like a pure “compound”
Gas Chromatography: Results
In a modern gas chromatography instrument, the results are displayed and analyzed using a computerized data station. It is no longer necessary to calculate peak areas by triangulation; this determination is made electronically.
Our analysis will be conducted on a modern data station.
Compounds in mixture: boiling points.
Cyclohexane 80 oC
Toluene 110 oC
Mixture separates by distillation according to the boilingpoint. Compounds with the lower bp come off first! The same is true on the gas chromatographic column; the lower boiling compound comes off first!
How to identify the components in your unknown
mixture
Use the retention time information from your gas chromatograms to provide a positive identification of each of the components in the mixture.
Don’t rely on the distillation plot to determine the composition of your mixture!
Calculation of percentages from the data for fraction 2
area counts/response factor = adjusted area
Cyclohexane area = 42795/1.133 = 32104Toluene area = 18129/1.381 = 13127Total area 45231
Note: this calculated area is different than thatshown on the data sheet! Use this calculated area!
Percent cyclohexane = 32104/45231 x 100 = 71.0%Percent toluene = 13127/45231 x 100 = 29.0 %
Round off numbers so that the total equals 100%
Calculation of percentages from the data for fraction 4
area counts/ response factor = adjusted area
Cyclohexane area = 57546/1.133 = 43170Toluene area = 191934/1.381 = 138981Total area 182151
Note: this calculated area is different than thatshown on the data sheet!
Percent cyclohexane = 43170/182151 x 100 = 23.7 %Percent toluene =138981/182151 x 100 = 76.3 %
Round off numbers so percentage = 100%