Fractional Distillations

Topics: Distillation, Chromatography, Benzene Pages: 7 (839 words) Published: August 13, 2014
Sample Lab Report
Simple and Fractional Distillation
Unknown # 2
Purpose
In this experiment we aim to demonstrate that we can separate two volatile compounds from a mixture due to the different chemical properties of each compound. We will accomplish this by a separation procedure known as distillation, which relies on each compound having a distinct and separate boiling point. Our pure products will be analyzed with gas chromatography to determine the success of the distillation. Procedures

The experiment was performed as stated in the course textbook: Pavia, D. L., Lampman, G. M., Kriz, G. S., Engel, R. G. Introduction to Organic Laboratory Techniques: A Microscale Approach. 2007, 4th Ed. Pp 51—57. . Data

The distillation curves for our simple and fractional distillation (See page 3) clearly demonstrate that fractional distillation separates the two compounds more completely. The boiling point (bp) of our unknown compounds was taken from the flat regions of the fractional distillation curve. Our unknown mixture contained hexane (bp 69 ºC) and toluene (bp 110.6 ºC). Analysis via gas chromatography allowed us to determine the relative percentage of hexane and toluene at fractions near the beginning and end of our distillations. Relative percentages have been recorded in the table below, and our calculations are shown on page 5.

Simple Distillation
Retention time (s)
Fractional Distillation
Retention time (s)

Cyclohexane
Retention time (s)
Toluene
Retention time (s)
Cyclohexane
Retention time (s)
Toluene
Retention time (s)
Fraction 1
(4th mL)
15
25
14
24
Fraction 2
(17th mL)
14
26
16
25

Simple Distillation
Area (s)
Fractional Distillation
Area

Cyclohexane
Retention time (s)
Toluene
Retention time (s)
Cyclohexane
Retention time (s)
Toluene
Retention time (s)
Fraction 1
(4th mL)
15551
400
14444
248
Fraction 2
(17th mL)
222
16558
166
25995

A figure of the gas chromatography data has been included on page 4. Results and Calculations
Simple Distillation Fraction#1
Cyclohexane Total Area 15551 + 400 = 15951% Cyclohexane = 15551/15951 X100 =97.49232% %Toluene = 400/15951 X 100 = 2.5077 %
Simple Distillation Fraction#2
Cyclohexane Total Area 222 + 16558 = 16780% Cyclohexane = 222/16780 X100 =1.3230% %Toluene = 15951/16558 X 100 = 96.334 %
Fractional Distillation
This calculation was done the same as the Simple Distillation Fraction #1 (4mL)
%Cyclohexane 98.312%%Toluene 1.5548
Fraction #2 (17mL)
%Cyclohexane 0.63453%%Toluene 99.365%
Fractional Distillation
Calculation similar to Simple Distillation %
% Cyclohexane
% Toluene = 0.63453%
Conclusion
This week we utilized two methods of distillation (simple and fractional) to separate a mixture of two volatile compounds. We found that while the simple distillation separated the majority of the two compounds near the beginning and the end of the distilling process, fractional distillation produced much more pure fractions. In simple distillation the column was shorter, allowing less room for the two different compounds to fully separate. While heating the round-bottom flask the hexane molecules gain higher kinetic energy faster than the toluene molecules due to their lower molecular weight and lesser intermolecular forces. The longer fractional distillation column allowed the hexane molecules with higher kinetic energy to separate from the lower-energy toluene molecules. After viewing the gas chromatograph data from the fractional distillation we saw that our unknown compounds had almost completely separated, while the fractions from the simple distillation were less pure. Although distillation is a good separation technique it is still necessary to ensure that proper steps are taken to reduce the chances of error. Failure to add a boiling chip to the round-bottom flask could allow the mixture to heat unevenly. This could allow molecules with a higher...
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