Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
Purpose:
DCP 6
CE 5
To separate a mixture composed of 15 mL n-hexane and 15 mL cyclohexane by
employing a simple distillation method, and then graphing temperature versus the
changing volume of the distillate.
Background:
Simple distillation is a method of physically separating mixtures based on the
different volatilities of the mixture’s components. Volatility is the tendency of a
substance to vaporize. If a substance has a high volatility, it will therefore have a
high vapor pressure. By definition, a substance reaches its boiling point when the
vapor pressure, which acts on the surface of that substance, exceeds the
surrounding atmospheric pressure in magnitude. Therefore, we can say the more
volatile a substance is, the faster it will boil. In this experiment, n-hexane will be
mixed with cyclohexane, both in the liquid state. N-hexane has a boiling point of
69.0 ˚C while cyclohexane has a boiling point of 80.7 ˚C. To separate this mixture by
simple distillation, the following apparatus will be used.
The organic mixture is placed in the distillation flask with boiling stones and is
subject to heat. N-hexane will vaporize first, as it has the lower boiling point. The
vapor will travel through the condenser and transform back into the liquid state to
be collected by the receiving flask. To measure the amount of n-hexane being
collected, a graduated cylinder will be used instead of the receiving flask. Once all
the n-hexane is in the cylinder, the temperature should increase until cyclohexane
begins to vaporize. Cyclohexane will then condense as well, entering the graduated
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
cylinder as a liquid. If the temperature is measured with respect to the growing
volume in the graduated cylinder, a graph can be obtained that shows the relative
boiling points of the two components. The parts of the graph in which the slope
flattens (or tends to 0) describe be the boiling points of the two organic
components.
Source: General Chemistry 4th Edition Whitten, Gailey, Davis
Hypothesis:
If the simple distillation of mixture containing n-hexane and cyclohexane is
successful, then the trend line that describes the temperature versus the changing
volume of the distillate will be a polynomial of the 4th order. I expect this because
when a substance reaches its boiling point, the temperature of that substance stays
relatively constant (because of the enthalpy of vaporization) while the substance
changes phases. As we are dealing with two boiling points, I expect the temperature
to rise and flatten twice, which is indicative of four changes of slope on the graph,
and therefore indicating a 4th ordered polynomial equation.
Data Collection:
Boling point of n-hexane: 69.0 ˚C
Boiling point of cyclohexane: 80.7 ˚C
Source: ChemSpider
An organic mixture containing 15 mL of n-hexane and 15 mL of cyclohexane was
made. The components and their uncertainties are shown in Table 1.
Table 1 Title???
Volume of n-hexane
Volume of
cyclohexane
15.0 ± 0.1 mL
15.0 ± 0.1 mL
The components were mixed in the distillation flask with boiling stones and placed
in the apparatus. The heat was turned on and temperature was recorded against the
volume of the distillate. This data is shown in Table 2.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
Table 2 Title??
All Trials
V / ± 0.1 mL
Trial 1
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
70.0
70.5
71.0
71.0
71.0
71.0
71.5
72.0
72.0
72.5
73.0
73.5
74.0
75.0
75.0
Trial 2
Trial 3
T / ± 0.5 °C
70.5
70.5
71.0
71.0
71.0
71.0
71.0
71.0
71.5
71.5
72.0
72.0
72.0
72.0
73.0
72.0
73.0
72.5
74.0
73.0
75.0
74.0
76.0
74.5
76.0
75.0
76.0
77.0
76.5
77.0
Trial 4
71.0
72.0
72.0
72.0
73.0
73.0
73.0
73.0
74.0
74.0
74.0
76.0
77.0
77.0
78.0
Trial 5
71.0
72.0
72.0
72.0
72.0
72.0
73.0
73.0
73.0
74.0
74.0
75.0
76.0
78.5
78.5
Qualitative Data:
Both n-hexane and cyclohexane were clear organic liquids with very sharp and
poignant smells. The smell resembled that of isopropyl alcohol. The two organic
liquids mixed seamlessly to form a uniform mixture. The white, brittle boiling chips
were placed in the flask, and the flask was placed in a beaker of distilled water
above the hot plate. The heating process then started, and bubbles started to form
inside the distillation flask, indicating that the n-hexane had started to boil. Drops of
n-hexane started to form in the graduated cylinder quite rapidly, and the alcohol
smell permeated the classroom during this process. During the last 4-6 milliliters,
the rate at which the temperature was increasing started to slow down, probably
due to the small amount of organic liquid left in the distillation flask.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
Data Processing:
To create a temperature versus volume graph, the data in Table 2 will be averaged.
A sample calculation of how the data was averaged is shown below, using the
temperature values with respect to volume at 2 mL.
TV 2 

TT1  TT 2  TT 3  TT 4  TT 5
5
 TV 2 
(70.0  0.5C)  (70.5  0.5C)  (70.5  0.5C)  (71.0  0.5C)  (71.0  0.5C)
5
 TV 2 
353.0  2.5C
5
 TV 2 
353.0C 2.5C

5
5
 TV 2  70.6  0.5C
This mathematical process was repeated to get all average temperatures for each
volume. The average temperature of all trials recorded against volume is shown in
Table 3, which is shown on the next page.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
Table 3 Title???
V / ± 0.1
mL
Average T / ± 0.5 °C
2.0
70.6
4.0
71.3
6.0
71.4
8.0
71.4
10.0
71.8
12.0
72.0
14.0
72.3
16.0
72.6
18.0
72.9
20.0
73.5
22.0
74.0
24.0
75.0
26.0
75.6
28.0
76.7
30.0
77.0
The uncertainty of the volumes still reflects the precision of the graduated cylinder
used to measure the organic liquids. The uncertainty of the average temperature
still reflects the precision of the thermometer used. Table 3 will now be plotted on
an X-Y graph below.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
The R2 value indicates a very strong, 4th order polynomial regression. Both the
horizontal and vertical error bars reflect the precision of the apparatuses used to
measure temperature and volume. The horizontal error bars are not visible, as their
magnitudes are too small relative to the scale of the graph.
Knowing that the boiling points on the graph is represented by the slope tending to
0, we can make a good approximation of the experimental boiling point by
considering the lines marked in red on the graph. These lines represent the point at
which the slope of the graph was closest to 0.
Thus we get:
Experimental Boiling Point of n-hexane: 71.4 ˚C
Experimental Boiling Point of cyclohexane: 72.7 ˚C
I must stress that these values are visual approximations derived from the graph.
They have no mathematical backing.
Percent Error and Uncertainty:
There are two percent errors for this experiment, the percent error of the boiling
point of n-hexane, and the percent error of the boiling point of cyclohexane. The
calculations are shown below.
%e 
exp erimental  theoretical
 100
theoretical
%en hexane 
71.4  69.0
 100  3.5%
69.0
%ecyclohexane 

72.7  80.7
 100  9.9%
80.7
To find the percent uncertainty for this experiment, the worst-case scenarios from
Table 3 will be considered. The worst-case scenario value for the volume
measurements is 30.0 ± 0.1 mL and the worst-case scenario value for the
temperature measurements is 77.0 ± 0.5 °C. Therefore, the percent uncertainty for
the experiment is:
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
77.0  0.5C and 30.0  0.1mL considered
 %u  (
0.5
0.1
 100)  (
 100)
77.0
30.0
 %u  0.65%  0.33%
 %u  1%

Therefore, the percent error values are considerably larger than the percent
uncertainty value, and thus more systematic errors were present.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
Conclusion:
The purpose of this lab was to separate a mixture composed of 15 mL n-hexane and
15 mL cyclohexane by employing a simple distillation method, and then graphing
temperature versus the changing volume of the distillate. The purpose was achieved
with great precision, as the percent uncertainty for the entire lab was about 1% and
the regression coefficient of the 4th order polynomial trend line was 0.99598.
However, the lab is lacking in accuracy, as the percent errors for the boiling points
of n-hexane and cyclohexane, respectively, were 3.5% and 9.9%. Furthermore, these
values in themselves are not mathematically concrete, as they were obtained by
visual approximation of a graph. Needless to say, there was a lot of systematic error
in the lab.
Evaluation:
(1) One underlying weakness, a systematic error, of this lab was the fact that the
boiling points of n-hexane and cyclohexane were too close; 69.0 ˚C and 80.7 ˚C
respectively. This caused the temperature vs. volume curve to appear flatter in slope
than it should have, as there was not a precise moment in time where n-hexane
stopped boiling and when cyclohexane started boiling, it just appeared that the two
liquids continuously boiled.
(1)´ To improve this weakness, the two organic liquids used should have had
substantially different boiling points, with a minimum difference of 25 ˚C. This
would have yielded a graph with a steeper slope, as the temperature would have
increased more rapidly from the lower boiling point to the higher one, since they
would have been farther apart. The problem with this is that both organic liquids
must still have boiling points under 100 ˚C, as this is the temperature at which water
boils, and water was being used to heat the distillation flask.
(2) Another systematic weakness was that a small, 20 mL graduated cylinder was
used to collect the condensated distillate, even though the final volume of distillate
would amount to 30 mL. That means that we had to switch to another graduated
cylinder during the experiment after the first one was completely filled. This could
have obscured the accuracy of the temperature readings, as the volume was
measured in two different graduated cylinders.
(2)´ To fix this weakness, a graduated cylinder that can hold up to 30 mL of liquid
should be used. This way, the volume may be measured more accurately, as only one
apparatus would need to be used during the procedure. The entire distillation
apparatus should also be manipulated to work with the bigger graduated cylinder,
ensuring more accurate results.
Bernardo Gouveia
Mansur
IB Chemistry HL Y2
3/6/16
Distillation of an Organic Mixture
(3) A final limitation, which could be viewed as systematic as it relates to percent
error, is that there was no reliable % error value for this lab, as the way we
approximated the experimental boiling points was a mathematical crime.
(3)’ A solution that would fix this would, again, be to use two organic liquids that
differ to a greater magnitude in boiling points. This would have made the graph
easier to approximate, as the point at which the slope tends to 0 would be more
obvious. However, these systematic errors are in the nature of a simple distillation
experiment. We will never achieve the ideal slope of 0 because boiling points in real
life vary greatly and are more of a range than an exact value.