Liquids: Simple Distillation, Fractional Distillation, and Gas Chromatography

I. Introduction

A. Objective

The purpose of this experiment is to compare the efficiency of simple distillation versus fractional distillation for separation of a mixture of toluene and cyclohexane. The mixture will first be separated using fractional distillation. A fractional distillation apparatus will be set up and a Thermowell heater containing sand will be used as the heat source. The mixture will be brought to a boil and slowly condensed. The temperature as a function of the number of drops will be recorded along with boiling point versus the number of drops distilled. Three fractions will be collected representing “pure cyclohexane”, changeover fraction, and “pure toluene”. These three fractions and the undistilled sample of the original mixture will then be analyzed using gas chromatography. A simple distillation apparatus will then be set up and a Thermowell heater containing sand will also be used as the heat source. The mixture will be slowly heated and condensed. The temperature as a function of the number of drops will again be recorded. Only one fraction will be collected for the simple distillation.

B. Materials and Safety

Chemical Name	Molecular Formula	Molecular Weight (g/mol)	Liquid		Solid	Solubility	Potential Hazards
Chemical Name	Molecular Formula	Molecular Weight (g/mol)	b.p. ºC	Density g/mL	m.p. ºC	Solubility	Potential Hazards
Cyclohexane	C₆H₁₂	84.16	80.74	0.779	6.55	Immiscible	Flammable, eye irritant, harmful to environment
Toluene	C₇H₈	92.14	110.6	0.86	-95	Insoluble	Flammable, irritant

C. Experimental Procedure

Fractional distillation will take place first. A 10 cm column will be packed with steel sponge. Then 3 mL of the unknown mixture of cyclohexane and toluene will be added to the 5 mL short-necked flask along with several boiling chips. The unknown mixture given will be recorded. The fractional distillation apparatus will be set up with a cooled vial as a receiver. A Thermowell heater containing sand will be used as the heat source. The mixture will be brought to a boil over the hot sand bath. When the vapor reaches the thermometer bulb, heat will be reduced by shifting some of the sand. The distillation is to take place at a rate no faster than five drops per minute. The temperature as a function of the number of drops will be recorded along with boiling point versus the number of drops distilled. Three separate fractions will be collected representing “pure cyclohexane”, changeover fraction, and “pure toluene”. The receiver will be changed for the first time after the temperature begins to rise after a plateau. The receiver will be changed for the second time after the temperature begins to plateau after the rise. The distillation will be stopped when about 0.3 mL of mixture remains in the flask.

Next, gas chromatography will be used to analyze the three fractions from fractional distillation along with an undistilled sample of the original mixture. The procedure for this part of the experiment will be described during the laboratory lecture.

Finally, simple distillation will take place. A simple distillation apparatus will be set up will a cooled vial as a receiver. A 3 mL sample of the unknown mixture of cyclohexane and toluene will be added to the 5 mL short-necked flask along with several boiling chips. A Thermowell heater containing sand will again be used as the heat source. As the vapor reaches the thermometer bulb, the heat will be reduced by shifting some of the sand. The distillation will take place at a rate no higher than five drops per minute. The temperature as a function of the number of drops will be recorded along with boiling point versus the number of drops distilled. Distillation will be stopped when about 0.3 mL of mixture remains in the flask.

II. Experiment and Results

A. Data

First, a simple distillation took place. Four mL of an unknown mixture of cyclohexane and toluene labeled “unknown A” was dispensed into a 5 mL short-necked flask along with a few boiling chips. A simple distillation apparatus was set up using a sand bath as the heating source. A water cooled condenser was used to cool and condense the gas into a vial. The temperature was recorded as after each drop of the liquid into the vial. Distillation was ceased when there was only about 0.3 mL of unknown remaining in the flask.

Next a fractional distillation took place. Four mL of “unknown A” was again dispensed into a 5 mL short-necked flask along with a few boiling chips. A fractional distillation apparatus was set up using a sand bath as the heating source. The 10 cm column was fairly loosely packed with steel sponge. A water cooled condenser was used to cool and condense the gas into a vial. The temperature was recorded after each drop of the liquid into the vial. The temperature rose and plateaued at first, and after it started rising again a new vial was used to collect the liquid. After the temperature began to plateau for a second time, a third vial was used to collect the liquid. Distillation was stopped after there was only about 0.3 mL of unknown left in the flask. The three fractions collected were then analyzed and graphed using gas chromatography, which was performed by Dr. Murray.

Simple Distillation:

Drop #	Temperature (ºC)	Drop #	Temperature (ºC)	Drop #	Temperature (ºC)	Drop #	Temperature (ºC)
1	76.5	26	84	51	91	76	101
2	77.5	27	84.5	52	91.5	77	101.5
3	78	28	84.5	53	91.5	78	102
4	78.5	29	85	54	92
5	79	30	85	55	91.5
6	79.5	31	85	56	93
7	80	32	85.5	57	94
8	80	33	86	58	93
9	80.5	34	86	59	92
10	80.5	35	86	60	91.5
11	81	36	86.5	61	93
12	81	37	86.5	62	94
13	81	38	87	63	94.5
14	81.5	39	87	64	94.5
15	81.5	40	87.5	65	94
16	82	41	88	66	95.5
17	82	42	88.5	67	96.5
18	82	43	89	68	98
19	82.5	44	89.5	69	98.5
20	82.5	45	90	70	99
21	83	46	90	71	99
22	83	47	90	72	99.5
23	83	48	91	73	100
24	83.5	49	91	74	100
25	84	50	91	75	100.5

Fractional Distillation:

Drop #	Temperature (ºC)	Drop #	Temperature (ºC)	Drop #	Temperature (ºC)
1 (Fraction A)	51.5	26	75	51	91
2	56	27	74	52	92
3	59	28	74	53	93
4	61	29	74	54	94
5	62	30	74	55	94.5
6	64	31	73.5	56	95
7	64.5	32	74	57	95.5
8	65	33	75	58	96
9	66.5	34	76	59	96
10	67	35	77	60	96
11	67.5	36	77	61	96
12	68	37	77	62 (Fraction C)	97
13	69	38	76	63	97
14	70.5	39	75	64	97.5
15	71	40	77.5	65	97.5
16	71.5	41	78	66	98
17	72	42	78	67	98
18	72.5	43	79	68	98.5
19	73	44	81	69	98.5
20	71.5	45 (Fraction B)	83	70	98.5
21	73	46	84.5	71	98.5
22	74.5	47	86.5	72	98
23	75	48	88	73	97.5
24	75	49	89	74	96.5
25	75	50	90	75	94

	Volume (Drops)
Fraction A	44
Fraction B	17
Fraction C	14

	Percentage Cyclohexane	Percentage Toluene
Initial Composition	46.1%	53.9%
Fraction A	73.4%	26.6%
Fraction B	5.5%	94.5%
Fraction C	0.28%	99.72%

B. Graphs

$fractional distillation$

$comparison of simple and fractional distillation$

III. Conclusions

It appears that the fractional distillation was more efficient than the simple distillation. The simple distillation graphed basically as a straight line, but the graph of the fractional distillation actually somewhat shows the plateau where mostly cyclohexane is being condensed and then the rise and second plateau where mostly toluene is being condensed. This can not be really seen in the graph of simple distillation. This may be due in part that the simple distillation was heated too fast. I know that for the fractional distillation it seemed like we heated it at a very steady rate, but the simple distillation was harder to control. This may be why our data did not plot so well.

However, in theory the fractional distillation should be more efficient because of the steel sponge. The sponge acted as surface area for gas to condense on. This prevented some of the toluene from condensing into the vial during fraction A because it would condense onto the sponge and drip back down into the flask. The boiling point of toluene is higher than the boiling point of cyclohexane, so the cooler surface of the sponge helped condense toluene, but the cyclohexane still made it to the vial because of its lower boiling point. The sponge helped improve efficiency of the distillation. The time required for fractional distillation was greater than the time needed for simple distillation, but it was a much more accurate distillation.

The initial composition of the unknown was 46.1% cyclohexane and 53.9% toluene. For fraction A, we ended up with a mixture of 73.4% cyclohexane and 26.6% toluene. I feel that this is a decent result, but it could have been a little better. I think we probably should have switched the vials a few drops earlier, which would have left us with a higher percentage of cyclohexane and lower percentage of toluene. Fraction B ended up with a mixture of 5.5% cyclohexane and 94.5% toluene. The mixture should have ended up more around 50% of each, so this means that we should have switched from vials between fraction B and fraction C sooner. Toward the end of fraction B, pretty much pure toluene was being condensed into the vial, which is probably what threw our numbers off so much. For the final fraction, the mixture ended up being 0.28% cyclohexane and 99.72% toluene. This means that basically pure toluene was all that was left, which is what we were hoping for during the last fraction.

We collected 44 drops for fraction A, 17 drops for fraction B, and 14 drops for fraction C. This does not seem proportional considering the original composition was 46.1% cyclohexane and 53.9% toluene. It should have ended up being about an equal volume for fraction A and fraction C, with a lower volume of fraction B. Something like 27 drops for fraction A, 15 drops for fraction B, and 33 drops for fraction C would have been ideal.

Overall I feel that what probably cause the most error in this lab was getting the right heat for the sand bath and discerning when to switch vials during fractional distillation. It took a long time before the temperature rose at all, so we could not tell if we were heating the sand bath enough. If we turned the temperature up too much, then the temperature would shoot up when it did begin to rise and that would make it hard to get accurate data. When switching the vials, it seemed that the temperature would hover in a 2 to 3 degree when it plateaued, then it would begin to rise, and I think we let the temperature rise a few degrees too many before switching vials. It was difficult to tell if it was done plateauing or rising.