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Lab 2: Distillation

Elizabeth Bellizio Tyler Hamby Lisa Nguyen

August 30, 2012

Introduction Distillation is the technique used to separate and purify liquid mixtures. There are several different kinds of distillation. All distillations involve vaporization and condensation (Hill, T4-1). In distillation, the liquid mixture is heated to a boiling point where the one compound’s vapor pressure is equal to the external pressure over the liquid. The vapors above boiling point can be removed, condensed, and collected as a liquid distillate. Simple distillation is used to separate liquids whose boiling points are around 100 °C (Hill, T4-2) apart and is also used to separate pure liquids from nonvolatile impurities. To complete a successful simple distillation, the liquid mixture is placed in a distillation flask which is fitted with a distillation head. A condenser is attached to the distillation head and leads to a receiver. The liquid is heated until boiling point. The vapors rise through the distillation head and enter the condenser. The condenser’s walls should be cooled by a jacket of cool water so the vapors will condense back to a liquid. This collected liquid is a distillate. In the following experiment, simple distillation was used to separate methanol and water. Fractional distillation is used to separate liquids whose boiling points are closer together, approximately 25 °C apart (Hill, T4-4). To complete a successful fractional distillation a fractional column packed with steel wool must be added to a simple distillation apparatus. In this experiment, copper turnings will be used. The packed column increases the surface area through which the vapors must pass, forcing the vapor to repeatedly condense and re-vaporize (Hill, T4-4). The more this happens, the more micro-distillations occur, thus allowing the distillate to be as pure as possible. The efficiency of the fractionating column depends on the column’s length and surface area and can be measured in theoretical plates (each plate is equal to one simple distillation). The more theoretical plates used, the better the separation of the two liquids (Hill, T4-6). In the following experiment, fractional distillation was used to also separate methanol and water. A second fractional distillation was performed in order to identify the two compounds in an unknown solution. Table of Physical Constants Name

Chemical Formula

Molecular Weight (g/mol)

Boiling Point (°C)

Melting Point(°C)

Density (g/ cm3)

Acetone

C₃H₆O

58.042

56-57

94

0.773

Chloroform

CHCI₃

119.378

61

63

1.5

Cyclohexane

C₆H₁₂

84.160

81

6.5

0.791

Ethanol

C₂H₆O

46.068

78

130

0.78

Heptane

C₇H₁₆

100.202

98

91

0.695

C₄H₈O₂

88.105

77

84

0.899

CH₄O

32.042

65

98

0.753

H₂O

18.015

100

0

0.999

Ethyl Acetate Methanol Water

Table 1: Table of Reagents Experimental The fractional distillation of the MeOH and water solution was begun by mixing methanol and water together in a 2:3 ratio. 2 mL MeOH was added to 4 mL of water in a long neck flask. Two boiling chips were added to the flask. The micro-scale distillation apparatus was assembled in the fume hood. A heating mantel with a sand bath was prepared as part of the distillation apparatus. The fractioning column was carefully packed with copper turnings. The flask containing the solution was completely submerged in the sand bath and connected to the fractioning column. A mercury thermometer was used to record the temperature during the distillation. The collecting flask was placed in an ice bath. The heating mantel was turned on and set to 30 % of maximum heating capacity. The apparatus was continuously observed until drops of distillate began dripping into the collecting flask. The temperature was recorded for each drop. After the fractional distillation of the MeOH and water solution was completed, the equipment was cleaned thoroughly The fractional distillation of unknown solution #4598 followed the same procedure as the first distillation. The distillation apparatus was rinsed out with some of the unknown solution. The remaining 4 mL of unknown was added to the distillation flask. Two boiling chips were then added to the flask of unknown solution. The fractioning column was packed with copper turnings. The flask was carefully and completely submerged in the sand bath and the heating mantel was set to 30 % of maximum heating capacity. The collecting flask was placed in an ice bath. A mercury thermometer was used to record the temperature of the distillation. The apparatus was again observed. The temperature at which each drop of distillate fell into the collecting flask was recorded. After the temperature of each drop was recorded, the distillation apparatus was dissembled and cleaned. The recorded data for the simple and fractional distillations of the MeOH and water solution was then exchanged with other students.

Results The simple distillation of the MeOH and water solution produced drops of water at temperatures ranging from 58 ˚C and 98 ˚C. The two plateaus observed for this distillation were at 68 °C and 98 °C. The fractional distillation of the MeOH and water solution produced drops at temperatures ranging from 65 ˚C and 98 ˚C and the two plateaus were at 66 °C and 98 °C. The fractional distillation of the unknown solution #4598 produced drops of liquid at temperatures ranging from 66 ˚C and 95 ˚C. The plateaus were observed at 68 °C and 95 °C.

58

68

68

68

72

94

98

60

68

68

68

73

94

98

62

68

68

69

74

96

98

64

68

68

70

74

98

64

68

68

70

76

98

66

68

68

70

76

98

66

68

68

71

79

98

68

68

68

72

90

98

Table 1: Temperature of Drops of Distillate during Simple Distillation of Methanol and Water Solution

Simple Distillation of Methanol and Water Mixture 100 95 Temperature (°C)

90 85 80 75 70 65 60 55 50 0

5

10

15

20

25

30

35

40

45

50

Volume (Number of Drops of Distillate)

Figure 1: Simple Distillation of Methanol and Water Mixture

65

65

66

66

94

55

65

65

66

66

98

65

66

66

66

98

65

66

66

66

98

65

66

66

66

98

65

66

66

66

98

65

66

66

66

98

65

66

66

66

Table 3: Temperature of Drops of Distillate during Fractional Distillation of Methanol and Water Solution

Fractional Distillation of Methanol and Water 100

Temperature (C)

95 90 85 80 75 70 65 60 0

10

20

30

40

Volume (Number of Drops of Distillate)

Figure 2: Fractional Distillation of Methanol and Water

66

68

68

69

70

72

74

76

78

80

86

94

95

67

68

68

70

71

72

75

77

78

80

88

94

95

67

68

68

70

71

73

75

77

79

81

89

94

95

67

68

68

70

71

73

75

77

79

82

90

95

95

67

68

68

70

71

73

76

77

79

83

92

95

95

68

68

68

70

72

73

76

77

80

83

93

95

68

68

69

70

72

73

76

77

80

84

93

95

68

68

69

70

72

74

76

78

80

86

94

95

Table 4: Temperature of Drops of Distillate during Fractional Distillation of Unknown Solution #4598

Fractional Distillation of Unknown #4598

Temperature (˚C)

100

91

83

74

65 0

28

55

83

Volume (Number of Drops of Distillate)

Figure 3: Fractional Distillation of Unknown Solution #4598

110

Discussion The results of the simple and fractional distillation lab are as expected. For the simple distillation of the methanol-water mixture, the two plateaus observed were at 68 °C and 98 °C. The distillation occurred at 68 °C then increased sharply to 98 °C. The first plateau indicates the boiling point of the compound with a lower boiling point. Therefore, after the simple distillation of the methanol-water mixture, it would appear that the boiling point of methanol is 68 °C. The accepted literature value for the boiling point of methanol is 65 °C.

The simple distillation experiment results had 4.6% error. This is a low percent error. The error may have been because the variance was turned up too high and the thermometer was recording temperatures that were higher than they should have been. This higher heat may also have caused the distillation to occur too quickly. The second plateau in the simple distillation experiment was at 98 °C, implying that the boiling point of water is 98 °C. The accepted literature value for water’s boiling point is 100 °C.

The 2% error for the fractional distillation is very low. The observed differences may have been caused by the frigid temperature of the laboratory, possibly affecting the reading of thermometer. Another reason might have been that the thermometer was not placed low enough in the distillation flask so it was not reading the most accurate temperature of the distilling vapors. The fractional distillation for the methanol-water mixture results were more accurate than the results from the simple distillation, which was to be expected. The distillation occurred steadily at 66 °C before distillate began to form at 98 °C. This first plateau would imply that the boiling point of methanol is 66 °C. As previously stated, the accepted boiling point of methanol is 65 °C.

The recorded 1.5% error in this fractional distillation is very low. These results would imply that the fractioning column used in this experiment was very efficient. The second plateau in this fractional distillation was recorded at 98 °C for the boiling point of water. This means there is, again, a 2% error.

The error may have been caused by a poorly assembled apparatus in which the thermometer was not appropriately placed to record the most accurate temperatures of the distilling vapors. Comparing the results from these two distillations demonstrate the higher accuracy of fractional distillations over simple distillations. Though the observed boiling point of water remained the same (and very close to the accepted value), the boiling point of methanol was observed to much closer to the accepted value during the fractional distillation than during the simple distillation. The copper turnings in the fractioning tube are indeed efficient at forcing the distilling vapors to recondense and re-vaporize for more accurate data. In the fractional distillation for unknown solution #4598, the temperatures recorded for the distillate ranged from 66 °C to 95 °C. The distillation occurred steadily at 68 °C for 17 drops before it began increasing steadily and finishing off at 95 °C for 10 drops. These results imply that the unknown solution #4598 consists of chloroform and heptane. The accepted literature value for the boiling points of chloroform and heptane are 61 °C and 98 °C, respectively.

The 11% error for the boiling point estimation of chloroform is very high. This may be because the variance was set too high and the heating occurred too rapidly. This may have caused the distillation to occur too quickly with vapors that were too hot, thus distorting the thermometer readings. The 3.1% error for the boiling point of heptane is not very high. The lower recorded boiling point may have been caused by the frigid temperatures of the laboratory. Also, the thermometer may have been shifted during the experiment such that the bulb was not low enough in the apparatus to obtain more accurate temperature readings of the distilling vapors. Based on this fractional distillation, unknown solution #4598 had approximately a 2:1 ratio of chloroform to heptane.

Conclusion In the simple distillation, the observed boiling points for methanol and water were 68 °C and 98 °C, respectively. In the fractional distillation, the observed boiling points were 66 °C and 98 °C. The reported literature values for the two compounds are 65 °C and 100 °C, respectively. The data from the fractional distillation of unknown solution #4598 suggests that the solution is composed of chloroform and heptane in a 2:1 ratio. References Richard k. Hill and John Barbaro, Experiments in Organic Chemistry, 3rd ed; Contemporary Publishing Co. of Raleigh, Inc., 2005; T4-1,T4-2,T4-3,T4-4,T4-5,T4-6,E2-1,E2-2 Questions 1. Briefly define the following terms: a. bumping: an instantaneous, violent expulsion of vapor from the liquid b. azeotrope: a mixture of two liquids in which the volatility of each component is affected by the presence of the other component c. theoretical plate: equivalent to one simple distillation; a unit used to measure the efficiency of a fractionating column d. Raoult’s Law: at a given temperature, the partial vapor pressure of a component (e.g., PA) of the mixture is equal to the vapor pressure of that pure component (P°A) multiplied by its mole fraction (C) in the liquid (where the mole fraction of compound A, CA, is the number of moles of component A in the mixture divided by the total number of moles in all the components of the mixture):

2. During the distillation of a mixture of two miscible liquids with very different boiling points, the temperature rises to a constant reading and then drops off before returning to a higher reading. Explain the likely cause of this fluctuation in temperature readings. When the component of the mixture that has the lowest boiling point has completely evaporated and the temperature has not yet reached the boiling point of the other component, the temperature will decrease due to loss of condensation. 3. What effect does (a) decreasing the pressure or (b) increasing the pressure above a liquid have on the liquid’s boiling point? a. By decreasing the pressure above a liquid, its boiling point can be lowered. b. By increasing the pressure over a liquid, its boiling point can be raised. 4. Explain why flooding the fractioning column will lead to a poor separation in a fractional distillation. The mixture will travel through the column too quickly, which will cause fewer theoretical plates to form. This will cause the results to be off because the distillate will not be a pure liquid. 5. Calculate the mole fraction of each compound in the following mixtures: a. 100 g water and 100 g of acetone Water – 100 g H2O Molecular weight = 18.0 g / mol

Acetone – 100 g C3H6O Molecular weight = 58.1 g / mol

b. 2.25 g of ethanol, 34 g of methanol, and 19 g of 2-propanol Ethanol – 2.25 g C2H6O Molecular weight = 46.1 g / mol

Methanol – 34 g CH4O

Molecular weight = 32.0 g / mol

2-propanol – 19 g C3H8O Molecular weight = 60.1 g / mol

6. At 25 °C the vapor pressure of acetone is 230 mm Hg and the vapor pressure of toluene is 284.5 mm Hg. What is the total vapor pressure at 25 °C above an equimolar mixture of these two miscible liquids?

7. At 80 °C the vapor pressures of pure liquids X, Y, and Z are 385, 190, and 66 mm Hg, respectively. What pressure must be exerted on the surface of a solution containing one mole of X, three moles of Y, and two moles of Z for boiling to begin at 80 °C?

8. A mixture of miscible liquids containing 6 moles of A and 4 moles of B is distilled at 1 atm (760 mm Hg) of pressure. The initial distillate was analyzed and found to contain 20 mole percent A and 80 mole percent B. a. What are the initial vapor pressures of A and B in the initial mixture?

b. What are the vapor pressures of pure A and B?...