2019-organic chemistry lab1(chem223)-lab three PDF

Preview

Summary

2019-organic chemistry lab1(chem223)-lab three...

Description

Lab #3: Distillation Introduction In general, the process of distillation allows a mixture to be separated and purified based on the different boiling points of its components. Distillation starts with the heating of a liquid mixture and follows the transition of liquid into vapor. Since the vapors are influenced by volatility, the component of the liquid mixture that is higher in volatility at a given point in time will be more concentrated in the vapor that will be condensed (Pavia 38). The volatility of the components allows for the identification of a component through temperature changes. This experiment utilized simple and fractional distillation to allow us to determine the percent composition of methanol and water in an unknown sample. While simple distillation works faster and more effectively for liquids with a boiling point difference of 100 degrees Celsius or more, fractional distillation takes longer and works better for liquids with a boiling point difference of 30 degrees celsius or more (Domzalski, Lab #3 - Distillation).

Methanol:

Methods/Procedures (Explained in Pre-Lab) Results/Observations/Analysis of Results Unknown M (Experimental Simple Distillation)

mL

Temperature (Degrees Celsius)

1

80

2

80

3

80

4

81

5

82

6

82

7

84

8

85

9

87

10

88

11

89

12

92

13

95

14

97

15

100

16

100

17

100

Unknown M (Experimental Fractional Distillation) mL

Temperature (Degrees Celsius)

1

67

2

69

3

70

4

62

5

64

6

64

7

64

8

69

9

90

10

90

11

100

12

100

13

100

Discussion/Conclusion Graphs from experimental data:

Percent Composition of Unknown M Fractional Distillation

Methanol:

Water:

9𝑚𝐿 60 𝑚𝐿

51𝑚𝐿 = 60 𝑚𝐿

= 0.15 × 100 = 15% methanol

0.85 ×100 = 85% water

From our experimental data, the values at the beginning of fractional distillation started off with 67°C and rose up to 70°C before it started to decrease to 62°C and settled at the boiling point of methanol, 64°C. Due to the high initial temperature readings, this can indicate a source of error due to the fact that we heated up the heating mantle too high in the beginning due to the time constraint for fractional distillation. This source of error could have also altered the results of our percent compositions, because according to the procedure, the apparatus should have been heated slowly to ensure proper separation. If the component of the mixture that has a lower boiling point will not have enough time to boil, this can skew the results. Another source of error could have been the placement of our thermometer, as it could have been placed too low in the apparatus. This placement could have displayed a higher temperature that was not correct. Despite these outliers of the fractional distillation data, the general shape of the graph is similar to the theoretical graph, as the apparatus displayed the temperature at a constant of 64°C after the inaccurate high temperature readings in the beginning, displayed a drastic increase in temperature to 90°C and then heated to 100°C, which is the boiling point of water. Fractional distillation was able to give us a rigid distinction between methanol and water due to the fact that it is meant for solutions with components that have a boiling point difference of ≥30°C. As this procedure is more precise, we were able to conclude that methanol concluded its vaporization at the 9 mL mark, as at this data point, the temperature drastically increased to 90°C. Since we concluded that 9 mL of our 60 mL solution was methanol,

the remainder, 51 mL, that heated to 100°C, was concluded to be water. Based on this the percent composition of methanol and water was 15% and 85%, respectively. The simple distillation experimental data on the other hand did not require as much time for the procedure, so the heating mantle was gradually heated and there were no significant outliers in the temperature data. The graph that was made from our data follows the general trend of the theoretical graph, but due to the fact that simple distillation is performed for mixtures where the components have a boiling point difference of ≥100°C, it did not show a rigid separation of methanol and water, like fractional distillation. The boiling point difference between methanol and alcohol is 36°C, so a process like simple distillation which is made for separating components that have higher boiling point differences will not be as effective and precise as fractional distillation. Simple distillation provided a general linear trendline for where the temperatures were increasing. Despite the differences between the fractional and simple distillation, they both displayed the increasing temperature as the components were separated, but with different precision, as fractional distillation allowed for the calculation of the percent composition. Post Lab Questions Pavia 4th Edition Questions 1. In the accompanying chart are approximate vapor pressures for benzene and toluene at various temperatures.

a. What is the mole fraction of each component if 3.9 g of benzene (𝐶6𝐻6) is dissolved in 4.6 g of toluene (𝐶7𝐻8)? 3.9 𝑔𝑟𝑎𝑚𝑠 78.1 𝑔𝑟𝑎𝑚𝑠

i.

Benzene =

ii.

Toluene =

iii.

Mole Fraction of Benzene =

iv.

Mole Fraction of Toluene =

4.6 𝑔𝑟𝑎𝑚𝑠 92.1 𝑔𝑟𝑎𝑚𝑠

= 0.049

= 0.049 0.049 𝑔𝑟𝑎𝑚𝑠 0.098 𝑔𝑟𝑎𝑚𝑠

0.049 𝑔𝑟𝑎𝑚𝑠 0.098 𝑔𝑟𝑎𝑚𝑠

= 0.5

= 0.5

b. Assuming that this mixture is ideal, that is, it follows Raoult’s Law, what is the partial vapor pressure of benzene in this mixture at 50°C? i.

𝑃𝑏𝑒𝑛𝑧𝑒𝑛𝑒= (mole fraction of benzene ×vapor pressure of benzene at 50°C )

ii.

(0.5 ×270) = 135 mm Hg

c. Estimate to the nearest degree the temperature at which the vapor pressure of the solution equals 1 atm (bp of the solution). i.

Total Pressure of Solution at 100°C = (1340 mm Hg × 0.5) + (560 mmHg×0.5) = 950 mm Hg

1. This answer shows that at around 100°C the solution would boil because the solution’s total pressure is greater than the atmospheric pressure d. Calculate the composition of the vapor (mole fraction of each component) that is in equilibrium in the solution at the boiling point of this solution. i.

Mole fraction of benzene =

𝑃

𝑏𝑒𝑛𝑧𝑒𝑛𝑒

𝑃

𝑡𝑜𝑡𝑎𝑙

ii.

=

135 𝑚𝑚 𝐻𝑔 950 𝑚𝑚 𝐻𝑔

= 0.14

Mole fraction of toluene = 1 - 0.14 = 0.86

e. Calculate the composition in weight percentage of the vapor that is in equilibrium with the solution i.

Pressure of Benzene = Vapor pressure of Benzene × Mole fraction of Benzene 1. 120 mm Hg × 0.5 = 60 mm Hg

ii.

Pressure of Toluene = Vapor pressure of Toluene × Mole fraction of Toluene 1. 37 mm Hg × 0.5 2. 18.5 mm Hg

iii.

Benzene % composition =

iv.

Toluene % composition =

60 78.5

18.5 78.5

× 100 = 76.4%

× 100 = 23.6%

2. Estimate how many theoretical plates are needed to separate a mixture that has a mole fraction of B equal to 0.70 (70% B) in Figure 15.3.

a. To separate a mixture that has a mole fraction of B equal to 0.70 (70%) B, you would need three theoretical plates. On the graph from Figure 15.3, if a line is drawn where B=0.7 (70%B), this shows three theoretical plates needed. 6.

Describe the behavior upon distillation of a 98% ethanol solution through an efficient column. Refer to Figure 15.10. a. Since a 98% ethanol solution is an azeotrope which has a fixed composition of ethanol and water than cannot be changed, it does not obey Raoult’s law which states that two miscible liquids would form an ideal solution (Domzalski, Lab #3 - Distillation). Once 78°C is reached during distillation, the ethanol with start to evaporate and condense but will not be a pure distillation due to the fact that there is still 98% ethanol and 2% water that will not be separated. This solution would distill as if it were a pure solution that has no impurities.

9. Two components have boiling points of 150°C and 130°C. Estimate the number of theoretical plates needed to separate these substances in a fractional distillation. a. Ten plates would be needed to separate these two components because a substance the components have a boiling point difference of 20°C, according to table 15.2 in Pavia.

Pavia Lab Manual Questions 1. Why is the ability to separate 2 liquids by fractional distillation drastically reduced if heat is applied too rapidly to the distillation flask? a. In fractional distillation, the difference in boiling point of the components of the mixture can be greater than 30 degrees celsius. Since this range is much lower, if the heat is applied too quickly to the distillation flask, the component of the mixture that has a lower boiling point will not have enough time to boil, as it can end up boiling completely, which would skew the data. 2. The bulb of the thermometer placed at the head of a distillation apparatus should be adjacent to the condenser. Explain the effects on the temperature recorded if the thermometer were placed (a) well below the exit to the condenser and (b) above the exit. a. If the thermometer is placed above the exit, the temperature measurement would be closer to room temperature due to the fact that the thermometer is not low enough to measure the temperature of the vapor before it condenses. b. If the thermometer is placed below the exit, the temperature would read as higher because there would be an excess of vapor surrounding the thermometer. 3. Answer the following question using the table below as a reference. The components of a methanol- water and carbon tetrachloride-toluene mixture differ

from each other in boiling point by about the same amount. However, the methanol-water mixture can be separated more efficiently by fractional distillation. Explain. SOLVENT

B.P.

Heat of Vaporization*

Acetone

56.5

125.3

Methanol

64.7

261.7

Hexane

68.7

79.2

Carbon Tetrachloride

76.7

46.4

Benzene

80.1

93.5

Cyclohexane

80.7

93.2

Water

100.0

536.6

Toluene

110.6

86.8

* In Calories per gram at the boiling point (b.p.) a. Due to the fact that Toluene has a high volatility, the carbon tetrachloride-toluene mixture is distilled effectively with simple distillation. In fractional distillation, Toluene would vaporize faster and mix into the distillation tube, while methanol-water would not do so. Even though they have similar boiling point difference ranges, the volatility of the components determines which type of distillation would be more effective. References CS ChemDraw Pro. Computer software. Cambridge, MA: CambridgeSoft Corp., 1997.

Domzalski, Allison. Lab #3 - Distillation. September 2019. PowerPoint Presentation Hunter College. Chemistry 223 Lab Manual. Fall 2019 Pavia, Donald L. A Small-Scale Approach to Organic Laboratory Techniques. Belmont, CA: Brooks/Cole Cengage Learning, 2016. Print....