- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 1 (47.29 KiB) Viewed 115 times
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- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 6 (43.14 KiB) Viewed 115 times
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- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 8 (63.98 KiB) Viewed 115 times
- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 9 (43.37 KiB) Viewed 115 times
- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 10 (31 KiB) Viewed 115 times
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- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 12 (41.2 KiB) Viewed 115 times
- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 13 (52.98 KiB) Viewed 115 times
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- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 17 (33.28 KiB) Viewed 115 times
- 14 Distillation Distillation Is A Method Of Purification Where A Compound Is Selectively Turned Into A Vapor And Then C 18 (40.64 KiB) Viewed 115 times
pentane (P = 507 tort) to pure hexanc (P.... = 148 torr) (Raoult's Law), and how the total vapor pressure of the mixture changes as well (Dalton's Law). 0.0 600 0.2 0.4 0.6 0.8 1.0 600 P pen 500 500 P 400 400 P total Vapor Pressure (tor) 300 300 200 200 100 100 TIITVE 86 0.2 0.4 0.6 0.8 18 mole fraction hexanes in pentane Figure 14-1: Partial and total vapor pressures for pentane/hexane mixtures From the Ideal Gas Law, the composition of the vapor above the mixture can be determined from the partial pressures of each component: PX Protel The consequence of these trends is that for an ideal solution, the vapor is enriched in the more volatile component compared to the solution. In our pentane/hexane example, the vapor will always have a higher mol% of pentane compared to the solution. This is the principle that allows miscible liquids to be partially separated by distillation. If the vapor can be collected, and then condensed back to a liquid, it will be enriched in the more volatile component. This behavior is often described using a temperature composition diagram: Figure 14-2 shows one such diagram for the pentane/hexane system: 64
00 70 0.2 0.4 0.6 0.8 10 70 Vapor 65 65 Liquid 60 60 55 Temperature 55 50 50 45 45 40 40 34.0 135 0.2 0.4 0.6 0B mole fraction hexanesin pentane Figure 14-2 The bottom curve represents the boiling point of the liquid as its composition varies. Pure pentane (0 mol% hexane) boils at 36 *Cat I atm), and hexane boils at 68. As the amount of hexane (the less volatile component) in the mixture increases, the vapor pressure of the mixture decreases (cf. Figure 14-1), a higher temperature is needed to bring P...to 760 torr (1 atm), and thus the boiling point of the mixture increases. The top curve represents the composition of the vapor above the liquid at that temperature. It shows that, for a given mixture, the vapor is more enriched in the more volatile component (pentanc). For example, 80 mol% hexane in pentane boils at 59 "C(see Figure 14-3). Tracing horizontally from that intersect, we see that the composition of the vapor at that temperature is 60 mol% hexane. 0.0 0.2 0.4 0.6 0.8 1.0 70 970 Vapor 65 65 Liquid 60 60 55 55 Temperatur 50 50 45 45 40 40 3500 35 1.0 0.2 0.4 0.6 0.8 mole fraction hek nesin pentine Figure 14-3 65
If that vapor were condensed back to a liquid, we would now have a liquid (the distillate that is 60 mol% hexane. In other words, we converted a liquid that was 20 mol% pentane into 40 mal pentane. As the distillation progresses, and the "pot" is depleted of pentane, the mass of hexane rises, the boiling point rises, and less and less pentane appears in the distillate. So, over the course of a distillation, the following trend is seen: 80 70 70 60 60 Temperature 50 50 40 40 30 30 volume distillate Figure 14-4 The initial distillate has the lowest boiling point, and thus the highest percentage of pentane. As the distillation progresses, more and more of the less volatile component (e.g. hexane) comes over as well, and towards the end of the distillation the distillate is primarily the less volatile material, Thus, although complete separation of the two components isn't achieved, we can get enrichment in the more volatile component at the start of the distillation, and in the less volatile component towards the end One single evaporate/condense cycle, as shown in Figure 14-3. is referred to as a "theoretical plate", and it is used as a measure of distillation efficiency. The term gets its name from distillation towers, such as those used in the petroleum industry, that consist of a series of plates that vapors collect on. If your apparatus allows vapors to repeatedly evaporate and condense as they pass through, before being cooled and collected, greater efficiency of separation is achieved. For example, if 80 mol% hexanes on average evaporated and condensed three times before collection (i.e. the apparatus had an efficiency of three theoretical plates), the initial distillate would be only 14 mol hexane: the purity of the pentane will have gone from 20 to 86 mol% (Figure 14-5). 66
00 70 0.2 0.4 0.6 08 10 70 65 Vapor 65 60 Liquid 60 55 Temperature SS 50 50 45 45 40 40 35 6.0 0.2 0.4 0.6 0.B mole fraction hexanesin pentane Figure 14.5 10 20 30 40 80 SO 80 ideal 70 70 fractional simple 60 60 Temperature ro 50 50 40 40 30 10 5030 40 20 30 volume ditatem Figure 14-6 The plots of T vs. Van for a less efficient (low theoretical plate) distillation and a more efficient (high theoretical plate) distillation of a mixture of 25 ml.pentane and 25 ml.hexane are shown in Figure 14-6. Note that in the more efficient distillation, the initial distillate is closer to the boiling point of pure pentane (indicating a higher purity of pentane), and the final distillate closer to the point of pure hexane (indicating a higher purity of hexane). Also, the temperature rise is sharper, which indicates a cleaner separation of the two components. If infinite theoretical plates were possible, the curve would approach the ideal show by the grey line: 100% pentane boils off first, then the temperature spikes to the boiling point of hexane, then the hexane boils off. This is the unattainable ideal. However, there are special apparatus that have thousands of theoretical plates that can almost completely separate such a mixture cleanly into its constituents. 67
Technique Simple Distillation when there is a large difference in the boiling points of the components to be separated, a simple distillation apparatus can provide adequate separation. The apparatus is shown in Figure. A flask (the "pot") containing a source of agitation to promote even boiling (e.g. a magnetic stir bar, or boiling stones) is fitted with a "distillation head" with a sidearm that fits to a water-cooled condenser. The distillation head is fitted with a thermometer, whose bulb is in line with the bottom of the sidearm. The flask is heated (eg. with an electric mantle) to a boil. The thermometer measures the temperature of the vapors as they spill over into the condenser, where they are cooled to a liquid. A receiver such as an Erlenmeyer flask is placed under the condenser spout to collect the distillate. Secure clamping is critical! The entire apparatus should be elevated off of the bench top, and its weight should be entirely supported by clamps when the support of the heating mantle and stirrer are removed. This is because the heat source must be removed before the contents of the flask go dry. Never distill to dryness! Without boiling solvent moderating the temperature of the flask's contents, residue left in the flask can then heat up and decompose, possibly violently. This is a common cause of laboratory accidents. Check that all ground-glass joints are fit together snugly, with no air gap to cause leaks. Often during clamping a piece of glassware such as the condenser will be forced to fit a clamp, unseat, and cause a leak Figure 14-7: Simple distillation apparatus in the system. Make the clamp fit the position of the glassware, and not vice versa. Because the distance between the pot's contents and the sidearm is short, boiling too vigorously can cause the contents to "bump" over into the condenser. To avoid this, start with a low setting on the Powermite heat regulator and gradually increase the temperature until a steady boil is achieved. During the course of the distillation, further slight increases of the Powermite setting may be required to maintain an adequate rate of distillation. 68
Fractional distillation: The apparatus, shown in Figure 14-8, is similar to that for a simple distillation, except that a "fractionating column" is placed between the pot and the distillation head. The fractionating column has a design giving it a large interior surface area. Vapors of the boiling mixture repeatedly evaporate and condense on the surface as they progress up the column; employing a fractionating column thus provides more theoretical plates of efficiency. Commonly encountered fractionating columns include those with glass beads or helices, or with numerous glass distillation apparatus "fingers" projecting inside (known as a Vigreux column). Another Figure 14-8: Fractional solution is to pack a condenser with metal pot scrubber, which is the case in Figure 14-8. (Note: the only reason a water condenser is used for making this pot-scrubber column is because they're convenient! No water should be circulated through the fractionating column.) Microscale simple distillation: Another way to perform simple distillations is to set up a reflux apparatus, but place a reservoir such as a Hickman still head in-between the pot and the condenser. Distillate will pool in the still head until it overflows back into the pot. If the volume of the still head isn't large enough to hold all of the desired distillate, a side arm with a screw cap allows distillate to be periodically removed by Pasteur pipette or by syringe. The Hickman still head is very fragile and prone to breakage. In particular, the lower male joint is weak where it meets the still head, and the sidearm will break if the screw cap is over tightened. Clamp the apparatus at the Hickman still head to minimize stress on the joint, and be careful that the heating block isn't moved sideways while the upper part remains clamped. CSS Figure 14-9: Microscale distillation using a Hickman still head 69
B. Distillation of a Mixture of Immiscible Liquids: Steam Distillation Theory The physical laws describing the distillation of a mixture of immiscible liquids differ from that of miscible liquids. In the Organic laboratory, the most common scenario where we have two immiscible liquids is when one is water and the other is an organic compound. In these cases, we describe distillation of this mixture as a steam distillation. The key difference between steam distillation and distillation of an ideal solution is that the percent composition of the mixture does not affect the boiling point of the mixture, or the composition of the distillate. For a mixture of immiscible liquids, the total vapor pressure is simply the sum of the vapor pressures of the individual components: PPP At the temperature where P = atmospheric pressure, the mixture will boil. The mol fraction of the organic component in the vapor, Yr is equal to the ratio of that component's vapor pressure to atmospheric pressure: Yorg Peotel For example, if iodobenzene (bp 188 °C) and water (bp 100 °C) are combined, at 98 C the mixture will co-distill. At that temperature: P = 714 tort Pr46 torr P = 760 torr and the mixture will boil at 760 torr (1 atm). The composition of the vapor will be: Phi: 46 Corr X 100% = 6.1% 760 torr 714 torr Water: x 100% = 93.9% 760 tort One use of steam distillation is the isolation of relatively non-volatile organic compounds from natural sources. For example, eugenol, the main component of clove oil, has a boiling point of 254 "C. At that temperature, it would be difficult to distill it directly out of cloves without thermal decomposition. However, by adding water to cloves and performing a steam distillation, eugenol will co-distill over at just under 100 °C. Many natural spice and herb oils can be obtained in this manner. 70
Technique yriqargonmout a Although there are more elaborate apparatus that can be used that inject steam into a mixture, the easiest way to perform a steam distillation is to simply use a simple distillation apparatus and add water to the pot (Figure 14-10). Because the organic component is usually only a small fraction of the distillate, it may be necessary to add more water to the pot over the course of the distillation, until no organic phase is detected in the distillate. Figure 14-10: A simple distillation apparatus being used to steam-distill catnip oil from the leaves of the plant 71
You will use the values provided by the previous zero-point quiz to complete this assignment. For the distillation of an isopropanol/hobutanol IPOH/BUOH) mixture, you were provided • the composition of the mixture to be distilled (volume percent isobutanol in isopropanol . the temperature at which the initial distillate came over. A phase diagram for this system is provided an a POP IPOH IBUOH phase diagram od The working assumption is that you will print out the diagram work on it ( STRONGLY suggest using pencil so you can correct any mistakes), and then scan and upload it as well as your other work calculations stated number of theoretical plates If you do not have a working printer, you can plot a version on graph paper, or place paper on your monitor and trace lt, etc. Let your TA know this is what you're doing so they can check and make sure it's legible. As long as it's clear how you determined theoretical plates, the accuracy of your reproduction of the graph won't matter. Additional useful information: The molecular weight of isopropanol is 60.10 g/mol . The density of isopropanol is 0.786 g/ml • The boiling point of isopropanol is 82.6 °C • The molecular weight of isobutanol is 74.12 g/mol • The density of isobutanol 0.802 g/mL. • The boiling point of isobutanol is 1079" You will use this data, and the dogram to determine the number of theoretical plates the distillation apparatus provided To do this, you will need to know the composition of the initial mature, and the initial distillate. In mole fraction. Mole fraction de tole percent divided by 100 For the initial mixture to be distilled, you will convert von IBOH to molt fraction BOH Hint start your calculation me you love any convenient volume of solution lee 100 mL) and go from there. You must show your calculations For the initial distillate, use the phase diagram and interpolate the mole fraction by tricing from the temperature on the yard to the internet with the liquid phase dower) curve, down to the xxs Show this by drawing these lines on the graph hindi
• The molecular weight of isopropanol is 60.10 g/mol . The density of isopropanol is 0.786 g/ml The boiling point of isopropanol is 82.6 °C . The molecular weight of isobutanol is 74.12 g/mol . The density of isobutanol is 0.802 g/mL. • The boiling point of isobutanol is 107.9 °C You will use this data, and the diagram, to determine the number of theoretical plates the distillation apparatus provided. To do this, you will need to know the composition of the initial mixture, and the initial distillate. In mole fraction. Mole fraction is mole percent divided by 100. For the initial mixture to be distilled, you will convert vol% BOH to mole fraction IBUOH. Hint start your calculation assuming you have any convenient volume of solution les 100 mL) and go from there. You must show your calculations. For the initiat distillate, use the phase diagram and interpolate the molt fraction by tracing from the temperature on the y-axis, to the intercept with the liquid phase (lower) curve, down to the x-axis. Show this by drawing these lines on the graph. Then on the graph trace evaporate/condense cycles (theoretical plotes) from the initial mixture composition until you meet or pass the initial distillate composition Report the number of theoretical plates that the distillation apparatus provided, eg. "about 2" "between 2 and 3" "not quite 3' etc. Your description must match the work you show on the graph
CHEM325 Simple and Fractional Distillation Grade: Name: Section: Date: / 1. Initial Volume of 1:1 methanol:water ml 2. Distillation you performed (circle one): simple fractional 3. Name of partner: 4. Distillation partner performed (circle one): simple fractional 5. Your T vs. vol. data: vol (mL) T("C) T("C) T("C) T("C) 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 vol (ml) 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 20 vol (ml) 20.5 21 21.5 22 225 23 23.5 24 24.5 25 25.5 26 26.5 27 27.5 28 28.5 29 29.5 30 vol (L) 30.5 31 31.5 32 32.5 33 33.5 34 345 35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40
Grade: Name: Section: Date: 1 1. Complete a hand-drawn graph of T. vs. vol. as described on p. 19 of your lab manual, and . include it with this sheet when you turn it in to your TA. 2. Compare the initial temperatures, final temperatures, and curve shapes between the two distillations. Based on your data, which distillation did a better job at separating methanol from water? Explain your reasoning. 3. Were there any contradictions between the data you and your partner obrtained, and the expected result? If so, what source(s) of error were likely responsible?
4. A phase diagram for methanol-water is provided on the next page. Use it to determine the number of theoretical plates of resolution your distillation apparatus provided. Note: the initial composition of the liquid requires you to calculate the mol% of water in the aqueous methanol you were provided with (which was 50 vol% methanol in water). The composition of the initial distillate can be interpolated from the temperature of your initial distillate, using the lower liquid curve. On this page, show your calcuations to determine the initial mol% of water in the water/methanol mixture.
100 95 90 85 T("C) 80 -liquid vapor 75 70 65 60 0.1 0.2 0 0.3 0,4 0.6 0.7 0.8 0.9 0.5 mol fraction water 1
Grade Name: Section: Date: // 1. Mass of cloves used: 8 2. Mass of empty round-bottom flask: 3. Mass of round-bottom flask + clove oil from extraction: 4. Mass of clove oil isolated: 5. Summarize your observations for your potassium permanganate test: 6. Summarize your observations for the iron(III) chloride test:
Name: Section: Date:/ 1. Using your data, calculate the percent by mass of the clove oil in the powdered cloves you used. 2. a) Clove oil may contain eugenol, cugenyl acetate, and/or caryophyllene (as well as other, minor components). In the table below, for each possible component and each chemical test, indicate whether it should give a positive or negative test result. Also indicate what your actual test result was for your clove oil. Compound KMnO, test result ("+" or "-") expected actual FeCl, test("+" or "") expected actual eugenol cugenyl acetate caryophyllene b) What can you conclude about the composition of your clove oil, based on these test results?