Distillation. Simple Distillation

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1 Experiment: Distillation Distillation is the most commonly used technique for purifying an organic liquid or for separating two or more organic liquids from each other. The technique consists of heating the liquid being distilled in a flask until it starts to boil. The resulting vapor rises up a vertical tube, called a distillation column, attached to the flask. At the top of the tube, the vapor enters another tube called a, where it is cooled and condensed, forming a liquid that is collected in a container called a receiver. Mechanically, this technique is rather straightforward, boiling a liquid, transporting the resulting vapor to a location where it is condensed and collected. How distillation brings about the separation of two or more compounds is a more complex issue. Such a distillation is called fractional distillation. Simple Distillation A simple distillation is one in which only a short, unpacked distillation column is used. It is effective for separating volatile organic liquids from non-volatile ones or from solids. When the boiling point difference between the two liquids in the sample is 80ºC or more, separation by simple distillation is very effective. (Note: Some sources state that simple distillation can be used in cases where the boiling point differences between the liquids in the solution are at least 25ºC apart. In practice, this boiling point difference is too small for use in simple distillation. A better rule of thumb is to use 80ºC as noted above.) Fractional Distillation Fractional distillation is used to separate organic liquids whose boiling points are closer to each other than 80ºC. How does fractional distillation work? Manipulation of Raoult s and Dalton s laws, two fundamental laws of solutions, leads to the conclusion that in any mixture of liquids, the vapor phase in equilibrium with the liquid phase will have a higher percentage of the more volatile (lower boiling) liquid than the liquid phase will. Said another way, the vapor phase in equilibrium with the liquid phase will be richer in the more volatile component than the liquid phase. Consider a solution of two organic liquids, X and Y, where X is the more volatile component. The solution mixture consists of 50% X and 50% Y by volume. When heat is applied to this mixture, the vapor in equilibrium with the remaining liquid will be richer in the more volatile X than the liquid will. Let s say that this vapor is now 60% X and 40% Y. If this vapor condenses at some point up the distillation column, the condensed liquid will now have a composition of 60% X and 40% Y. As more heat is applied, this liquid too will partially vaporize and again the vapor formed from it will be richer in X than the liquid left behind, let s say 70% X and 30% Y. As this vapor travels up the distillation column, it too condenses, forming a 70% X and 30% Y liquid. This vaporization condensation process continues, each time enriching the vapor phase in the more volatile component X. If the distillation column is long enough and if it contains enough sites where these vaporization condensation steps can take place, by the time the vapor reaches the

2 , it will be 100% X. The pure X will condense and flow through the and into the receiver, and the distillation will have been successful in separating X and Y. Fractionation columns, as these distilling columns are called, may be long or short. They contain sites where condensation of the vapor can occur. These sites may be built into the column as with the glass prongs (alembics) of a Vigreux column, or they may be introduced to the column in the form of pieces of steel scrubbing sponge or glass helices. The closer together the boiling points of the solution components are, the longer and/or more fully packed the distillation column must be to bring about a good separation. As you distill a mixture with different boiling points, you will be able to track quality of separation by monitoring the temperature. Again considering liquids X and Y, assuming X boils at 40 C and Y boils at 150 C, in a perfect separation you would first collect pure X while your thermometer steadily reads 40 C. Once all of the X has distilled out, the temperature on the thermometer will drop for a time while no vapor is contacting the tip of the thermometer. Then, as soon as enough heat has been put into the flask, the Y will begin to vaporize, rise up the column and begin containing the thermometer tip. At this point you would have a new flask ready to collect pure Y at a steady temperature of 150 C. In reality, however, separations are rarely perfect and, therefore, you will instead observe a gradual increase in temperature as you transition from compound X to compound Y. The more gradual the increase, the poorer the separation. In the experiment below, you will not actually be separating the two compounds. Instead, in this study of distillation techniques, you will track temperature and volume data while collecting all of the distillate in one graduated cylinder. The goal is to graph temperature volume data from your distillation and two other types of distillation data from your peers. Using this data, you should be able to determine which type of distillation gave the best separation of ethyl acetate and butyl acetate. Pre-lab Preparation 1. Describe how a distillation column is able to separate a mixture of ethyl acetate and butyl acetate. Your description must describe the effect of repeated evaporation-condensation steps on the composition of the vapor rising through the column. 2. Look up the physical constants (bp, RI and density) of ethyl acetate and butyl acetate. Write these constants in a table format in your notebook along with the chemical structure for each. 3. Based on your data above, which compound will you collect first in your distillation? Explain briefly why this compound will distill first.

3 Experimental Procedure! Safety Considerations! Ethyl and butyl acetate are not dangerous compounds but it is a good idea to keep from breathing large quantities of organic vapors. Keep containers stoppered until you are ready to use them, and when you are finished with them, dispose of them properly according to your instructor's directions. As you are distilling, check to be certain that your ground glass joints are tightly sealed so that hot vapors don't escape into the lab.! Use boiling stones.! Don t plug sand baths directly into the wall; use Variac controllers.! Never distill to dryness. In this lab, you will do a distillation of a 50% ethyl acetate 50% butyl acetate solution. You will be told to do this distillation in one of three ways listed below: 1. Simple distillation with no column 2. Fractional distillation with Vigreux column 3. Fractional distillation with steel sponge column You will compare the results of your distillation with those of students using a different distillation column, in an effort to determine which column was most effective for separation of the mixture. 1. Setting up the Distillation Apparatus After you are assigned a particular type of distillation, assemble a distillation apparatus as shown in the diagrams below. For safety, be certain to clamp the distilling round bottom flask. You should also adjust the height on the ring stand to allow easy raising and lowering of heat and easy emptying of the receiver. Pay close attention to the following details. If it is difficult to insert the temperature probe, use glycerol to lubricate the rubber adapter. Position the thermometer so that the tip of the metal probe is approximately one inch (2.5 cm) below the bottom of the side arm of the distilling head. This is critical. If the vapor does not come in contact with this much of the probe, you will have an erroneous reading. Check to make sure you have all of the clamps and water hoses connected as shown. Verify that your water hoses are securely attached and turn the water on slowly to prevent pressure surges from popping off the hoses and drenching your area.

4 After you have set up your own apparatus, do not turn on the heat to your heating mantle until your instructor has checked your apparatus. However, you may put the solution to be distilled into the distilling flask along with 3-4 boiling stones before your set-up is cleared for work. Simple Distillation (water out) distillation head 50 ml r.b. flask can be heated with a sand-bath or small heating mantle (water in) vacuum take-off (used only for vacuum distillations) receiver (graduated tube) Clamp Keck clips incidated by this style of arrow: Note: Always use a clamp for the hot distilling flask. Keck clips are insufficient.

5 Fractional Distillation (water out) distillation head fractionating column 50 ml r.b. flask with heating mantle (water in) vacuum take-off (used only for vacuum distillations) receiver (graduated tube or r.b. flask) Keck clips incidated by this style of arrow: If you are doing fractional distillation, you will be assigned a column, either a steel sponge column or a Vigreux column. If you are assigned a simple distillation, there will be no fractionating column present (see previous photo). With your set-up, either simple or fractional, you will distill a mixture by volume of ethyl acetate and butyl acetate. As the experiment progresses, you will collect temperature - volume data every 1 ml. Collect your distillate in a 50-mL graduated cylinder. The temperature will change as you collect distillate. You will plot this temperature-volume data to see how well each of the columns separates the mixture. It is important to remember that distillation is used to separate two liquids, so in a normal distillation, you would be trying to separate ethyl acetate from butyl acetate. You would collect a fraction of material boiling from 76ºC to 80ºC, consisting mostly of ethyl acetate, a hopefully small fraction boiling between 80ºC to 120ºC, and a third fraction boiling from 120ºC to 126ºC consisting mostly of butyl acetate. In this case, we are collecting all of the distillate in one graduated cylinder to determine from the temperature-volume data which distillation apparatus works best. This will probably be the last time you ever collect distillate in a graduated cylinder. In the future you will collect it in flasks, one for each boiling point fraction

6 2. Distillation Into the 50-mL round bottom flask pour 25.0 ml of the 50% ethy1 acetate - 50% buty1 acetate solution (already prepared for you). Add two or three boiling stones, and connect the flask to the rest of the apparatus. After your instructor has checked your apparatus, verify that the water is flowing. Set the Variac to begin controlling the current into the 50-mL heating mantle. You should start an intermediate setting and gradually increase the current as needed to vaporize the two components. Distill the liquid, collecting temperature-volume data every one milliliter of distillate collected. Once it is clear that you have been collecting only butyl acetate (based on temperature), be sure that you have collected 4-5 ml of it, and then stop the distillation by lowering the heating mantle until it is a few inches below the round bottom flask. After the distillation apparatus has cooled to room temperature, take it down carefully, removing the thermometer first. Before leaving for the day, be sure to get temperature-volume data from two other students who have done the fractional distillation with the two other types of columns. Post-Lab and Report Requirements 1. Tables of temperature-volume data for the Vigreux column distillation, the steel sponge column distillation, and the "no column" distillation. 2. On a computer, make a graph with temperature on the vertical axis and the volume of distillate collected on the horizontal axis. Use the Excel spreadsheet available on the Science Learning Center website. Go to the Montgomery College Homepage and scroll to the very bottom. On the right, under Counseling and Advising select Learning Centers (tiny font at the very bottom right of the page). Select the Germantown Campus Science Learning Center. From the left column, select Online Study Materials. Select CH203 and then scroll down to open Excel Graph Distillation Template. Type the temperature data into the correct columns and print. Compare the curves for the three distillation columns. A perfect distillation would give a curve like the one shown below. 126 C Temp 77 C At ~12.5 ml (all ethyl acetate should be gone) Volume of Distillate

7 3. A student from another college (MC students would never do this) is distilling his sample of butyl acetate and is unable to reach the required distillation temperature for this substance of 126 o C. The highest temperature he recorded was only 118ºC. What possible things could he have done wrong with his distillation? 4. Which of the distillation columns, for which you have just plotted date, separates the ethyl acetate from butyl acetate best? Discuss all three methods and explain how you decided which is best. 5. What would be the impact of the following factors on the quality of separation of two components in a fractional distillation? a. Heating the distilling mixture more rapidly. b. Using a longer distilling column. c. Using a column with better packing. Explain why you answered as you did. Your explanation should relate to the number of evaporation-condensation steps that take place in the distillation. 6. What would be the impact of the above factors on the speed with which the distillation could be carried out? Explain your answer. 7. Explain how your answers to 5 and 6 above relate to your decision about how to carry out an effective distillation in a reasonable amount of time. Is fractional distillation always the best choice? 8. An organic liquid decomposes at 240 o C, before its boiling point can be reached. What steps can be taken to distill this liquid to separate it from some solid impurities dissolved in it? As always, write a concise summary/conclusion for your work done in this lab.