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Distillation_Techniques

Distillation_Techniques - Chemistry 251 2nd Lab Period...

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Unformatted text preview: Chemistry 251 2nd Lab Period DISTILLATION TECHNIQUES Reading Assignment: F&F, pp. 77-88; 93-96 Preliminaries. Before coming to lab, you are expected to have read the reading assignment and to have prepared your notebook for entering data from this experiment. Since keeping a notebook is an important technique wherever research is done, it is worth mastering. Because this is your first experiment, you will receive a score on your notebook pages, but it will not count toward your final grade. In addition, before you begin your work in lab, you must submit to your TA a completed preliminary report (attached) for this experiment. Background. Distillation is one technique used to separate a mixture of two (or more) miscible liquids. The quality of the separation depends on the difference between the boiling points of the liquids. Separation becomes more difficult if the boiling points are similar. In an ideal distillation, the liquid with the lower boiling point would boil (vaporize) first. As it passes through the condenser, it cools and again becomes a liquid. The component with the higher boiling point would remain in the distilling flask. In this ideal scenario, the temperature would remain constant until all of the lower boiling component is removed, at which point the temperature would begin to increase until the second liquid starts to boil. Experimental Setup. From this experiment and the associated lecture you should gain an appreciation of the relative importance of vapor pressure in the "real world" separation of mixtures. In one form or another, this concept underlies various physical separation techniques. For this experiment, you will be divided into groups of four. One person will carry out a simple distillation experiment (see below), and the each of the others will set up one of three fractional distillations. Each of you should take data from all of the experiments. Check each others experimental setups. Arrange operation of the stills and data acquisition as you find convenient. You will compare the effectiveness of simple distillation and fractional distillation in separating the same mixture, 35 mL of a 50 : 50 methanol-water mixture. You will follow the course of the separation by reading the temperature at frequent intervals, and recording this temperature as a function of the volume of distillate collected. Comparison of the temperature-volume curves for the four distillations will demonstrate the relative efficiency of your setups. Simple Distillation. A large scale simple distillation apparatus is shown in Figure 5.7, p. 85 of F&F. However, you will use a small scale distillation setup using the combined distillation head found in your drawer. The combined small scale distillation apparatus (see the diagram on the following page) includes the distillation head, the condenser and the vacuum adapter in a single unit. For this experiment, a 10 mL graduated cylinder should be used as a receiver instead of the flask shown in the illustration. Choose a round-bottomed flask for the distillation pot that will be between 1/2 and 2/3 full at the start of the distillation (this is a general rule in distillations of all types). Simple Distillation Setup Note Position of Thermometer Bulb Open to air Flask should be replaced by 10 mL graduate cylinder for this experiment. Procedure. Obtain two clean and dry 10 mL graduate cylinders to use in collecting the liquid as it comes out of the condenser. Weigh one of these and record the weight in your lab notebook. You will use this pre-weighed cylinder to collect the first 10 mL that come out of the distillation. Place 35 mL of a 50 : 50 methanol-water mixture in the boiling flask and add a few boiling chips. Before turning on the Thermowell, check the following: thermometer position, direction of water flow, properly lubricated joints, tightness ofjoints. boiling chips in liquid. Turn on the Thermowell to a setting of about 40 volts and heat the mixture until it boils. You may need to increase the heat during the distillation as the more volatile component is removed. It is important to maintain a fast enough distillation rate that the thermometer is constantly immersed in hot vapors, but a low enough rate that these vapors are not superheated (heated beyond their boiling point). Record the temperature as the first drops enter the graduated cylinder and then at intervals of every 2 mL, or more often during periods of rapid temperature rise. Also, make a notation with each reading that indicates whether or not there is liquid visibly condensing and dripping off the thermometer bulb at the time of the reading. After you have collected the first ten mL, switch the graduate cylinders and immediately weigh the first cylinder with its liquid contents again. (Be sure there is exactly 10 mL of liquid in the cylinder. If not, use a pipet to either add (from the new cylinder) or remove some liquid. You will use the mass of this 10 mL of liquid to calculate the density of the liquid in your report. Distill a total volume of ~25 mL. Do not distill until dryness. An efficient notebook layout for recording your data would be a table with one column for the simple distillation and a separate column for each of the fractional distillations. Fractional Distillation. For the fractional distillation, you will use one of three columns: a Vigreux column or a column packed with either glass beads or carborundum chips. Begin by setting up a simple distillation apparatus and insert the Vigreux or packed column between the still pot and thermometer head. (See the illustration below). Securely clamp both the still pot and the column. Distill 35 mL of a 50:50 methanol- water mixture, recording the temperature and volume as directed in the simple distillation instructions. When packing an empty column with either glass beads or carborundum chips you will need to put a small piece of steel sponge in the bottom of the column to keep the packing from falling through into the boiling flask. DO NOT use glass wool for this purpose. Use scissors to cut a small piece of the steel sponge from the bigger pad - DO NOT try to pull a piece from the bulk with your hands or you will end up with shredded fingers. Fractional Distillation Setup Open toair Vigreux —-=-'— (or packed) column Thermowell heater Clean-up. When you are finished with the simple and/or fractional distillations, dispose of the distillate and the residue in the still pot by washing them down the sink. 35359 33:00 mad md mmd ad I. fig? - 8.0:. u A it. 35:8 .3, $522, a: 1022 .x. ON 9. 0o om Iouemew % Chemistry 251 Due 2nd Lab Period Your Name TA, Day, Time and Location Preliminary Report for Distillation Technigues Place answers on this page. 1. Often a student will forget to add boiling chips before commencing a distillation, and finds that no boiling occurs even though the contents of the flask seem hot enough. Why is it dangerous to add boiling chips to the hot liquid? Why must a distillation apparatus have an opening to the atmosphere? Suppose you are given ten different known mixtures of liquids A and B and the boiling points of the two pure liquids. Assuming an ideal binary system, how would you construct the lower (liquid) curve of a boiling point-composition diagram for A and B similar to the one on p. 83 of F&F (Figure 5.5)? Where will you dispose of the methanol-water mixture upon completion of the lab? On the back of this sheet, sketch the apparatus that you will use for fractional distillation, and name the individual pieces of equipment that make up the assembly. Chemistry 251 Your Name _. TA, Day, Time and Location REPORT ON SIMPLE AND FRACTIONAL DISTILLATION I. Record all of the temperature/volume data in a table like this in your notebook: Volume of Distillate (mL) Temp °C Temp °C Temp °C Temp °C Simple Distillation Fractional~Vigreux Fractional-Glass Fractional- Beads Carborundum Chi 5 ll. Using the weight of the first ten mL of liquid collected in each distillation, calculate the density of the liquid in gms/mL (ie. divide the weight of the liquid by its volume). Then, using the plot (or the equation) on the last page of this handout, calculate the purity (as %—methanol) of this initial fraction for each of the distillations. Distillation type Weight (first 10 mL) Density (first 10 mL) Purity or %-CH30H Ill. On a single sheet of graph paper, plot the results of all four distillation experiments along with the curve expected for an ideal distillation. Be sure to label each curve in some way. Using these curves, rank the four types of distillation as to relative efficiency. Is this the order you expected? Are your plotted results consistent with the purities calculated above ? Comment on your results. (Computer generated plots are acceptable). Chemistry 251 Page 2 IV. Using the DISTILL program on the computers in Baker G02, estimate the number of theoretical plates obtained from each of the four distillations. Number of Theoretical Plates Using this data and assuming that each of the fractional distillation columns was 15 centimeters in length, calculate the HETP value for each of the column packings. HETP Value IV. Explain how each of the following factors could be adjusted to give the most efficient fractionation (separation): a) Length of the distilling column b) Size of the glass beads in a packed column c) Rate of distillation (drops/minute) d) Size of the distilling flask ...
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