Properties of Liquids

Transcription

1 Experiment: Properties of Liquids Many of the organic compounds you will be studying this year will be liquids, and in lab, you will frequently have to identify unknown liquids and confirm the identity of others. There are three quick and easy physical measurements that you will explore in these lab periods, measurements that you will make repeatedly as the course proceeds. Let s look at them one by one. Boiling Point A liquid boils at the temperature at which its vapor pressure equals the atmospheric pressure. For all liquids, as heat is applied, molecules move with greater kinetic energy. More and more molecules have the energy to overcome the intermolecular forces (London dispersion forces, dipole-dipole forces and hydrogen bonds) that keep them close together in the liquid state. As these molecules achieve this energy, they break away from surrounding molecules into the gas state where they are very far apart from other molecules. Here in the gas state, the vapor, like all gases, exerts a pressure, called the vapor pressure. As more heat is added to the liquid, more and more molecules escape to the gas phase, and the vapor pressure rises. When the vapor pressure reaches the pressure exerted by the atmosphere, boiling occurs. Boiling points of liquids vary as the atmosphere pressure changes. Reduced atmospheric pressure lowers the boiling point of a liquid. Chemists take advantage of this fact to more easily distill high boiling liquids or liquids that decompose before their atmospheric boiling points are reached. [Note: Distillation is the process by which liquids are purified by separating them from other substances in a solution, using the different boiling points of the components of the solution as the basis of the separation. The liquid components are boiled, separated in the gas state, and condensed separately in the different parts of the distillation apparatus. You will learn more about distillation in another lab.] For very high boiling liquids, lowering the pressure inside the distillation apparatus lowers the boiling points enough to enable distillation to occur at more easily achievable temperatures. For you this year, however, all boiling points will be determined at atmospheric pressure, which varies so little that the boiling points will not noticeably change from the given boiling point values. At atmospheric pressure, the boiling points of liquids will be constants you will use all throughout the course to help you identify them. If the liquid sample is pure and you have measured the boiling point correctly, the boiling point should be within two or three degrees of the listed value.

2 Index of Refraction Light changes its direction as it passes from one medium to another. This phenomenon is called refraction. When the first medium is air and the second medium is another substance, one measure of how much refraction occurs is called the index of refraction, sometimes known as the refractive index. Air i Liquid r i = incident angle r = refracted angle sin i Refractive index = sin r The Abbé refractometer measures the refractive index, n D, directly, automatically correcting for the white light used and giving readings as if light from the D line of sodium were being used. The refractive index of a liquid is dependent on temperature. As temperature increases, the refractive index decreases and vice versa. Most reference books list refractive indices taken at either 20 o C or 25 o C. Hence, if your refractive index is measured at 22 o C or 19 o C, you must apply a correction factor to correct to the temperature that matches your reference. A good rule of thumb to use is that the refractive index changes per degree deviation from the reference temperature. n D ref = n measured + ( / o C )( T measured - Reference o C ) The following examples show you how to apply this correction. Example 1: n D at 22 o C is Determine n D at 20 o C (to match your reference value). n D 20 = (.00045/ o C)(2 o C) = Note the n D 20 shows the reference temperature in this example is 20 o C. Be sure to indicate the reference temperature when tabulating your data. Note also that this number is a unitless ratio. Example 2: n D at 19 o C is Determine n D at 25 o C.

3 n D 25 = (.00045/ o C)(6 o C) = Here the n D 25 shows the reference temperature in this example is 25 o C. Refractive indices are constants that you will be using all year to help you identify organic liquids. If the liquid sample is pure and you have measured the refractive index correctly, the refractive index should be within of the listed value. Density Density is a measure of the mass of a sample per unit of volume. density = mass volume Density, too, is a constant that can be used to help identify different classes of organic liquids. Even if the liquid sample is pure, the errors inherent in measuring the density are such that this result, while useful, should not be relied upon as much as the boiling point or refractive index. In this lab, you will measure the boiling points, refractive index, and the density of a known and unknown liquid. You will compare the experimental data for your known with the data given for it in the Handbook of Chemistry and Physics, and you will identify your unknown by comparing your data with that of compounds listed in the Handbook of Tables for Organic Compound Identification. Pre-lab Preparation 1. Carefully read over the Appendix 1 and background information. Understand the theory behind the experimental procedures, and be comfortable with the procedures themselves. 2. Look up the physical property values for 3-pentanone and write them in your notebook. This should include boiling point, refractive index (including reference temperature) and density. Don t forget to list the reference source and to use a table format.

4 Experimental Procedure! Safety Considerations! Use boiling stones for you boiling point measurement.! Don t plug sand baths directly into the wall; use Variac controllers.! Keep unknown liquids off of skin and avoid breathing the vapors.! Dispose of waste organics in the waste container provided, not in the sink. 1. Boiling Point of Known Sample: 3-Pentanone In a boiling point tube, a short fat test tube, place enough 3-pentanone so that the liquid level in the bottom of the tube is approximately one centimeter. Also drop in a small boiling stone or two to assure even boiling of the liquid. Insert one of the digital thermometers, which will be set out for you, into a cork which has a groove cut into its side to allow vapors to escape from the tube. Be careful inserting the thermometer into the cork. If you have difficulty, try lubricating the thermometer tip with some glycerol before inserting it into the cork. If you use glycerol, clean the thermometer before placing it into the tube containing the liquid to be tested. Otherwise, the glycerol will contaminate the boiling sample and affect the boiling point of the liquid. Place the cork and thermometer into the boiling point tube and adjust the tip of the thermometer so that it is one or two centimeters above the surface of the liquid. vented cork 30 ml fat test tube temperature probe should be no more than 1-2 cm above liquid

5 Heat the boiling point tube and liquid in the sand bath, pushing the tube into the sand far enough so that the liquid in the tube boils vigorously. As the liquid starts to boil and vapor begins to rise up in the tube, the temperature read by the thermometer will increase. When the liquid is boiling vigorously and vapors and condensing liquid are clearly visible as a wavy line of condensing liquid about 1 inch (2.5 cm) above the tip of the thermometer, the temperature readings should level off. This steady temperature reading will be the boiling point of the liquid. Further heating should not result in a further increase in temperature as long as there is still liquid in the tube. Read the highest temperature recorded while there is still liquid in the tube. (Be careful not to let all of the liquid evaporate or the temperature will rise well above the boiling point of your material.) Once you have your reading, raise the tube out of the hot sand bath to allow the sample to cool without boiling to dryness. Then, compare the boiling point you obtained for 3- pentanone with the one you recorded in your pre-lab. If it does not agree within 2 degrees, repeat the process. Use a fresh boiling stone for each boiling point determination you make. Note: The most common error in determining boiling points results from not heating the liquid until the vapor-liquid condensation line is sufficiently high above the tip of the thermometer. The temperature registered on the thermometer may level off because not enough heat has been added to the liquid, and the boiling point you record will be lower than the liquid s actual boiling point. Don t be afraid to push the boiling point tube deep into the sand bath where the temperatures are hottest. The liquid will boil more vigorously and the true boiling point will be reached. 2. Refractive Index of Known Sample: 3-Pentanone Read the procedure in Appendix 1, which explains how to correctly use the refractometer. Your lab instructor will assist you when you take the refractive index of 3-pentanone. Record the temperature, and correct the experimental refractive index to the correct reference temperature. Compare this corrected value with the reference refractive index listed in your pre-lab. The first three digits should be in perfect agreement with the reference value. 3. Density of Known Sample: 3-Pentanone Carefully determine the mass of a small volumetric flask ( ml) and its stopper. You may need place a small beaker on the balance to keep the flask upright. If so, be sure to tare (zero) the balance with the beaker in place or get an exact mass of the beaker to subtract later. Be careful not to touch the flask with your fingers. Then, using an eyedropper, carefully fill the flask with enough of the sample so that the meniscus of the liquid sits on the line of the neck of the flask. Stopper the flask immediately and weigh it. Determine the density from the mass of the sample and its volume, taking care to consider the correct significant figures. Look up the density or specific gravity of 3-pentanone in the Handbook of Chemistry and Physics. Calculate the percentage error in your average experimental density. (experimental density-book density) % error = 100 book density

6 4. Unknown Measurements Take a vial of your unknown liquid alkane, and record its label number in your notebook. Determine the boiling point, the refractive index, and the density of the unknown, using the same procedures that you used with 3-pentanone. Note that you will be given a limited amount of the unknown. If you need more, you may be penalized one letter grade on your lab report for this experiment. [HINT: Since you are going to have only this limited amount of sample to do one boiling point determination, one refractive index determination, and one density determinations, you will want to plan the order in which you are going to collect your data. It may not be the order in which you collected data for your known sample]. When you are finished with your unknown determinations, discard the unused unknown into the appropriate waste jar. Do not throw unknown down the sink. Post-Lab and Report Requirements Your unknown report should contain the following: 1. A definition of what a boiling point is in terms of vapor pressures. 2. A brief description of refractive index. 3. Create a table showing the experimental boiling point, refractive index, and density of 3-pentanone. Briefly comment on the quality of your results compared to the reference values. 4. Create a table showing the experimental boiling point, refractive index, and density of your unknown, and the boiling point, refractive index, and density, from the Handbook for the Identification of Organic Compounds, of the alkane you think your unknown is. If you had trouble choosing between several alkanes, give the data for all alkanes you considered, but you must select one as the best identification of your unknown. Be sure to write down the number of your unknown in tables and on the report sheet. 5. Explain why you identified your unknown the way you did. What problems did you encounter in identifying your unknown? Which data was most valuable? 6. Explain why the boiling point of a liquid decreases when the atmospheric pressure decreases. 7. Explain why a volumetric flask was used for density determination instead of a graduated cylinder. 8. The refractive index of a sample is determined to be at 21.0 o C. Determine the value of refractive index of this sample at 25.0 o C. Don t forget to write a conclusion/summary. See the file, Writing a Lab Notebook, for examples and guidance.