Physical Properties: Identification of a Pure Liquid

Transcription

1 EXPERIMENT Physical Properties: Identification of a Pure Liquid 4 Prepared by Edward L. Brown, Lee University and Verrill M. Norwood, Cleveland State Community Collee To identify an unknown liquid by comparin the experimental and theoretical boilin points and densities. OBJECTIVE Balance disposable lass pipette A P P A R A T U S Bunsen burner rin stand APPARATUS AND CHEMICALS thermometer (-10ºC to 150ºC) rin clamp 1 mm cross section of rubber hose wire auze test tube (10 15 cm lon) boilin stone test tube clamp striker (lihter) Thermometer (or test tube) clamp 1000 ml beaker C H E M I C A L S Unknown liquid (~ 3.5 ml) Pure compounds can be obtained after a chemical reaction (or a series of reactions) usin a purification process. This purified product can be evaluated by comparin its physical properties with those of the same substance in pure form. Physical properties are those characteristics of a substance that can be observed or measured without chemically chanin the substance. Taste, odor, and color are examples of physical properties that are more observable than measurable. Measurable physical properties include meltin point, boilin point, refractive index, density, solubility, and viscosity. If the product of a chemical reaction is not pure, many of these physical properties will reflect the presence of impurities. In addition, careful determination of only a few of these physical properties can discriminate between two possible compounds. Many scientists devoted their lives to collectin and tabulatin the physical properties of the elements and compounds. Two Copyriht 2005 Chem21 LLC. No part of this work may be reproduced, transcribed, or used in any form by any means raphic, electronic, or mechanical, includin, but not limited to, photocopyin, recordin, tapin, Web distribution, or information storae or retrieval systems without the prior written permission of the publisher. For permission to use material from this work, contact us at info@chem21labs.com. Printed in United States of America.

2 books have emered as references in this area: N. A. Lane s Handbook of Chemistry and The Chemical Rubber Company s Handbook of Chemistry and Physics. The handbook for this lab has been condensed to Table 1 eiht oranic liquids and their densities, meltin points and boilin points. With meticulous laboratory care, your collected data and this table will allow you to identify your particular unknown. The boilin point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure of the atmosphere above the liquid. At the boilin point, bubbles of vapor form within the liquid and rise unimpeded to the top of the liquid, breakin the surface tension and releasin the vapor into the atmosphere above. If the pressure of the atmosphere above the liquid is exactly 1 atmosphere, then the temperature at the boilin point is called the normal boilin point. Because boilin points are pressure dependent, certain corrections to the boilin point measurement must be made if the pressure is sinificantly different from 1 atmosphere. Fortunately for this lab, as lon as the atmospheric pressure is 760 mmh +/- 10 mm H, then the correction will be less than 0.3º C (well within the error rane of readin a thermometer). The density of a substance relates two common measurements, mass and volume, as a ratio that is unique (thus meaninful) to the substance bein studied. Whether one has only a speck of a pure substance or a warehouse, the density will be the same. M D= V Equation 1 In this lab, mass will be determined usin a balance. Many balances have a TARE button which allows you to place a sheet of weihin paper (or beaker / watchlass) on the balance and then tare the mass to Once tared, the balance will report the mass of the sample you place on the weihin paper (or in the beaker / watchlass). It is IMPERATIVE that you use the SAME BALANCE when makin measurements durin the SAME LAB!! We are enerally interested in the differences in mass and this measurement is accurate only if the SAME INSTRUMENT is used for both measurements. Typically, liquid volumes are determined usin calibrated lassware such as raduated cylinders, volumetric flasks, burettes, and pipettes. In today s lab, a reen chemistry approach to determinin the density and boilin point will be employed. In this reen chemistry approach, the total volume of liquid unknown you will be iven is ~ 3.5 ml. CHEM Experiment 4 4-2

3 After the density and boilin point of the liquid is determined, there will be ~ 1 ml of chemical waste that must be placed in the labeled waste container in the hood. A. Thermometer Calibration PROCEDURE Useful measurements result only from usin equipment that is precise and accurate. Precision enerally results when a reat deal of care has been used by a student to reproduce a measurement. However, even careful students will obtain bad results when their equipment is inaccurate. Calibration is a process where equipment inaccuracies are exposed and corrections are determined. Durin any calibration process, it is critical that students exercise the reatest amount of precision possible in order to obtain an accurate instrument. At the front of the class are two lare beakers of water one containin ice and water and the other containin boilin water. Each student will measure the temperature of water in the two beakers to calibrate their thermometer. 1. Place your thermometer in the beaker containin ice and water. Make sure the beaker has plenty of ice (at least ½ ice) and carefully stir the contents with your thermometer to make sure the entire solution is at the same temperature. 2. After 1 minute, record the temperature (make sure the thermometer is still in the ice water when you read the temperature don t lift it out) to the nearest 0.2 C [Data Sheet]. 3. Next, place the same thermometer in the beaker containin the boilin water. Make certain the water is boilin and do not allow the thermometer to touch the sides of the beaker. 4. After 1 minute record the temperature to the nearest 0.2 C [Data Sheet] - make sure the thermometer is still in the boilin water when you read the temperature. B. Density of a Liquid Unknown 5. Obtain a numbered vial containin a liquid unknown Do Not Share This Unknown With Anyone Else In The Class! 6. Record the four diit number of your unknown liquid [Data Sheet]. 7. Construct a micropycnometer accordin to the directions below and those iven by your Instructor. If you are unable to construct the micropycnometer after 1-2 tries, you may obtain one that is already made from the Instructor s desk. CHEM Experiment 4 4-3

4 Obtain a clean dry Pasteur pipette 1. you will construct a micropycnometer accordin to Fiure 1. Grasp the pipette at its ends with both hands and place it in 2. Pull Apart an oxidizin (nonluminous) flame 1 cm away from the tapered end. Rotate the pipette at the tip of the 3. Micropycnometer inner blue cone to evenly distribute the heat. After a minute, the lass will bein to soften. When it has softened to the point that it is difficult to rotate the pipette without twistin the lass (Do Not Twist The Glass), remove the pipette from the flame and immediately (within 0.5 seconds) pull the two ends apart (~ 25 cm or about 10 inches). Hold Fiure 1 this position 3-5 seconds to allow the lass to cool and the capillary to become riid. 8. At ~ 0.5 cm from the bottom of what will become the micropycnometer, heat the capillary stronly to cut the lass into two pieces. 9. Continue heatin the bottom of the micropycnometer to form a rounded end. 10. Allow the micropycnometer to cool before weihin it. Place a small beaker on the balance first and TARE its mass to Then, place the micropycnometer into the beaker and record its mass [Data Sheet]. 11. Place a rubber pipette bulb on the pipette you constructed when you made the micropycnometer and use this pipette to transfer your unknown liquid into the micropycnometer. Avoid air bubbles Make sure the micropycnometer is completely filled with liquid Don t break the lass tip of your pipette inside the micropycnometer 12. Place a small beaker on the balance first and TARE its mass to Place the micropycnometer filled with unknown liquid in this beaker and record its mass [Data Sheet]. CHEM Experiment 4 4-4

5 13. Remove the unknown liquid with the same delivery pipette Place This Liquid In A Clean, Dry Test Tube (this will be used in Step 17)!! 14. Return the vial containin any extra unknown liquid to the Instructor s Desk. 15. Fill the micropycnometer to the top with distilled water usin the pipette you constructed. Avoid air bubbles Make sure the micropycnometer is completely filled with water Don t break the lass tip of your pipette inside the micropycnometer 16. Place a small beaker on the balance first and TARE its mass to Place the micropycnometer filled with water in this beaker and record its mass [Data Sheet]. C. Boilin Point of an Liquid Unknown 17. Add a boilin stone to the Unknown liquid already in the Pyrex test tube from Step 13 [see Fiure 2]. 18. Secure the test tube with a three-finered clamp and lower it into a beaker filled with tap water. 19. Heat the water rapidly with a Bunsen burner until the unknown liquid beins to boil. 20. Clamp (or hold) your thermometer approximately 1 cm above the boilin unknown liquid for about 20 seconds before recordin the temperature to the nearest 0.2 C [Data Sheet] the liquid should be drippin off the end of your thermometer when you read the temperature. Lift the test tube out of the hot water immediately after recordin the boilin point to prevent the vaporization of your unknown into the lab s atmosphere. Fiure 2 Boilin Stones CHEM Experiment 4 4-5

6 D. Unknown Identification 21. Compare your unknown s density and boilin point to those recorded in Table 1 to identify your unknown. Report the identity of your unknown in your online report sheet. Substance Chemical Formula Density (/ml) Meltin Point ( C) Boilin Point ( C) Acetone (1) C 3 H 6 O ,3-Butanedione (2) C 4 H 6 O Cyclohexanol (3) C 6 H 12 O Chloroform (4) CHCl Bromoform (5) CHBr Hexane (6) C 6 H Isopropyl alcohol (7) C 3 H 8 O Methyl alcohol (8) CH 4 O Table 1 Waste Disposal: Place any liquid remainin from the boilin point determination in the waste beaker in the hood. You should have returned the unknown vial (alon with any unused liquid unknown) to the Instructor s desk. Lab Report: Once you have turned in your Instructor Data Sheet, lab attendance will be entered and lab attendees will be permitted to access the online data / calculation submission part of the lab report (click on Lab 4 Identification of an Unknown Liquid). Enter your data accurately to avoid penalty. The lab proram will take you in order to each calculation. If there is an error, you will be iven additional submissions (the number and penalty to be determined by your instructor) to correct your calculation. CHEM Experiment 4 4-6

7 CHEM Lab 4 Student Data Sheet Actual Freezin Point of Water 0 ºC Actual Boilin Point of Water 100 ºC Freezin Point of Water recorded on Your Thermometer ºC Boilin Point of Water recorded on Your Thermometer ºC Unknown Number (liquid) Mass of micropycnometer Mass of micropycnometer + unknown liquid Mass of micropycnometer + water Boilin Point of the Liquid Unknown ºC CHEM Lab 4 Instructor Data Sheet Actual Freezin Point of Water 0 ºC Actual Boilin Point of Water 100 ºC Freezin Point of Water recorded on Your Thermometer ºC Boilin Point of Water recorded on Your Thermometer ºC Unknown Number (liquid) Name: Mass of micropycnometer Mass of micropycnometer + unknown liquid Mass of micropycnometer + water Boilin Point of the Liquid Unknown ºC CHEM Experiment 4 4-7