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exp1w08 - Chem 6A AN INTRODUCTION TO ORGANIC LAB TECHNIQUES...

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Unformatted text preview: Chem 6A AN INTRODUCTION TO ORGANIC LAB TECHNIQUES, COLLIGATIVE PROPERTIES, & SPECTROSCOPY A guide to teaching assistants and their students Modus Operandi (1-4) 1. TA responsibilities. Safety: For safety, TAs permit a maximum of eighteen students per section and require their students to wear lab goggles and closed-toed-shoes. Instruction and score: For each experiment, TAs give a brief review of the theory and procedure for the daily activities, give a quiz, grade the pre-lab, and inlab parts, and assign technique grades as necessary. After each experiment, TAs will request a photocopy of the entire experiment (pre-lab, in-lab and post-lab write-up). TAs are to complete their grading within one week of receiving the lab report. During each experiment, TAs may assign 2-3 low technique scores (50) and 2-3 outstanding technique scores (90) to students. ALL TAs use the same spread sheet to track their two sections. Each grade sheet will be normalized to a similar average and letter grade spread. Absence and make-up: TAs do not have the authority to cancel class or permit a student to skip a lab. If a student does not attend, TAs should inform them to contact the instructor immediately with their reason. The instructor will contact the chem 6A TAs who will provide a "permission" email informing students of potential make-up times (2 max). If a TA misses a lab, they must inform the instructor and make approved arrangements. 2. General procedures for students in each lab. Each class begins with a 10 minute quiz that emphasizes important aspects of the current experiment or specifics from the past Monday lecture. The TAs will give a 10-30 minute overview of the experiment, including the theory behind the practice; take notes as explained. While you are setting up your experiment, the TA will grade the pre-lab of your notebook in red ink, assigning a score with the date and their initials. Before leaving lab, your TA will again initial the in-lab portion of your notebook to verify your record of the experimental procedure and observations. You should work independently on ALL the experiments. Course Outline NINE EXPERIMENTS Exp. 1 (basic characterization of organic compounds: d, mp, ri) Exp. 2 (separation of liquids by distillation) Exp. 3 (liquid-liquid extraction and separation of A, B, N) Exp. 4 (isolation and purification of a natural product, caffeine: solid-liquid extraction and sublimation) Exp. 5 (rxn 1: photoisomerization, TLC) Exp. 6 (rxn 2: E1 dehydration, GC) Exp. 7 (rxn 3: carbonyl reduction, IR) Exp. 8 (rxn 4: ether synthesis, MS) Exp. 9 (rxn 5: bromination, NMR) Winter 2008 Head TA: L. Miller [email protected] Instructor: Dr. Pettus [email protected] 1 Timeline If your section cannot finish all nine experiments because of holidays, the last two labs can be run on the same day. However, students should write-up the experiments as separate reports. Week 2 Lecture 2: Lab 2: Week 6 Lecture 5: Lab 6: Week 10 Lecture 8: No Lab: Week 3 No Lecture (1/21/08) Lab 3: Week 7 No Lecture (2/18/08) Lab 7: Final EXAM http://www.registrar.sa.ucsb.edu /finals.htm Week 4 Lecture 3: Lab 4: Week 8 Lecture 6: Lab 8: Week 1 Lecture 1: Check in and Lab 1: Week 5 Lecture 4: Lab 5: Week 9 Lecture 7: Lab 9: 3. Lab Reports. Mohrig, Chapter 3 General: The laboratory notebook is a record of all work performed in the laboratory. It is a legal document that gives testimony of performed work. It should be concise, written clearly and neatly, so as to allow a future experimentalist to reproduce the experiment. Your ability to keep a good notebook can affect also your technique grades. Notebook: Students should use a non-perforated notebook. Do not use a spiral/ loose-leaf notebook, and never remove pages from your notebook. The notebook should begin with a table of contents (see page 3 marginal note), including the title of every experiment and the corresponding page number, listed in sequential order. All entries should be written in ink, whether done in advance of the experiment or while making observations. You should never type reports. All pages are to be numbered sequentially. The lab record (pre-, in- and post-labs) is written on only right hand pages of the notebook. This leaves the left-hand pages for notes and extraneous calculations. Contents: Reports consist of three components. The pre-lab (Part I) is completed before coming to lab. During lab, TA lecture notes are then written into the notebook (left-side) and the actual in-lab procedure (Part 2a) is completed on the right-side. The post-lab experimental analysis (Part 2b) is completed after the experiment is done. Part I will be initialed, dated and graded by the TA at the beginning of each experiment. Part I includes: A) a title, the date, your name, you perm number, and your TAs name; B) a purpose/objective; C) a reaction diagram or flow chart; D) table of reagents and products (experiments 5-9); and E) a brief intended procedure. Throughout the experiment, annotations and changes can be made to Part I and Part 2a on the left hand page in ink. Part 2a is written during the lab. It is comprised of A) the actual experimental procedure and B) experimental observations. Before leaving lab you must get your TAs initials Grading Notebooks [20%] part I (30 pts) part II (70 pts) Technique [5%] 50/100 pts (below average) 70/100 pts (average) 90/100 pts (above average) Quizzes [25%] Final Exam [50%] Curved Course Average set to B- / B break 2 to verify you have composed an experimental procedure and have maintained experimental observations during lab. Part 2b, the final post lab write-up, is written after lab. It includes A) your results, B) a discussion of your results, C) a conclusion, and D) references for any literature used. Any loose paper (spectra and quizzes) and TLC should be neatly taped into the notebook following to the appropriate lab. Submission: <lab report for each lab> A photocopy of Part 1 and Part 2a/b including spectra are submitted to the TA for grading by Friday for the previous Monday and Tuesday labs and by Monday for the previous Wednesday, Thursday and Friday labs. Your TA can reduce your notebook grade by 10% for each day it is late. Experiments not submitted by the beginning of exam week will receive a score of zero. <whole lab notebook> You are required to turn your notebook over to your TA at the end of the quarter. It will be used to assist the instructor in assigning your final letter grade. If a student does not turn in their notebook, they should be prepared to lose at least 2/3 of a letter grade from their final letter grade. At the end of the course please write one page about the course, its strengths and weaknesses, as the last page of your notebook. Make suggestions about what you would you do to improve it. Detail of notebook preparation: ---Pre-Lab [Part 1]--- (30%) : need to be done before the beginning of lab A. Title, date, name, student perm number and TA overseeing the experiment B. Purpose/Objective Give a brief introduction to the purpose of the experiment and the approach to be used. Demonstrate that you understand the objective and the key concepts of the experiment. Do not copy directly from the laboratory manual. Usually, one or two paragraphs will be adequate (less than 1/2 a page). Use only the third person, present tense, passive voice when writing the introduction. (ex) Correct: Cyclohexanol is converted to cyclohexene using...... Incorrect: In this experiment, I will be performing an acid catalyzed dehydration... Example: Cyclohexene is prepared from purified cyclohexanol by acid catalyzed dehydration. Trace acid is neutralized with sodium carbonate and the product is salted out of the aqueous extract using brine. The organic layer is dried over magnesium sulfate as a drying reagent. Purified cyclohexene is obtained by simple distillation and it is characterized using IR and NMR spectroscopy by monitoring the loss of -OH and the gain of an alkene. ideal notebook 2 dollars at U-cen <Chem 6a example Index> "STUDENT'S NAME" "PERM NUMBER" Summer 2006 Monday 8:00 am-12:00 pm "TA's NAME", TA in charge (Table of Contents) Experiment 1 page XX Density, melting point and refractive index .......... ............................................... Quiz Score (0-100) .......... ................................ (Part I) Score (0-30) Pre-Lab .......... ............ and Post-Labs (Part 2a-b) Score (0-70) In Composite Notebook Score (0-100) .......... ............................. Technique Score (50,70 or 90) Experiment 2 page XX Separation of water and ethanol (simple and fractional distillation) .......... ............................................... Quiz Score (0-100) .......... ................................ (Part I) Score (0-30) Pre-Lab .......... ............ and Post-Labs (Part 2a-b) Score (0-70) In Composite Notebook Score (0-100) .......... ............................. Technique Score (50,70 or 90) Experiment 3 page XX Acid, base and neutral separation (liquid-liquid extraction and drying) .......... ............................................... Quiz Score (0-100) .......... ................................ (Part I) Score (0-30) Pre-Lab .......... ............ and Post-Labs (Part 2a-b) Score (0-70) In Composite Notebook Score (0-100) .......... ............................. Technique Score (50,70 or 90) continued.... C. Reaction Diagram For the reaction lab (labs 5-9), relevant balanced, and fully labeled, chemical equations should be included. D. Table of Reagents and Products Table of reagents needs to be completed before Lab starts. Especially for the reaction lab (labs 5-9), relevant balanced, this is very important. E. Intended Procedure with Flow Chart Demonstrate that you are prepared for lab by giving a brief description of what you actually intend to do in lab experimentally. A "game plan" or checklist, written in your own words, will save you time in lab. This can be written in paragraph form or as a bulleted list. Do not copy directly from the laboratory manual. The flow chart is helpful to understand the exact procedure of the lab. Beautiful drawing of the equipments is highly recommended. (ex) Weigh lauric acid. Add to 3mL conical vial with stir bar. Add EtOH. Add AcCl. Reflux with water condenser and drying tube for 1 hr... an example of flow chart ---In-Lab-[Part 2a]-- (20%) A. Actual Procedure This is an account of what really was done. Do not regurgitate the laboratory manual. Students need to write whole the procedure as exactly carried out. If the procedure has been modified, or changed in any way from the original way written in lab manual, note the changes here. Remember that the procedure section should be sufficiently detailed, such that another student would be able to repeat the whole experiment based on your report. Keep the following points in mind: (i) Use the third person, the passive voice, and the past tense. Correct: The solution was heated on a hot-plate for 30 minutes. Incorrect: I heated the solution on a hot-plate for 30 minutes. Incorrect: The solution is heated on a hot plate for 30 minutes. 3 (ii) Avoid the "recipe format". Incorrect: Heat the solution on a hot-plate for 30 minutes. (iii) Incorporate your observations into the procedure. (ex) The solution was heated on a hot-plate for 30 minutes, during which time the color of the solution changed from red to green. (iv) Should be written concisely written. Avoid unnecessary detail. Correct: 20 mL of hydrochloric acid (3M) was added to the solution with constant stirring. Incorrect: 20 mL of 22.5 C hydrochloric acid (3M) was poured from a graduated cylinder into a 100-mL beaker containing the solution. During this process the solution in the beaker was stirred with a 15- cm long glass rod having a diameter of 5 mm. How to write a Post-Lab Results: Label and title all attached flowcharts, spectra etc. The results section gives you an opportunity to discuss the significance of your results, to assess the validity of the method, to indicate possible reasons for a poor yield, and so on. Do not over-comment on IR spectra, just pick out and comment on the spectral peaks of importance. Show sample calculations. Remember there is a difference between % recovery yield calculations and % Yield calculations. In the latter, you must determine limiting reagents and a theoretical yield (cf. Exp. 6-9). Discussion: an example A clear colorless liquid with a slight alcohol odor, corrected bp 196-201 C, and refractive index of 1.5262 (at 20 C), was obtained from the reaction of...[also draw and name structure of product here]... The yield of 1-phenylethanol was 13.2 g of clear, colorless liquid, and the % yield was 56%. The theoretical yield for the reaction was calculated to be 23.57 g, but this assumes that all the limiting reagent (acetophenone) reacted and that no byproducts formed (styrene). Thus, this is a fairly good yield for this reaction, which normally gives yields of product around 85% (ref: textbook pp#). The product appears to be pure. According to the CRC Handbook the product should be a clear, colorless liquid, with a bp of 203 C. The product obtained was clear and colorless with a (barometric pressure corrected) bp of 195-201 C. The boiling point of the product was 2 C below the literature value, indicating some impurity and/or error, and boiled over a range of 6 C, which definitely means some impurities are still present. The refractive index of the product was 0.0010 below the literature value of 1.5272, indicating again that some slight impurities are present. The infrared spectrum for the product shows good purity. All the signals for an aromatic/aliphatic alcohol were present; O-H stretch @ 3350 cm-1, aromatic C-H stretch @ 3080 cm-1 and alkane C-H stretch @ 2850-2950 cm-1, C=C stretching @ 1600, 1500 and 1450 cm-1, and C-O stretch for a alcohol @ 1077 cm-1. No bands due to reasonable impurities were observed in the infrared spectrum. The HPLC chromatogram showed high purity, 99.54%, with only traces of acetophenone and styrene being present. The boiling point of the product was 2 C below the literature value, however an uncalibrated thermometer was used to take this reading. This may account for why the temperature reading was low, but does not explain why the product boiled over a range of 6 C. The refractometer used in this experiment was uncalibrated. This is a practical source of error for the experiment. And might partly account for why the RI was 0.0010 below the literature value of 1.5272. Perhaps, rinsing the flask more during transfers would have improved the overall yield. B. Observations Prepare a simple flow chart of the procedure, and record any observations alongside. This will show your scientific engagement and any points that need to be considered as important matters. Correct: The reaction mixture turned green and a precipitate formed. The crude product, a yellow crystal, weight 15mg. ---Post-Lab-[Part 2b]-- (50%) A. Results: This is one of the most important section of your report. Wherever possible, tabulate your data, such as the melting/boiling point with its range. Analyze the IR and/or NMR spectra, and any other observations or measurements. Include all the spectra, which will be provided from your TA as standards, with your interpretations and peak assignments. Especially, show clearly how did you calculate the % yield. B. Discussion This section should be completely based on your results (measured or calculated values) and observations. The values you record are your data, and you need to consider the meaning of your data. You also need to show your understanding of the experiment. Often you need to site references where you can obtain the supporting informations. First, your discussion should state what you've made (draw the structure and name it) and what it appears like (was it as expected, compared to a standard or the literature, e.g. white shiny crystalline solid). Next, discuss the yield and purity of the product(s) you recovered/synthesized. Qualitatively assess the performance. A discussion should quote actual experimental values and not talk in vague terms. Correct: The product obtained was found to be fairly pure, as it had a mp of 110-112 C, a mp range of only 2 C. This result was 3 degrees below the literature value of 115 C for 'compound X'. This also shows that the product was not completely pure. Correct: The infrared spectrum of the alkene product (see page xx of this report) had the absorption bands of the expected alkene, 3050 cm-1 sp2 C-H stretch and 1650 cm -1 C=C sharp absorption. No broad alcohol band was observed at 3300 cm -1, indicating no reagent alcohol remains and that the reaction resulted in the conversion of the alcohol to the alkene product. Incorrect: The product obtained was found to be pure. Data interpretation should demonstrate a clear understanding of the technique, the experiment, and the spectra. The next section of your discussion covers sources of error and loss. Try to think of at least two sources of each. Sources of error include theoretical sources, such as the reaction did not go to 100% completion, and practical sources, such as the instrument or glassware used was not calibrated. Sources of loss include theoretical sources, such as reaction byproduct formation, and practical sources, such as surface adhesion, loss on glassware, and mechanical transfer loss (a spill). Finally, mention at least one way to improve the experiment. C. Conclusion Give your signature and a pledge that all of the observations and conclusion herein are your own and that you believe them to be correct. Then have another person witness your pledge with their signature. D. References: You should reference any literature used in your report, i.e. melting points, spectral data, etc. Use an acceptable scientific journal style/format for your references. Be consistent. Author name (surname, initials.), year published. Title, publisher name, publisher location, page numbers 4. Technique Score. During each experiment, technique scores can be assigned; 3-4 low technique scores (50pts) and outstanding technique scores (90pts) will be assigned. The technique score can be very subjective. Your TA can give you a low score if you 1) are wearing inappropriate clothing, 2) have a messy area, 3) use incorrect disposal techniques, 4) are extremely inefficient in lab, 5) have an unorganized unkept notebook, 6) are not wearing your safety goggles, 7) are unprepared for lab. Failure to listen, learn, and comply will be reflected in your technique grade. Your TA can give you an outstanding score if you 1) clean up a dirty area, 2) ask good questions, 3) have a very neat, organized notebook, 4) are efficient during lab. TAs should strive to have everyone finish the lab course with an average technique score. Therefore, if you receive a low score, you should make use of opportunities (preferably in the same lab) to zero it out with a high score. Conclusion: an example To the best of my knowledge all of the conclusion and observations herein are true [signed], [witnessed], dated. 4 5. Grading. (see page 2 marginal note) Grades are based on technique; in class quizzes; notebooks; final exam. TAs will be entering nine quiz scores, nine notebook scores (total of Part 1+ Part 2a/b), and up to nine technique scores. TA's have been instructed to set the average grade of their section to a 2.67 (B-). All TAs will use the same grading schemes and the same Excel chem 6A spread sheet. The grade sheet is emailed to the instructor at the beginning of dead week. No reports or notebooks are accepted after the end of dead week. The grades in this class are curved! Students generally assume that curving means an upward adjustment of low test scores. However, the basis of this practice derives from assumptions about statistical distributions of scores. If you assume that scores should fit a normal bell curve. Because curving each score could lead to compounding distortions compromising accuracy, the curve is not applied until after the final exam. Consider the past final exam (S06). The average was 67%, the high was a 90% and the low was a 30%. The absence of scores above 90% suggests that I am indeed a hard grader or that I am living up to my standard of challenging students. The averages in each TA sections were then normalized to my average and to my top spread of 23 points ( spread between average and highest score). Some students received A+. A few failed. I am not alarmed by apparently low numbers, but these results do tend to worry conscientious students who are conditioned to think in terms of 90% = A, 80% = B, etc. This can undermine class morale, and low class morale can undermine student ability. That does concern me, so I offer some suggestions for dealing with these anxieties. Evaluate your standing with your TA in terms of how your section is doing. Where are you relative to your class mean? For example, if you have a 75% average, but the section average is 67%, you are doing better than your score might suggest to you. However, you should not assume, that every class will have a certain percentage of A's -- or F's. You might be a strong person in a relatively weak group, or you might find yourself in fast company. Sometimes there are no [A] students in a section. This is also a real-life view of how work is evaluated and is not unique to professors and exams. Grades speak to the world, not just to parents or campus. Grades should mean something more than flattery. The grades in my example, like the grades of this university, are skewed toward the higher end of the curve. Average is a definition for C, but average at UCSB is not C, it is the C+/B- break. This is called grade inflation, and it is not as flattering as most students would like to believe. Grade inflation lowers standards, discounts achievement and hurts students in the outside world more than students realize. If 50% of students receive a grade of A, or graduate cum laude (as reported at some Ivy League schools), what does that tell you about challenge and standards? How much did those students grow during their college years? Did those students obtain an education, or just a credential? How much weight does such an inflated credential carry? Eventually performance shows and counts. I encourage students to sharpen skills now by rising to the challenge. You, and your family, are paying a lot for a real education. Of course, if a student fails to complete the lab work, turn in their reports, skips class or the final exam then little choice is left in my evaluation of their work. 6. Missing Lab. If you miss a lab, you must contact the instructor by email (not the TA) within 24 hours and inform them of your excuse. If accepted, the instructor will make email arrangements for a make-up experiment in another TAs section. TAs should not give unexpected students entry. Students must make-up the experiment with minimal guidance from the new TA, who may be directing a different experiment. Because of waste disposal issues, if you do not make up the experiment within one week, you will receive "0's" for the notebook score. The missed quiz will not count toward your grade. If you make-up the lab, be sure that your TA enters scores for Part 1 and 2a of the make-up into their grade-book. However, even if you make-up a missed lab, TAs are instructed to highlight your records. If you attempt to make-up three or more labs, you will probably receive an [F] as your final letter grade. 7. Rules for the Disposal of Reaction Wastes. Improper disposal of reaction wastes can effect your technique score! Dilute aqueous wastes containing only acids, bases, or salts (which have been neutralized!) may be disposed in sinks. Wet methanol, acetone and ethanol are considered aqueous waste. When mixed with dry organic, however, these solvents are considered organic waste. All organic wastes must be poured into the bottles provided. Please note that there may be separate bottles for halogenated and non-halogenated wastes. Bottles with specific labels for each experiment's waste will be available in the hood. Solids, such as drying agents, are placed in 5 the plastic bags provided or in solid waste containers. TA's will independently dispose of your product vials after grading. Do not leave any unlabeled vials with chemicals in your drawer. At the end of the quarter, you must clear all chemicals out of your locker. When in doubt about how to dispose of something, ask your teaching assistant. Also, see the comments concerning waste given at the end of each experiment. 8. Laboratory Safety. (Mohrig, Chapter 1) Safety can effect your technique score! Dealing with chemicals requires an alertness and awareness of the problems associated with the handling of volatile, flammable, corrosive and toxic materials. Many generations of organic chemists have learned how to do chemistry both safely and enjoyably. It is necessary to be cautious, but not to the detriment of performing the experiments expeditiously. Learn to be aware of the safety requirements, but then to enjoy the experience of preparing materials and analyzing them as efficiently as possible. Syringe disposal. Your TA will give you 1-2 capped needles during the experiments requiring syringes. Upon dispersal, 10-20 points will be subtracted from your quiz grade as an "insurance." When you return the needle, 20-30 points will be re-added to your quiz grade. TA's will put waste needles in the assigned disposal jars. DO NOT PUT NEEDLES IN THE TRASH!! REPORT NEEDLES IN THE TRASH TO THE TA. Safety glasses. Safety glasses must be worn in the laboratory. You will not be admitted into the laboratory unless your eyes are and remain protected. Visitors must also wear safety glasses. Do not wear contact lenses in the laboratory. Organic fumes may harm them, and caustic reagents cannot be washed from the eye if contact lenses are worn. Gloves. Gloves should be worn if you are handling corrosive materials. Surgical gloves will not protect you against strong acids, but they are available for your use at other times. Heavy rubber gloves are available for handling extremely corrosive materials. You may wish to purchase your own rubber gloves and keep them in your locker. Shoes. Sandals or open shoes are forbidden in the laboratory. Clothing which leaves your legs exposed should not be worn, unless a laboratory coat or apron is worn as well. Hair. Your hair should be pulled and tied back from the face so that it cannot be caught in equipment or open flames. Glassware. Glassware is cleaned easiest after every use. Most organic materials are removed from glassware with acetone and water. Soap may not be necessary. Heating. Heat is an ignition source. Only use heat in a working hood. Never heat a closed system! Keep flammable solvents away from flames and heat. Ether has a very low flash point and may be ignited by a hot-plate. Cleanliness. Your locker and bench-top should always be ordered and neat. Do not store chemical or reagents in your locker, with the exception of labeled products and recrystallizations. The balance-area should be cleaned after every use. Clean up any spills immediately. Organic solvents often dissolve plastics and rubber items. Accidents. Safety showers and eye washes are available in the case of accidents. The eye wash fountains at the front of the lab are for flushing the eyes with water after an accident. 9. Laboratory Instruments The IR: The FT-IR is a powerful, but easy to use instrument. First, you will need to acquire background scans by selecting "Background" from the Scan menu. Make sure that nothing is in the sample chamber when this is done. To run a sample, place the plate in the V-shaped sample holder that permanently resides within the FT-IR. Select "Sample Scan" from Scan menu. From the File menu, select "Plot". In the resulting window, select "Plot" once again. Click "Done" once plotting has begun. Because of their expense and moisture sensitivity, the single crystal salt plates should be handled carefully by the edges. You will mostly use "the thin film technique" for your dry samples. To do this, set one salt plate flat on a clean surface (e.g. paper towel, kimwipe, etc.), put one small drop of "neat" (undiluted) sample on the plate, and one drop of methylene chloride to evenly disperse the sample. Let it dry on the plate. You can now run your sample. After recording your spectra, clean the salt plate with dichloromethane and kimwipes, touching the edges only. Put the plate back into the desiccator, or give it to the next student in line. KBr pellets are obtained by using the metal hexagonal nut and tightening the bolts with a torque wrench to approximately 30-40 lbs/square inch. The most important part of the preparation of the pellet is to see that your dry sample is ground well (approximately 5 minutes grinding) and that about 10 times as much dry KBr is added to the mortar and mixed well. If everything is dry, when you carefully remove the screw on the die, you will see an almost transparent pellet that may be mounted by placing the die on the plastic cell holder. The GC: Your TA will show you how to use the syringe, the recorder, and the gas chromatograph for the separation of your products during the elimination experiment. Meet the Toys! melting points Distillations extractions drying agents chromatography NMR & IR spectroscopy polarimetry 6 Experiment 1: (Density, melting point and refractive index) Mohrig, Chapter 10, 13 Theory: These topics have been covered in general chemistry. However, in Chem 6A we use these measurements to identify organic compounds, and in some cases, approximate purity. A. Density (sample identification) Density can be measured in g/mL or mg/mL. It is a very useful property for identification purposes. Be sure to discuss how to measure the volumes of gases, liquids and solids and how to measure the masses of solids, liquids and gases. Density is determined by finding the weight of a specific volume of a liquid. To do this, we will use 1-mL plastic disposable syringes [no needle] to precisely measure 1 mL. The technique involves 1) weighing the empty syringe, 2) filling the syringe to 1 mL with any of the organic liquids (toluene, cyclopentanone and ethyleneglycol) supplied in the laboratory, and then 3) weighing the liquid plus the syringe. The difference is the weight of 1 mL of the liquid or the density is given by the weight of 1 mL. This part of the experiment also involves determining the density (mg/mL) of a 10 wt% salt water solution? What is the density (mg/mL) of a 1M salt water solution? B. Melting point (sample purity and composition) This experiment is designed to 1) introduce students to the use of a typical "melting-point apparatus," which you will use repeatedly throughout Chem 6A-B, 2) to demonstrate that pure compounds have "sharp" melting points and melt over a small temperature range, 3) to demonstrate how an impurity lowers the melting point of a substance and broadens its melting range, and 4) to explain how a "mixed melting-point" procedure can be used to find the eutectic point. The diagram that is constructed can be used to de- Mp apparatus termine the ratio of components in binary mixtures. This part of the experiment involves determining the melting point of ninemixtures of cinnamic acid (mp. 133-134 , MW = 148.16 g/mol) and urea (mp. 132-135, MW = 60.06 g/mol). Students will construct a melting point versus composition diagram. Fill a closed capillary with one of the samples to be measured (about 1/8 to 1/4 inch of crystals), and determine the melting point range (show your students how to fill the capillary). Determine the melting point range of a pure compound and the melting point range of a mixture of that compound with another compound. Construct a melting point vs percent composition diagram (either "percent by weight" or "mole percent"). A mixture has a wider melting range, and the starting temperature is considerably lower than for pure material. Inevitably, it is more difficult to determine when the melting starts as opposed to when it concludes. Students must remember to watch for shriveling, and then record the start at the first sign of a liquid at the edges of a crystal. The end point is recorded when the entire sample is a liquid. The following chart should help you translate the mol % and the wt % of the cinnamic acid/urea mixtures. The weight percent should be on the bottles. The eutectic point is 111 C with a 63 wt % which equals 41 mol % of cinnamic acid. The distance between the dashed and solid lines indicates the melting range. MW of urea is 60.06 and E-cinnamic acid is 148.16. wt. % of cinnamic acid A /urea B 0 (1000 mg B) 10 (100 mg A + 900 mg B) 25 (250 mg A + 750 mg B) 50 (500 mg A + 500 mg B) 63 (630 mg A + 370 mg B) 71 (710 mg A + 290 mg B) 85 (850 mg A + 150 mg B) 90 (900 mg A + 100 mg B) 100 (1000 mg A) mol % 0 4.3 12 29 41 50 70 78.4 100 Lecture Ideas Explain melting point Melting occurs when a compound is at the temperature where the solid and liquid phases are in equilibrium. Most pure organic compounds melt over a `sharp and narrow' range spanning 1-2 C. This melting range gives us an idea of purity, because the melting point decreases "almost" linearly as the amount of impurity increases. Explain the eutectic point The low point in the melting point phase diagram occurs at a very specific ratio of mixtures of compound A and B. This point is called the eutectic point or eutectic temperature. The eutectic mixture is the composition of the mixture of A and B at the eutectic point. At the eutectic point, both compounds are melting simultaneously, resulting in a sharp melting point rather than the broad melting point typically seen for impure compounds. A eutectic mixture is misleading in that it suggests a pure compound! Explain concept of a mixed m.p. Give access to the known pure substance, a mixed melting-point can be used to identity an unknown pure compound. Suppose we suspect the unknown to be benzoic acid (m.p. 120121 C). We find the mp to be 118119 C. This is close, but hundreds of compounds may have this melting-point. We mix a small amount of pure benzoic acid with our unknown. If the melting point is unchanged then we know that the pure unknown compound is benzoic acid. If the "unknown" was not benzoic acid, then benzoic acid would have acted as an impurity, and the m.p. would have been lowered and melted over a broader range. Explain Refractive Index (N). Defined as the relative speed at which light moves through a material with respect to its speed in a vacuum. The index of refraction, N, of transparent materials is defined through the equation shown above. c = 3X108, which is the speed of light in a vacuum and V is the speed of light in some other medium. Since the speed of light is reduced when it propagates through transparent gasses, liquids and solids, the refractive index of these substances is always greater than 1.0. If the refractive index of the product is 0.0010 below or above the literature value, it indicates that impurities are present. C. Refractive Index (sample purity and composition) Not used in many labs, but plays an important role in HPLC detection for compounds. Quiz ideas (1) Calculate the molarity from weight % for 40 wt % KOH / H2O. 7 ...
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