E10 Mixed Melting [JFM] Kai.docx

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CHM 17 Experiment #10 MELTING POINT DETERMINATION OF KNOWN AND UNKNOWN SUBSTANCES INTRODUCTION Melting point (MP) can be defined as the temperature at which there is a change of state from solid to liquid, and when these two phases are in dynamic equilibrium. In the case of phase change, dynamic equilibrium means that the processes of melting and solidification are occurring at the same time. Actually, melting point is not a single temperature value, but rather a melting range, that is, the temperature interval or range at which a solid completely changes from solid to liquid. A pure substance has an expected narrow melting range of 1.0 to 3.0 °C. For example, the reported MP of pure benzoic acid is 121-122°C. The presence of impurities would probably lower the melting point and widen its range to between 105-120°C, depending on the quantity of impurity present. The melting behavior of a sample thus gives a clue about its purity. Watch the brief video included here to illustrate how melting point is determined and what you would observe as a solid melts. https://www.youtube.com/watch?v=8T0OSYI5pYk (145) Mixed Melting points video project 1 Organic Chem lab - YouTube The determination of the melting point is used, among other tests, to identify a compound and to obtain evidence of its purity. Standard reference literature contains long lists of carefully measured physical properties, which include melting and boiling points for both organic and inorganic compounds. In this experimental simulation, you will be given 3 unknown compounds. Comparisons of the MPs of your samples with those listed in Table 1, would narrow the list of substances that your compound might be. Although only a few are on the list, there can be many substances that may have very similar melting points to those of your unknowns. The identity of an unknown compound may be determined by a method called the mixed melting point technique. It is used in this experiment to determine if two samples are identical or not. The requirement is that you are able to obtain a pure sample of the substance that you suspect your unknown sample to be. If a known substance is mixed together with your unknown, and there is no change in the mixed MP, you can assume that your unknown and the known substance are the same. But, if the two samples are, in fact, different substances, then there would be a marked difference in their mixed MP. This is because one substance behaves like an impurity in the other. Impurities will lower the melting point temperature and widen the melting range when compared to the melting point of the pure substance. Note: Only impurities that are soluble in the solid can impact the MP. Impurities such as paper fibers and dust have no effect on the MP. In this process, an unknown compound is mixed together in equal quantities with a known compound, and the mixture’s MP is determined. If the mixture melts at a lower temperature than the known compound or if the melting range is broad, then the known and unknown are not the same. If the mixture melts at the same temperature and range as the known compound, then the unknown and known are the same. Read the attached handwritten lab notes provided by your former lab partner, who left college to join an expedition in search of lost cities, where there are no lines of communications. Your challenge is to identify the 3 unknown compounds by using the notes she left behind. PROCEDURE 1

Melting Point Experiment I. Melting Points of Pure Substances 1. In this simulated experiment, you are given 3 samples (A, B, and C) of pure organic compounds, whose melting point ranges are: A. 79-81°C B. 111-113°C C. 165 to 167°C 2. Record these values in the proper blank spaces on top of page 4 of the attached report form. 3. In the Section, "Experimental Data" on page 4, record the names and melting point values of all the candidate compounds from Table 1 that fall into the same melting ranges of each of the unknowns. II. Mixed Melting Points 1. Carefully read your partner's notes that are in the Appendix. They contain the observations about her experiments studying the melting point ranges of samples A, B, and C when they were mixed with the candidate organic compounds listed in Table 1. 2. To help organize your thoughts about this experiment, collect the mixed melting point data from these lab notes and enter them into the appropriate spaces in the "Experimental Data" section of the attached report form. You must complete one form for each unknown on the Summary Sheets on pages 4 and 5. III. The Identification of An Unknown Substance by Mixed Melting Point Technique 1. For each unknown, you will learn that one of the mixed melting point ranges is narrow and close to the literature value of the unknown. The other mixed melting point ranges are broad and lower than the literature values. 2. Now you can identify your unknown samples by writing their names after each Summary Sheet in the blank lines, "Identity of Unknowns A, B, and C". IV. Deliverable Answer the supplemental questions on page 5, and submit it along with the report forms on pages 3 to 5 for this experiment. 2

Melting Point Experiment Experiment #10 – Melting Point Determination Report Form Name_____Kaisha Diaz_________ Section ____CHM 17_____ Date ____________ Table of Known Compounds Table 1 lists 33 pure compounds, several of which you will select to mix with each of your unknowns, so that you can determine the melting points of the mixtures, and thus the identity of the unknowns. Table 1. Known Compounds for Melting Point Experiment COMPOUND mp range ( o C) COMPOUND mp range ( o C) 1) 1,4-dimethoxybenzene 54-56 21) 4-nitroaniline 146-149 2) 2-phenylphenol 57-59 22) 1-napthoic acid 158-160 3) diphenylacetylene 59-61 23) salicylic acid 159-161 4) 2,4,6-trichlorophenol 64-66 24) triphenylmethanol 160-162 5) biphenyl 68-70 25) sulfanilamide 165-167 6) methyl 3-nitrobenzoate 78-80 26) 4-bromoacetanilide 165-169 7) naphthalene 80-82 27) itaconic acid 165-168 8) vanillin 81-83 28) d -mannitol 166-168 9) acetamide 79-81 29) acetaminophen 168-172 10) phenoxyacetic acid 98-100 30) succinic acid 184-186 11) resorcinol 109-112 31) hippuric acid 187-191 12) 3-toluic acid 111-113 32) 4-terphenyl 212-213 13) acetanilide 113-115 33) anthracene 210-215 14) 4-hydroxybenzaldehyde 112-116 15) (+/-)-mandelic acid 119-121 16) benzoic acid 122 17) 2-naphthol 120-122 18) 2-benzoylbenzoic acid 126-129 19) trans-cinnamic acid 132-135 20) urea 132-135 References: 1) Aldrich Handbook of Fine Chemicals , 2007-2008; 2) The Merck Index , 12 th edition; Budavari, S., Ed.; Merck & Co., Inc.: Whitehouse Sta., NJ, 1996. 3

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Melting Point Experiment In the blank spaces below record the names and the literature melting points of the 3 unknown compounds available to you. List these unknowns in order of increasing MP. Labels of the Unknowns Literature MP A 79-81°C B 111-113°C C 165 to 167°C Experimental Data Mixed Melting Points Summary Sheets For each unknown, fill in the data taken from your lab partner's notes in the Appendix. Unknown A: Melting point of Unknown A: 79-81°C Candidate compounds from Table 1: 1. methyl 3-nitrobenzoate_______________________________ M.P. 78-80°C 2. naphthalene________________________________________ M.P. 80-82°C 3. vanillin___________________________________________ M.P. 81-83°C 4. acetamide_________________________________________ M.P. 79-81°C Mixed melting point ranges with unknown A: Unknown + 1: methyl 3-nitrobenzoate_________________________ M.P. 68-73°C Unknown + 2: naphthalene__________________________________ M.P. 61-67°C Unknown + 3: vanillin_____________________________________ M.P. 62-71°C Unknown + 4: acetamide___________________________________ M.P. 79-81°C Identity of Unknown A: Acetamide Unknown B: Melting point of Unknown B: 111-113°C Candidate compounds from Table 1: 1. resorcinol_________________________________________ M.P. 109-112°C 2. 3-toluic acid_______________________________________ M.P. 111-113°C 3. acetanilide________________________________________ M.P. 113-115°C 4. 4-hydroxybenzaldehyde_____________________________ M.P. 112-116°C Mixed melting point ranges with Unknown B: Unknown + 1:resorcinol_____________________________________M.P. 110-113°C Unknown + 2:3-toluic acid__________________________________ M.P. 89-99°C Unknown + 3: acetanilide___________________________________ M.P. 95-103°C Unknown + 4: 4-hydroxybenzaldehyde________________________ M.P. 92-104°C Identity of Unknown B: Resorcinol_____________________________________ 4

Melting Point Experiment Unknown C: Melting point of Unknown C: 165 to 167°C Candidate compounds from Table 1: 1. sulfanilamide_______________________________________ M.P.165-167°C 2. 4-bromoacetanilide__________________________________ M.P. 165-169°C 3. itaconic acid_______________________________________ M.P. 165-168°C 4. d -mannitol________________________________________ M.P. 166-168°C Mixed melting point ranges with Unknown C: Unknown + 1: sulfanilamide_________________________________ M.P. 149-155°C Unknown + 2: 4-bromoacetanilide____________________________ M.P. 164-166°C Unknown + 3: itaconic acid__________________________________ M.P. 142-152°C Unknown + 4: d -mannitol___________________________________ M.P. 140-150°C Identity of Unknown C: 4-bromoacetanilide Complete the following exercises in the spaces provided: 1. Describe the effects of impurities on the melting point of a pure substance. The presence of even a small amount of impurity will lower a compound's melting point by a few degrees and broaden the melting point temperature range. 2. Based on your experience with this simulated experiment, what is a reasonable definition of a pure substance? The substances that are free from any kind of mixture and contain only one kind of particle are pure substances. 3. An unknown solid has a melting point of 132-135°C. A mixed melting point of the unknown with urea, gives a melting point of 120-126°C. Based on your understanding of the effects of impurities on melting points, is the unknown urea or not? Briefly explain your answer. If the MP range is lowered and widened, it means that the two are different compounds. If the MP stays the same it means that the two compounds are likely identical. This technique is known as a mixed melting point determination. Urea has a melting point of 132 -134°C but based on our understanding of melting point we can say that the unknown compound was not Urea. 4. a) An unknown solid has a melting point of 112-114°C. Based on data from Table 1 above and the mixed melting point technique of this experiment, you can narrow down the possibilities to three compounds. List the compounds. 3-toluic acid_______________________________________ M.P. 111-113°C acetanilide________________________________________ M.P. 113-115°C 4-hydroxybenzaldehyde_____________________________ M.P. 112-116°C 5

Melting Point Experiment b) A mixed melting point of the unknown with acetanilide gives a melting point of 113-114°C. Is the unknown acetanilide or not? Briefly explain your answer. The mixed melting point is closer to the actual melting point of pure acetanilide which is 114.3°C so according to what we know that if the MP stays the same it means that the two compounds are likely identical. So we can say that the unknown is Acetanilide. APPENDIX Name: Archie Ologist Class: CHM17 Section: Grad-U-8 Date: 12/12/2019 Observations - Mixed Melting Points SAMPLE A - mp = 78-81°C When A was mixed 50:50 with methyl-3-nitrobenzoate, mp was 68-73°C; with acetamide, mp was 79-81°C; with naphthalene, mp was 61-67°C; with vanillin, mp was 62-71°C SAMPLE B - mp = 111-113°C When B was mixed 50:50 with resorcinol, mp was 110-113°C; with 3-toluic acid, mp was 89-99°C; with acetanilide, mp was 95-103°C; with 4-hydroxybenzaldehyde, mp was 92-104°C SAMPLE C - mp = 165-167°C When C was mixed 50:50 with sulfanilamide, mp was 149-155°C; with 4-bromoacetanilide, mp was 164-166°C; with itaconic acid, mp was 142-152°C; with d-mannitol, mp was 140-150°C 6

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