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Melting Point Determination Lab Report

by: Loreal Williams

Melting Point Determination Lab Report CHE 302

Marketplace > Hampton University > Chemistry > CHE 302 > Melting Point Determination Lab Report
Loreal Williams
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I received a grade of a 94 on this lab report.
Organic Chemistry 1
Dr. Bump
Class Notes
Organic Chem Lab, Organic Chemistry, Melting, point, determination




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This 12 page Class Notes was uploaded by Loreal Williams on Tuesday July 5, 2016. The Class Notes belongs to CHE 302 at Hampton University taught by Dr. Bump in Summer 2016. Since its upload, it has received 8 views. For similar materials see Organic Chemistry 1 in Chemistry at Hampton University.


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Date Created: 07/05/16
Technique G: Melting Point Determination Loréal Williams June 1, 2016 Abstract           The temperature at which the intermolecular forces holding a crystalline solid together are  overcome is considered the melting point of the solid. At this point, the crystallized solid and  liquid are also at equilibrium. Each pure molecule has a unique melting point, with its melting  range usually being less than a degree. The technique of determining melting points of various  organic compounds was learned through this activity. These measurements were obtained  through the use of a thermometer, melting point capillary tube, Theile tube, oil, and Bunsen  burner apparatus. The observations in regards to the temperature range at which the compounds  liquefied were recorded in charts. Calculations to determine the average melting points were also recorded and compared. The technique was finally used to determine the melting points of  impure compounds. The results of the experiment suggest that when multiple compounds are  present together in the form of a solid, the melting point differs greatly from its melting point in  its purest form. Purpose/ Background           The ultimate purpose of this experiment was to quantitatively measure the melting point of organic solids by essentially observing changes in physical states of the compounds through the  use of a thermometer. The melting point is a physical characteristic of a molecule that can prove  helpful in identifying its presence and its purity. It requires that the intermolecular forces that  hold the solid together, including dipole­dipole interactions, hydrogen bonding, and London  1 dispersion forces, be overcome.  The temperature at which these forces are overcome, a  crystallized solid and liquid phases are at equilibrium, and at which the lattice energy of the solid 2 is broken down by heat to transition into a liquid is called the melting point.             The melting point of molecules can be determined by slowly heating a small sample of the  solid and observing the temperature range at which the solid first begins to develop into a liquid  to the point at which the solid sample is fully a liquid. A pure solid has a very small melting  range, usually of less than a degree. To determine if two compounds are identical, equal amounts of the compounds should be mixed and the mixture’s melting point measured the same way a  pure solid would be measured. If the mixture’s melting point is the same as those of the  individual compounds, the 2 compounds are the same. However, if the mixed melting point is  3 lower, one of the individual compounds was an impurity in the other.  This comes as a result of  the disruption of the crystal structure of the pure crystalline solid by the addition of an impurity,  making the lattice energy easier to break.  1 (Benoit, Causton, & Hunt, 2015) 2 (Bump, 2011) 3 (Bump, 2011) Reagents Compound Structures Literature  Melting Point  ( C) o Benzophenone 48 C Imidazole 90 C Benzoic Acid 121 C Cinnamic Acid 132 C Urea 132 C Table 1: Reagents; References 4­8 Procedure/ Observations Part 1: Thermometer Calibration           After being assigned to determine the melting ranges/points of benzophenone, imidazole,  benzoic acid, cinnamic acid, 75% cinnamic acid, and 25% cinnamic acid, the apparatus was set  up. Two clamps and fasteners were first attached to a ring stand with a thermometer then being  placed in a thermometer adaptor and attached to the top clamp. A small sample of benzophenone was then placed in a melting point capillary tube. A sample of imidazole of the same size was  placed in a second melting point capillary tube. The tubes were dropped in a West Condenser to  ensure that the sample was at the bottom of each tube. The two tubes were then held to opposite  sides of the thermometer by a rubber band so the samples were next to the thermometer bulb.            A theile tube containing cool mineral was attached to the lower clamp. Next, the  thermometer/sample assembly was placed into a Theile tube containing cool mineral oil at a  place where the rubber band on the thermometer was not touching the oil. The oil was then  heated above a Bunsen burner and the temperature was noted when each of the samples began to  liquefy and when they were completely liquid. This process was repeated again for  benzophenone and imidazole using new and cool oil. The average temperatures and midpoints  were calculated and recorded in the data chart below. Compound Lit MP Trial 1 Trial 2 Avg. Start Avg. End Midpoint Benzophenone 48 40­60 45­55 43 58 51 Imidazole 90 50­85 45­60 48 73 61 Table 2: Calibration of Melting Points of Benzophenone and Imidazole           After determining the melting points for the first two samples, the hot oil was replaced  with new and cool oil. Small samples of benzoic acid and cinnamic acid were collected in two  separate melting point capillary tubes and the procedure used for benzophenone and imidazole  was carried out. The melting ranges and average melting points were calculated and can be found in the data sheet below. Compound Lit MP Trial 1 Trial 2 Avg. Start Avg. End Midpoint Benzoic Acid 121 120­125 105­110 113 118 116 Cinnamic Acid 132 125­130 105­110 115 120 118 Table 3: Calibration of Melting Point of Benzoic Acid and Cinnamic Acid Part 2: Mixed Melting Point           After completing Part 1, a sample of 25% cinnamic acid and a sample of 75% cinnamic  acid were collected in two different melting point capillary tubes. The procedure used for the  previously used reagents in Part 1 was carried out. The data was calculated and recorded with the rest of the class and can be found in the data chart on the next page. Table 4: Mixed Melting Point  % Cinnamic  Desk  Melting  Midpoin Avg.  Acid Number Range t MP 100 % 115­120 118.0 118.0 75%  3 90­100 95.0 104.3 36 105­122 113.5 50%  20 95­100 97.5 88.3 44 70­90 80.0 61 75­100 87.5 25%  3 85­90 87.5 100.8 36 98­130 114.0 0% (Urea) 20 126­135 130.5 119.0 44 82­119 100.5 61 122­130 126.0 Part 3: Waste Disposal           After each of the trials, the used, heated mineral oil was placed into a bottle labelled “Hot  Mineral Oil” where it would stay to cool down for use again. Another sample of cool and new oil replaced the hot oil in the theile tube. The capillary tubes that had been used previously were  discarded into the broken glass container. At the end of the activity, the theile tubes were not  washed and returned to the box from which it was obtained.  Calculations/ Data To calculate the average start and end of each of the compounds’ melting range, the following  equation was used:                                  [(Start Temperature 1)+(Start Temperature 2)]/2=Average Start Temperature                                 [(End Temperature 1)+(End Temperature 2)]/2=Average End Temperature To calculate the midpoints (experimental melting points) of the reagents from the melting range,  the following equation was used:                                  [(Avg. Start Temp.) +(Avg. End Temp.)]/2=Midpoint To calculate the regression of the technique, the following was used:                                 al MP=0.88×Lit MP+0.72 Chart 1: Melting Point Determination of Tables 2 and 3 Literature Melting Point vs. Observed Melting Point Start 140 End 120 100f(x) = 0.88x + 0.72 Midpoint R²Linear (Midpoint) Observed Melting Point80oC) 60 40 Linear (Midpoint) 40 90 140 Literature Melting Point (oC) Calculations of Midpoint:  Benzophenone: ([(40+45)/2]+[(60+55)/2])/2=51 Imidazole: ([(50+45)/2]+[(85+60)/2])/2=61 Benzoic Acid: ([(120+105)/2]+[(125+110)/2])/2=116 Cinnamic Acid: ([(125+105)/2]+[(130+110)/2]/2=118 Chart 2: Mixed Melting Point Determination Mixed Melting Point 140 120 100 80 Melting Point (oC) 60 40 20 00 0.5 1 Composition Cinnamic Acid (%) Calculations of Average Melting Points: 100% CA (From calibration): ([(125+105)/2]+[(130+110)/2]/2=118  75% CA: (95.0+113.5)/2=104.3 50% CA: (97.5+80.0+87.5)/3=88.3 25% CA: (87.5+114.0)/2=100.8 0% CA: (130.5+100.5+126.0)/3=119.0 Discussion           After comparing the literature melting points and the observational melting points of the  pure compounds, it was seen that there was a slight difference, possibly indicating that there may have been an error in the reporting of the melting range. However, as expected, the  benzophenone liquefied completely before imidazole in both trials, showing that the  temperatures recorded were made for the appropriate compound. The experimental melting  points of benzoic acid and cinnamic acid were close together, almost identical. This may indicate possible error in reporting melting range. The lowered melting points that were recorded for the  different concentrations of cinnamic acid indicated the impurities present in the compounds.  Instead of a pure, 100% cinnamic acid solid with a melting point of about 132, the melting points were lowered when the concentration of cinnamic acid changed. This proved that melting point  is a good technique for identifying specific pure crystalline solids. References Benoit, W. L., Causton, A. S., & Hunt, I. R. (2015, August 17). CHEMISTRY 351 Laboratory  Manual Fall 2015. Retrieved from DEPARTMENT OF CHEMISTRY University of  Calgary. Bump, C. (2011). Technique G: Melting Point Determination. Organic Laboratory Experiments: Semi and Microscale Investigations in General Organic Chemistry, 85. Science Lab; Chemicals & Laboratory Equipment. (n/a). Material Safety Data Sheet Benzophenone MSDS. Retrieved June 2016, from  Science Lab; Chemicals & Laboratory Equipment. (n/a). Material Safety Data Sheet Benzoic Acid MSDS. Retrieved June 2016, from  Science Lab; Chemicals & Laboratory Equipment. (n/a). Material Safety Data Sheet Imidazole MSDS. Retrieved June 2016, from Science Lab; Chemicals & Laboratory Equipment. (n/a). Material Safety Data Sheet Trans­Cinnamic Acid MSDS. Retrieved June 2016, from  Science Lab; Chemicals & Laboratory Equipment. (n/a). Material Safety Data Sheet Urea MSDS. Retrieved June 2016, from msdsId=9927317


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