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BACKGROUND Melting Points and Intermolecular Forces: The melting point of a compound is the temperature at which a solid is in equilibrium with its liquid. The transition from the solid state to the liquid state occurs when molecules acquire enough energy to overcome the intermolecular forces holding them together. The stronger the intermolecular force, the higher the melting point of a compound. The strength of these intermolecular forces is determined by the structure of compounds. Intermolecular forces observed in organic compounds include London dispersion forces, dipole-dipole forces and hydrogen bonding. The weakest of the intermolecular forces is London Dispersion forces. These forces affect all molecules and result from attractions between induced, temporary dipoles. As a molecule’s size (surface area) increases, so does the strength of its London Dispersion forces. Dipole-dipole forces result from attractions between dipoles on polar molecules. As the polarity of molecules increases, so do the dipole-dipole forces. Amongst London dispersion forces, dipole-dipole forces, and hydrogen bonding, hydrogen bonding is the strongest intermolecular force. Hydrogen bonding occurs between the hydrogen atom in a polar bond (especially H – F, H – O, or H – N) and a nonbonding electron pair on a nearby electronegative ion or atom (usually an F, O or N in another molecule). The melting point range is the range of temperature from the point at which a crystalline solid first begins to liquify to the point at which it is completely liquid. Determination of the melting point range can aid in identification of a sample and describing its purity. Most pure organic compounds melt over a narrow temperature range typically 1 – 2 oC. In comparison, an impure compound will melt over a broad range of temperatures. Furthermore, the melting temperature range will be less than that of the pure compound. Melting Points of Mixtures: To understand the melting point behavior of impure compounds, consider a simple binary mixture of compounds X and Y and their melting point-composition diagram (Figure 1.1). This diagram shows melting point behavior as a function of composition. As compound X is added to pure Y, the melting point of the mixture decreases along the curve BA until a minimum temperature is reached at point A. This temperature is called the eutectic temperature, and it is the lowest possible melting point for a mixture of X and Y. The ratio of X and Y at the eutectic point is called the eutectic composition (40% X and 60% Y in this example). Consider a mixture composed of 25% X and 75% Y. In this case, X acts as an impurity in Y. As the mixture of solids is heated, the temperature rises, until the eutectic temperature is reached. At the eutectic temperature, X and Y begin to melt together at point A, the eutectic composition of 40% X and 60% Y. At the eutectic temperature, X and Y melt in the ratio equal to that of the eutectic composition. For instance, if 4 μmol of X melts, then 6 μmol of Y melts as well. The temperature remains constant at the eutectic temperature until all of the X melts. Once all the X is melted, the remaining Y is in equilibrium with the liquid mixture of the eutectic composition. As more heat is applied to the mixture, the temperature rises, and the remaining Y continues to melt along the curve AB. Finally, at the temperature corresponding to point B, all of the Y has melted such that the composition of the mixture is 25% X and 75% Y just like in the original solid sample. Note, in this example, the sample begins to melt at temperature A, and melting is completed at temperature B. This melting point range is broad and less than the melting points of either pure X or Y. This exercise could also be performed for a mixture in which Y is an impurity of X. If a mixture has exactly the eutectic composition, it will appear to have a sharp melting point at the eutectic temperature falsely indicating the presence of a pure sample. To determine whether a sample is pure or of eutectic composition, a small amount of pure X or Y can be added to the sample. If the melting point increases and broadens, the original sample was at the eutectic point. However, the chances of accidentally testing a eutectic mixture are small. To observe melting point broadening and depression, the impurity must be soluble in the compound. In other words, an insoluble impurity like sand will not affect the melting point. However, the impurity can be a liquid (like a solvent used during purification that is trapped in the crystalline lattice). Therefore, it is necessary to dry compounds before determining their melting points. Utility of Melting Point Determination: Determination of melting points is useful for proving whether two unknown samples with the same melting points are identical compounds. If the unknowns are identical, then a mixture of the two will have the same sharp melting point. If the unknowns are actually two different compounds, then a mixture of the two will reduce and broaden the melting point. Measurement of melting points is also useful for assessing compound purity. A melting point range ≥ 5 oC is indicative of an impure compound. Purification of the sample would cause the melting point range to narrow and increase. A maximized melting point range that narrows to 1 – 2 oC indicates that the sample is pure. Measuring Melting Points: Measurement of the melting point of a crystalline solid involves several steps. First, the sample must be prepared. The dry sample is ground to a fine powder and piled on a flat, hard surface. The open end of a capillary tube is pressed into the pile, and the sample is shaken into the closed end of the capillary. This process is repeated until the height of the sample reaches ~ 2 mm. Next, the sample-containing capillary is inserted into a device used to measure points. One such device is a Mel-Temp melting point apparatus (Figure 1.2). It consists of an electrically-heated aluminum block into which a thermometer and samples can be inserted. A light and magnifier allow for easy viewing of the sample(s). The heating rate can be carefully controlled. The low-end of the melting point range of a substance corresponds to the first appearance of liquid within the bulk of the sample. Sometimes the sample will shrink or sag upon heating, but this observation is not indicative of melting. Furthermore, traces of solvent may appear on the outside surface of the sample. This phenomenon is called sweating, and also should not be mistaken for melting. The high-end of the melting point range corresponds to the temperature at which no solid remains in the capillary tube. If the melting point of a compound is known, the sample can be heated quickly to within 10 – 15 oC of its melting point. When the temperature nears the melting point, the heating rate is slowed to increase at a rate of 1 – 2 oC per minute until the sample melts. If the melting point of a compound is unknown, it is convenient to measure an approximate melting point (called the “orientation melting point”) by applying heat such that the temperature rises at a rate of 10 – 15 oC per minute until the sample melts. Once the approximate melting point is known, the apparatus is cooled to approximately 15 oC below the orientation melting point. A new sample is placed in the device, and heat is applied again, but such that the temperature rises at a much slower rate (1 – 2 oC per minute). A slow heating rate is necessary because the thermometer’s reading of the temperature can lag behind the actual temperature of the sample. Common errors during measurement of melting points are usually due to a poor heat transfer rate from the heat source to the compound. Causes include (1) too much sample in the capillary tube, (2) coarselyground samples, and (3) excessive heating rates. Note, the numbers on the Mel-Temp dial do not correlate to temperatures. They correspond to the heating rates seen in Figure 1.3.EXPERIMENTAL PROCEDURES: Part 1: Melting Points of Pure Urea and Pure Cinnamic Acid: 1. Place a small sample of urea into a watch glass and crush it into a fine powder using the back of a spatula. 2. Push the open end of a melting point capillary tube into the powder and force the powder down into the closed end by tapping the closed end of the tube on the laboratory bench. 3. Repeat the cycle of loading and packing until ~ 2 mm of sample is loaded. 4. Place the sample-containing capillary tube into the Mel-temp device, and heat the sample to 117 oC at a rate of 10 – 15 oC per minute. 5. Continue heating the sample, but at a slower rate of 1 – 2 oC per minute. 6. Record the temperature at which melting begins and ends. 7. Repeat the melting point determination of urea by cooling the Mel-temp to 117 oC and repeating steps 5 & 6. If your measurements do not agree within 1 oC, repeat a third time. 8. Repeat steps 1 – 7 for cinnamic acidPart 2: Melting Point of a Urea-Cinnamic Acid Mixture: 1. Make a mixture of urea and cinnamic acid in a proportion of approximately 25% urea/75% cinnamic acid. 2. Crush the mixture into a fine powder and load it into 2 capillary tubes. 3. Determine an orientation melting point for the mixture by heating one of the samples quickly at a rate of 10 – 15 oC per minute until the sample melts. 4. Cool the apparatus to 15 oC below the orientation melting point and place the other sample of the mixture in the apparatus. 5. Heat the sample at a rate of 1 – 2 oC per minute and record the melting temperature range. 6. Repeat steps 1 – 5 for a mixture of urea and cinnamic acid in proportion of approximately 50% urea/50% cinnamic acid.Part 3: Melting Point of an Unknown: 1. Obtain an unknown sample, crush it into a fine powder, and load it into 3 capillary tubes. 2. Determine an orientation melting point using the first sample 3. Determine the actual melting point of the unknown using the second and third samples. If your measurements do not agree within 1 oC, repeat a third time. 4. Determine the identity of the unknown using Table 1.1. Special Waste Instructions:  Dispose of extra solid in the solid waste container.  Dispose of capillary tubes in the broken glass container located in the waste hood.Please fill out the following table for chemicals you need, mentioned in the written procedure? (5 points)Chemical Synonym MW density Tm Tb Safety Information (g/mol) (g/mL@ RT) (oC) (oC)ureacinnamic acidbenzoic acidsalicylic acidphthalic acid Science Chemistry