Melting Points and Mixture Melting Points PDF

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General Chemistry I CHEM 230- 085B

Jorge Padilla

Experiment #:1 Melting Points and Mixture Melting Points Experiment conducted on Friday, February 12th, 2021 Instructor: Professor Malleswari Ponnala

Objectives: The objective of this laboratory is to understand melting points, a method used to identify unknown samples through its melting temperature. A melting point technique is used to identify an unknown from a list of possible compounds through the temperature obtain from the unknown. Materials: a: Organic compounds: 4-hydroxyacetopheneore, acetanilide, benzoic acid, benzamide, malonic acid, salicylamide, salicylic acid, 4-hydroxyacetanilide, D-tartaric acid. b: Equipment: goggles, capillary tube, melting point apparatus, rubber gloves, watch glass, lab spatula. Methods: 1. At the lab the equipment was set up, and there will be 9 organic compounds as well . One of the compounds will be chosen by the instructor that will be labeled as the” mp unkown”. The procedure that was given by the lab manual was followed. 2. The compound was chosen and only a small amount, about 2-3 mg, and was placed on a clean dry watch glass. The compound was not in a form of a fine powder so it was crush with a lab spatula. A melting capillary tube was pushed by inserted the open end of the tube towards the crushed powdered. 3. Once the powder was packed onto the closed end it was dropped in a 2-3’ long piece of glass tubing. It was made sure that there was no more than 1-2mm of the powder at the bottom of the tube. If so this would interfere with the results, by giving a larger melting point range. 4. The tube was placed into the Thomas-Hoover apparatus. This determines what the approximate fast melting point, slow melting point and a mixed melting point.

The difference between slow and fast melting point is how fast the temperature ramp is increasing degrees per Celsius. The slow melting point is more accurate, when the temperature increasing is no more than two degrees Celsius per min. 5. The Thomas-Hoover apparatus was cooled down to about 69 degrees Celsius. That was because none of the compounds we had dose not melt below 69 degrees Celsius. The fast melting point was first done, so the tool was set plateau to 175 degrees Celsius. The plateau button was hold, while the up triangle button was hold as well until the tool was set to 175 degrees Celsius. After that was done that, hitting the start button will begin ramping up the temperature. 6. The compound started to melt at around 127 degrees Celsius. The apparatus then got to 137 degrees Celsius and that was when the compound was completely melted. That was then recorded and labeled as the “mp unkown,” After that the stop button was pressed. 7. To be able to get a slow melting point, which would allow giving a more accurate temperature the plateau was set to 120 degrees Celsius. 7 degrees lower when the compound started to melt, so then the apparatus can increase 2 degrees to give more time to absorb the heat. The compound began to melt at around 124 degree Celsius. It then completely melted at 126 degrees Celsius. By looking at table 1.1 we can eliminate all compounds giving us only benzamide and benzoic acid. 8. Some of the “mp unkown” sample was taken from the glass with a lab spatula and was place on a watch glass mixed with benzaoic acid, and some benzamide was place on another watch glass with a lab spatula as well with the some “mp unkown.” Both were crushed with the lab spatula until it was all completely form into powder. It was all mixed and was inserted into two separate capillary tubes.

9. Once both tubes was inside the apparatus it was determined that the capillary tube with benzoic acid and the “mp unkown” had no depression had a same exact melting point at 124-126 degrees Celsius. Compare to the other tube which had benzamide melted at 110 degree Celsius and it had huge depression.

Table 1.1 – Melting Point Knowns Organic Compound 4-hydroxyacetopheneore acetanilide benzoic acid benzamide malonic acid salicylamide salicylic acid 4-hydroxyacetanilide

Literature mp (°C) 109-111 113-115 121-123 125-128 132-135 140-144 158-161 169-171

D-tartaric acid

172-174

Questions 1. In the following table, you will find the name, observed melting point and literature melting points of several substances. Based on these data, what would you say about the purity of these substances?

c) 3-chloro benzoic acid is the purest compound in the following compounds because it has sharp melting point range and average melting point is slightly different (0.5 °C) from literature melting point. 2. Ibuprofen has a literature melting point of 75-77 °C. A student carried out a synthesis in which she hoped to obtain ibuprofen. She isolated a product having a melting range of 75-77°C. a) Many substances have their melting point in the range of 75-77 degree Celsius. So, we can't say that her sample is ibuprofen. We need more information to decide whether it is ibuprofen or not. Generally solids melt at a range of temperature instead of specific temperature. (b) We will take isolated compound and ibuprofen and mix both of them. Then we will determine the melting point of mixture. If isolated compound will be ibuprofen then melting point range of mixture comes out to be in range of melting point of ibuprofen. If isolated compound will not be ibuprofen then melting point range comes out to be lower or greater melting point range than the substance alone. By this way, we can identify whether she have ibuprofen or not. 3. The melting point of a pure compound is a unique property of that compound, independent of its source. Is this statement true or false? Explain your reasoning. True, a solid will have particular lattice structures. Thus the interactions are fixed and thus we have a specific melting point for such lattice structure. Melting means breaking of lattice strucres. So it depends on lattice structure but not how we arrived lattice 4. No two pure compounds can have the same melting point. Is this statement true or false? Explain your reasoning. False, for a pure compound the melting point is quite sharp only occurs over a 0.5 -1. 5. Explain why is it important to pack the capillary well when determining the melting point of the samples.

 It is important to heat the melting point bath slow and steadily because it will properly let the solution reach thermal equilibrium. If the solution is heated too fast, it will broaden the observed meltin  It is important to heat the melting point bath slow and steadily because it will properly let the solution reach thermal equilibrium. If the solution is heated too fast, it will broaden the observed meltin If the sample is not packed tightly, it will heat unevenly causing the sample to not reach thermal equilibrium and creating a broader melting point. 6. The compound X is phenyl succinate because when both of them are mixed there is no significant change in the literature melting point. Hence it is not O-toluic acid.

Discussion:

The melting point of a substance is a physical property that can be used for its identification. It is a measure of the amount of kinetic energy, heat that must be supplied to the particles of the substance in order to overcome the intermolecular forces that confine them to the solid state. The determination of melting points is particularly important to organic chemists, since they often work with solid molecular compounds that have low melting points is below 300 degrees Celsius and which can be conveniently measured. Organic compounds are used in this experiment for the same reasons. The melting point of a substance is one of the physical properties that chemists use to identify a substance. The melting point is the temperature at which a substance changes from a solid to a liquid state. A pure crystalline organic compound usually has a sharp and characteristic melting point range of 0.5 to 1 °C. The melting point range is determined by recording the temp at which melting first begins and the temp at which melting is complete. You may recall that impurities depress the melting point of a substance. They also increase the range of melting. When a sample melts at a lower than expected temperature over an extended range, this is a sign that the sample was not pure. Adding on, the melting point of a compound is also a criterion for purity and well as for identification. A technique known as a "mixed melting point" may be used as additional evidence in identifying a given compound. First, a melting point is taken of the unknown and a tentative identification is made using literature data. Then the unknown sample is mixed with some authentic sample of the suspected compound and the melting point is taken of the mixture. If the mixture shows no depression in the melting point, the two compounds almost certainly were the same and the identification of the unknown is confirmed. If the mixture shows a depression of melting point, the two compounds were not identical.

Often solid substances undergo some unusual behavior prior to actual melting. Compounds may decolorize, decompose, soften, or shrink. It is normal for compounds to appear to shrink immediately before melting. Actual melting begins when the first drop of liquid becomes visible. When researching your compounds, be sure that record any decomposition temperature listed, due to some solids decompose into other products before they melt. Conclusion: Overall we used melting points to determine the purity of organic compounds. Pure compounds were seen to have a narrow melting range, while impurities were seen to cause the melting range to become lower and broader. From the mixture melting points we were definitively able to identify our unknown benzoic acid....