Lab #1: Melting Points of Organic Compounds
May 15, 2014
1. A “Melting Point Range” refers to the range of temperature at which a solid melts into its liquid state. 2. Pure compounds have narrow or ‘sharp’ melting point ranges, 1°C or less if the compound is very pure. A less pure compound exhibits a broader melting point range, between 3°C to 10°C as well as a depressed or lower range. 3. Crushing the sample allows for greater surface area-to-volume ratio of the powder, this allows for more heat to penetrate the entire sample. This larger surface area-to-volume ratio allows for a sharper more accurate melting point. 4. Approximately 20 crystals should be loaded onto the coverslip to ensure that the sample is heated evenly. 5. The temperature of the stage should be 20°C below the expected melting point when you begin to heat 1-2°C per minute. 6. The melting point range would increase because the heat would not be able to properly penetrate the sample evenly. This would make the center of the large uncrushed sample have less exposure to the heat, and it would melt much slower than a small and evenly spread sample. 7. It is important because if the temperature is increased too rapidly at the melting point, the sample and the aluminum block will not be at the thermal equilibrium with the thermometer. 8. Rapidly heating the sample results in too high of a reading because the sample and aluminum bock are not at equilibrium with the mercury in the thermometer and this will result in a very high reading. 9. A mixed melting point analysis is preformed to determine the identity of an unknown sample. The unknown sample is placed in the heating apparatus and heated using a true (slow) melting point. Once the melting point of the unknown is determined, the known samples are mixed with the unknown to see which one matches the melting point of the unknown. If the unknown sample and the known sample melt at the same temperature as the unknown then they...
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