Melting Point Lab

Melting Point Lab - Melting Point The melting point of a...

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Unformatted text preview: Melting Point The melting point of a compound is the temperature at which liquid and solid phases coexist in equilibrium under atmospheric pressure. The melting point is recorded as a melting-point range. The first number of the range is the point when the crystals start to melt and the second number is the point at which the solid is completely melted to liquid. For example, benzhydrol has a melting point range of 65-67 °C, that means the compound melted over a 2-degree range. The melting point not only can be used in organic chemistry to identify an unknown but it can also be used to determine the purity of a compound. There are two ways to determine the purity of a compound using melting points. First, the purer the compound, the narrower the melting point range. A melting point range is generally accepted to have a range of 3 °C. Second, impurities also cause the melting point to be lower than that of a pure compound (melting point depression). Preparation: Mayo pg 52-56; View the "Melting Point Determination" DVD in the Vasche Library prior to this experiment. Purpose: Identify an unknown substance by first determining the melting point of the unknown, and then using mixed melting points to confirm the identity of the unknown. Procedure: Melting point of an unknown compound. Melting point determination is a simple, but critical skill for every organic chemistry student to learn. Therefore, this experiment will be performed individually. Obtain an unknown from the instructor and make sure to sign the “Unknown Sign-Up Sheet” in the appropriate spot. Record your unknown number in your notebook! Determine its melting point range (see below). The unknown will be one of the substances in the table below. It is advisable that you should run duplicate samples that show a consistent melting point range. After your unknown melting point range has been determined, mix a few mg of your unknown with known samples of comparable melting point ranges and use this information to identify the unknown. You need to mix your sample with at least two different known compounds. Use the Mel-temp apparatus to measure the melting point of your unknown and mixed substrates (Figure 1). (a) (b) Figure 1. Mel-temp apparatus (a) thermometer (b) Mel-temp capillary tubes Melting Point 1 First, obtain a capillary tube from the reagent bench or from your own drawer. Load the sample into the capillary tube by dipping the open-ended side of the tube into a bed of material and introduce about 1 mg of the sample. Gently tap the closed ended side of the tube on the bench top. The sample will slowly move down the tube. Be patient with this process since the capillary tube will break if you tap too hard. Insert the tube into one of the sample slot of the Melt-Temp with the closed-end down (Figure 2). (a) (b) Figure 2. Closeup view (a) slot for sample placement (b) view through eyepiece Each Mel-temp has three wells for capillary tubes, that means more than one melting point can be measured at once; share with other lab mates. Ideally, if you know the expected melting point range you can rapidly raise the temperature to about 20-30 °C below the expected range. At this point, the temperature should only be raised at about 2 °C/min. For this lab since you don't really know when your unknown will melt, you will set your Melt-Temp at about 30% - 50% maximum, which will raise the temperature at about 3-8 °C/min (Figure 3). Observe the sample through the eye-piece (Figure 2b). (a) (b) Figure 3. Closeup of power dial (a) power dial, 0 = min, 10 = max (b) on/off switch Once you know the melting point of your unknown you can make an educated guess from the known list of what your unknown might be. Consequently, you will make three melting point samples: 1. your unknown, again, by itself as the reference compound 2. your unknown mixed with the suspected compound 3. your unknown with another compound that has a similar melting point to your unknown. These three samples should be run at the same time to avoid melting point errors caused by using a different Mel-temp apparatus. If you run the samples at different heat settings, or with different Mel-temp apparatuses, the results may vary. Melting Point 2 Possible unknowns Compound benzophenone benzhydrol biphenyl vanillin acetate phenylacetic acid vanillin 9-fluorenone phenanthrene resorcinol acetanilide fluorene benzoic acid benzamide benzilic acid p -nitrophenylacetic acid salicyclic acid benzanillide Melting Point MP Range (°C) 48-49 65-67 69-72 77-79 77-78.5 81-83 82-85 99-101 110-112 113-115 114-116 121-123 128-130 150-153 154-155 158-161 164-166 3 CHEM 3012 - Spring 2009 Organic Chemistry Laboratory I Name ______________________________ Melting Point – Report Form Raw Data unknown # _______________ Part I: Melting Points unknown - trial 1: _____________ °C unknown - trial 2: _____________ °C Suspect Compounds suspect 1: ____________________ suspect 2: ____________________ Part II: Melting Points unknown: _____________ °C unknown + suspect 1: _____________ °C unknown + suspect 2: _____________ °C Observations appearance of unknown: appearance of suspect 1: appearance of suspect 2: Results unknown identity: ____________________ Melting Point – Report Form 4 Discussion (Must be duplicated in notebook. Limit yourself to the available space.) Conclusion (Must be duplicated in notebook. Limit yourself to the available space.) Melting Point – Report Form 5 Questions (Answer in complete, full sentences. Limit yourself to the available space.) 1. Three test tubes, labeled A, B, and C, contain substances with approximately the same melting points. How could you prove the test tubes contain three different chemical compounds? 2. One of the most common causes of inaccurate melting points is too rapid heating of the melting-point apparatus. Under these circumstances, how will the observed melting point compare with the true melting point? Explain your answer. Melting Point – Report Form 6 3. Melting-point ranges are ordinarily determined in a glass capillary tube. This means that the glass constitutes an impurity in the system. Why does the glass have no noticeable effect on the melting-point ranges? 4. A student suspected that an unknown was undergoing a chemical change at its melting point. Suggest a simple method for testing this hypothesis. Melting Point – Report Form 7 ...
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