Idea Transcript
Chem-105, Glendale Community College
Experiment 1: Determination of Melting Points of Pure Compounds and Mixtures Objective: in this experiment, the students will learn how to: • • •
Obtain a melting point of solid compounds and evaluate the identity and purity of the tested solid Determine the differences between melting behavior of pure solids and mixtures Evaluate differences between eutectic mixture and pure compound
Chemicals used: trans-cinnamic acid, urea, and their mixtures. Glassware and equipment used: Melt-point apparatus, melting point capillaries, spatula, weighing paper. Techniques: melting point determination. Target Molecules: O O OH H2N
NH2
urea
trans-cinnamic acid
Introduction: At room temperature, a compound can be found in solid, liquid, or gas phase. The transition from solid to liquid phase occurs at a characteristic temperature called melting point (mp). The melting point of pure compound is observed over a range of 1-2oC. It is known as the melting point range and is given by a lower temperature, at which compound just begins melting (the first drop of liquid noticed), and the higher temperature, at which all solid is converted into a liquid. Melting point can be used for: 1. Identification of compounds: each compound has its own mp and this physical characteristic can be used for a quick qualitative determination. If an unknown compound is mixed with its possible candidate, then the mixed melting point should correspond to that of the pure compound. 2. Determination of purity of compounds: mp can be used for determining whether the compound is pure or not based on the following The purer the compound, the higher is the melting point The purer the compound, the narrower the mp range. The melting point of a substance decreases as an impurity B is added.
Fig. 1. Melting point-composition diagram
This can be summarized in a mp composition diagram. Lets examine a mixture of compounds A and B (TA >TB, Fig. 1). We start with 100% B. As a pure compound, it will melt within a narrow range. As a small amount of compound A is added, the picture will change. Since compound B has a lower melting point, it will start melting first. As the first drops of B appear, compound A with a higher mp will dissolve in liquid A. As it dissolves, its mp will decrease because the crystal lattice of B is disturbed and it will take less energy to melt it. At the same time, the range will widen, because now it will take longer to
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Chem-105, Glendale Community College
completely melt the mixture with a higher mp component in there. As we increase the % of A in the mixture, the melting point will lower, and the range will become wider. However, at some point the solution of liquid A will become saturated with dissolved B. The point, at which the A and B are saturated by each other, is called an eutectic point, and this mixture will melt at lowest temperature possible for the given mixture. The melting point range for the eutectic point is sharp, so it is easy to confuse it with the mp of a pure compound. The solution to this problem is that when the other component is added to a pure compound, the melting point of the mixture will go down; however, in case of the eutectic point composition, addition of either component will increase the melting point. Before coming to the class, please review the melting point technique video.
Experimental Part I: Determination of Melting Points In this experiment you are required to determine melting points of trans-cinnamic acid, urea, and the mixtures of transcinnamic acid and urea (10%, 50%, and 90% urea respectively). Record mp ranges of all three samples. The melting point will be taken using melting point apparatus. In order to obtain an accurate mp of a compound, it has to be packed properly into a melting point capillary. Some moisture or a solvent can be trapped in a crystal lattice; when heated, this liquid will escape first and can be mistaken for a lower mp temperature (so-called sweating). 1. The compound has to be pulverized on a weighing paper into a fine powder using a spatula, and then tapped into a melting point capillary (do not use paper towels, filter paper, etc. to pulverize the sample). Make sure the height of the compound column is no more than 2-4 mm, otherwise it will take longer to melt a large amount of the compound which will result in a wider melting point range, even though if your compound is pure. 2. Now you need to pack the compound as tight as possible in the mp capillary, and for that you will need to pass it 3-4 times through a long glass tube. It is easier to pack all the samples in once. 3. Place the sample into a Melt-Temp melting point apparatus (notice that you can fit two samples at once). Digital melting point apparatus will allow you to set the temperature of the sample and then observe the melting process through magnifying window. 4. Record the temperature when you observe the first droplet of liquid. Then record the temperature at which the sample melts completely. Discard the capillary tube into designated broken glass container. Continue with the rest of the samples. Let the apparatus cool down between the measurements. raw sample
d just right
f
e too much
a pulverized sample too little
b
c
Melting point apparatus
Fig. 2. Melting point procedure: (a) raw sample; (b) pulverized sample; (c) amount of packed solid in the melting point capillary; (d) melting point apparatus; (e) first drop of liquid in the capillary; (f) the sample is completely melted.
Below are the instructions on how to operate Melting Point apparatus (SMP10): 1. Switch power on. 2. Use ‘plateau set’ and up and down arrows to set the plateau temperature (basically, literature value). 3. Insert sample tubes into block.
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Chem-105, Glendale Community College
4. 5. 6. 7. 8. 9.
Press ‘start’: heating light comes on. Wait until ‘plateau’ light is on. Press ‘start’: ramping light comes on. Watch for melting point. Press ‘stop’ to cool block down. Press ‘start’ to return to the set plateau temperature.
Note: once you are done using melting point apparatus, please disconnect the power cord, place it in the box provided on the bottom of the cabinet, and return the apparatus to the shelf. Prepare a table with three columns in your lab notebook. In the first column, enter the compound tested; in the second column the temperature at which you noticed the first drop of the liquid in the capillary tube, and in the third one the temperature at which the entire mixture was converted into a liquid. % composition of the mixture 0% urea (pure transcinnamic acid sample)
T1
T2
10% urea 50% urea 10% urea 100% urea (pure urea sample)
Waste disposal: Used capillaries should be placed in the broken glass container. Excess solids should be placed in the labeled beakers provided on the cart.
Guidelines for writing the report on ‘Melting Point/Recrystallization’ experiment (10 pt) Name, partner’s name Date Title of the Experiment: Target Molecule: The report should be not longer than two pages, double-spaced (font size 12, written in third person). Please follow the instructions given below: Abstract: The abstract should state the purpose of the experiment, the method used, and brief summary of experimental results. Be very specific and brief (3-4 sentences max). Tabulated data: tabulate data obtained for the known compounds and mixtures (make sure you report data to the correct number of significant figures). Make sure you follow the format shown in the above table: otherwise, points will be taken! Please construct an Excel graph of the data as a function of melting point vs composition of the mixture. Label the graph and insert it in your report (see example below). When constructing the graph, for x-axis please plot composition of the mixture by urea (note, the mp of the mixture of 0% urea corresponds to the mp of pure trans-cinnamic acid). Choose ‘Smooth Marked Scatter’ for the chart type.
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Chem-105, Glendale Community College
Results/Discussion: write in essay format, do not use bullets or number the discussion topics. • Compare the melting point ranges of the tested compounds with literature values and explain any differences. • Comment on your experimental results and explain how they meet your expectations. • Conclusion: summarize the results.
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