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).
Required Reading: Mayo 52-56
Purpose: Identify an unknown substance by determining the unknown' s melting point and compare the melting point with the known list.
Melting point of an unknown compound.
Obtain an unknown from the instructor and determine its melting point range. 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. First, obtain a capillary tube from the reagent bench or from your own drawer. Load the 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 three wells of the Melt-Temp with the closed-end down. 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. Observe the thermometer through the eye-piece.
Once you know the melting point of your unknown you can make an educational guess from the known list of what your unknown might be. Consequently, you will make three melting point samples: your unknown by itself as the reference compound, your unknown mixed with the suspected compound, and 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 cause by using different melt-temp. If you run the samples at different heating setting, the results may vary also.
|COMPOUND||MP RANGE (°C)||COMPOUND||MP RANGE (°C)|
|Phenylacetic acid||77-78.5||Benzilic acid||150-153|
|Vanillin||81-83||p-nitrophenyl acetic acid||154-155|
Questions: (Answer these questions in complete full sentences)
1. To get an accurate melting point range, the thermometer used to determine melting point has to be calibrated, Describe in details how you would calibrate the thermometer.
2. A student in the lab attempts to calibrate a thermometer. The student determines the melting points of four different compounds and the data are shown below. The student then measured an unknown melting point and finds it to be 111°C, what is the corrected melting point of the unknown?
|Compounds||Expected melting point( °C)||Experimental melting point (°C)|
3. How would the melting point of a substance be obtained if it sublimes before it melts?
4. If a compound is pure, how narrow should the melting point range?
5. If you have an unknown that has a melting point of 99-101°C, you mix the unknown with a compound A and obtain a melting point of 99-101°C. What can you conclude from this result?