The objectives of this lab are, as follows; to understand what occurs at the molecular level when a substance melts; to understand the primary purpose of melting point data; to demonstrate the technique for obtaining the melting point of an organic substance; and to explain the effect of impurities on the melting point of a substance. Through the experimentation of three substances, tetracosane, 1-tetradecanol and a mixture of the two, observations can be made in reference to melting point concerning polarity, molecular weight and purity of the substance. When comparing the two substances, it is evident that heavy molecule weight of tetracosane allowed it to withstand the increased heat as a solid for a longer duration. The melting point of the combined molecules was much lower than each in its pure form. The finding reinforces the idea of derivatives and their importance when identifying a substance based on melting point. The conclusions drawn from the data found in this lab will aid in the recognition of molecules based on the properties they illustrate.
The objectives of this lab are, as follows; to understand what occurs at the molecular level when a substance melts; to understand the primary purpose of melting point data; to demonstrate the technique for obtaining the melting point of an organic substance; and to explain the effect of impurities on the melting point of a substance. In this unit of study, I have learned that the physical properties of a molecule are commonly relied upon for identification. More specifically, a chemical’s melting point is often the physical property of choice for identification. The melting point of a substance is defined as the temperature at which the solid and liquid phases of a substance are in equilibrium with one another and both present at the same time. A molecule’s melting point is directly related to intermolecular forces within the substance; it describes its atoms attractive forces and how close the atoms are to one another within the compound. Using this knowledge obtained from the course work, I will experiment with two given organic compounds to enhance my understanding of melting points.
METHODS AND MATERIALS
Rubber bands (3)
Clean sheet of paper
Burner fuel stand
Thermometer- in- cardboard tube
Tetracosane Crystals- 0.2g
Tetradecanol Crystals- 0.2g
To begin this experiment, I must pour approximately 60mL of water into a glass beaker, allowing it to sit until room temperature is achieved. Next, I will pour the tetracosane crystals onto a clean sheet of paper crush them into a fine powder with the back of a spoon. A small amount of the powder will be collected and placed in the bottom of a glass capillary tube. My goal is to have the power occupy no more than 2mm of tube space; too much material will misguide a true melting point. Once inside the tube, I will secure the tube alongside a thermometer with a small rubber band; careful to place the bottom of the tube directly above the bulb of the thermometer. The finished structure is then placed into the water bath. It is critical at this point that water does not enter the glass tube.
Once the compound is secure in the beaker of water, the entire unit is placed onto the burner stand. Ignite the burner fuel and gradually increase the temperate while closely observing the compound for changes. Use caution in increasing the temperature gradually, as not to miss the correct boiling point and alter the data. A good standard to follow is increasing the bath temperature by no more than 5 degrees per minute. Note: a magnified glass is recommended to pay careful attention to the compound. Record the temperate when the first drops of liquid are noted, as the crystals begin to collapse; also, record the temperature when all the crystals have become liquid. These two measurements are the melting point range for tetracosane.
Once the range is obtained, remove the thermometer and beaker. Pour out the hot water and allow both the thermometer and the beaker to cool. Once cool, repeat the previous steps with the second compound, tetradecanol. At the conclusion of the second experiment, repeat the steps for a third and final time using an equal mixture of both compounds. Be sure to record the melting point range for all three experiments.
RESULTS AND DISCUSSION
Melting point range for tetracosane: 49-51 degrees Celsius
Melting point range for 1-tetradecanol: 37-40 degrees Celsius Melting point range for mixture: 34-37 degrees Celsius
I was instructed to record a melting point range for each compound, rather than a single temperature melting point. The reason for this instruction is that polymers and unpure substance melt over a range of several degrees. Based on the melting point ranges for both substances, it is difficult to differentiate the two based on its melting point data alone. The ranges of the substances are close enough to one another that one could argue for experimental error when presenting the data as a solo identifying property.
Based on these conclusions, I do not believe the tetracosane is a pure substance because I observed the compound melt over a range of degrees Celsius. If both compounds in the mixture were composed of tetracosane, its melting point would be theoretically, identical to the melting point of the original experiment. However, the melting point of the mixture was below that of the original data, reinforcing the knowledge that adding a derivative to a compound will lower its melting point.
During the experiment, heating the water too quickly would create inaccurate data. If the water is heated too quickly, the thermometer may have a hard time keeping up with the temperate, thus displaying an inaccurate measurement. It would be must harder to obtain an accurate range, if this were the case, as the substance may melt faster than observable. A molecule’s melting point is reflective of its physical properties. Melting points disrupt intermolecular forces, spreading atoms over a greater surface area; melting points do not affect covalent bonds, thus are not chemical changes.
At the conclusion of this experiment, I have determined that the molecular weight of the given molecules played a greater role in determining its melting point. 1-tetradecanol, an alcohol, contains the functional group –OH. This type of bond is referred to as a Hydrogen bond; hydrogen bonds are much stronger than typical dipole-dipole bonds found in many ionic compounds. The molecular formula for tetracosane is C24H50. The molecule contains no functional group, so the intermolecular bonds are dispersion forces; the weakest bond noted between atoms. Given this information regarding polarity and the molecular weights of the compound, it is easy to conclude that the weight of the tetracosane caused it to have a higher melting point.
At the conclusion of this experiment, I have a better understanding of the significance of organic compound’s physical properties. More specifically, I have a learned how melting points can aid in the identification of a molecule. It is important to remember that a molecules melting point alone is often now enough data to correctly identify the compound. This is why molecules are often melted using derivatives. I have also come to understand how a molecules polarity and molecular weight can influence its melting point. It was evident in this experiment that the heavier molecule withstood increased temperature longer. I was also able to recognize how mixed compounds will alter the melting point of the pure substance. My understanding of the data found in this lab will aid in my studies of organic chemistry as I become more accustomed to recognizing molecules based on the properties they illustrate.
Fryhle, C. B., & Solomons, T.W.G. (2011). Organic Chemistry (10th Ed). Hoboken, NJ: John Wiley & Sons Inc.