Lab Report
When a solid substance is heated, it liquifies, and at a certain higher temperature, all of the solid will have become liquid. The melting point (actually melting point range) of a compound is then defined as the temperature at which an observer can first see liquid forming from the solid to the temperature where the last particle of solid has become liquid. This experiment was performed in order to determine the melting range of Naphthalene accurately.
Determination of the melting points helps one to identify and have an idea of its purity and the accuracy of the experiment, which aids in bringing out the effect of the purity of a substance on its melting behavior. Pure substances usually have a melting point range of between two, while impure have wide ranges of melting point.
Therefore, a sharp melting point is the key determinant of a pure substance.
In this experiment, the melting point of naphthene and paracetamol (polymorphs I and ii) and their heat of fusion were determined after careful data analysis. Experimental values for the melting point and heat of fusion of Naphthalene was found to be 76. Its calculated heat of fusion was found to be 141.2j/g. The theoretical value is found to be at a melting point of 80.26 °C while its heat of fusion is found to be 19.0 Kj/mol.
The experimental value is found to deviate from the theoretical value due to some reasons related to the performance o the experiment. Some o the reasons may be due to impurities that may interfere with the results obtained. Common sources may be the use of defective instruments, environmental related, procedural, and human errors in handling this experiment, which leads to inaccurate data. All of these errors can be either random or systematic, depending on how they affect the results and outcomes. The instrumental error happens when the instruments being used are inaccurate, such as a balance that does not work.
There are a number of ways of improving the validity of the experiment, which include controlling various aspects o the experiment, improving measurement technique, increasing randomization to reduce sample bias, and blinding the experiment.
Controlling more variables is ensuring a few changes in the experiments. Measurement techniques can also be improved. To minimize human errors in measurements, you could use a computer and electronic devices the ensure the data collected is more accurate. Or perhaps instead of having one person measure the results, you could have multiple people take measurements and compare their answers.
Increasing randomization is a way to reduce a particular validity problem. That’s when the samples being investigated are not a representative sample of the population. For example, say your testing for the effect of a drug, and your trials contain mostly white males between 20 and 30 years old. That would not be a good sample due to the lack of breadth and age, and gender. Or perhaps you are testing weight-loss drugs on people who are of a healthy weight already. Increasing the randomization of the sample will reduce this problem.
The melting point for o the polymorphs o paracetamol was found to be 168°C, while its heat o fusion was found to be 245j/g. The other polymorph was found to have a melting point of 158.0 and heat of fusion of 176.3j/g. The two polymorphs are observed to melt at 176 and 189 degrees C and are known to be, a value for the heat of fusion (29.3 kJ/mol) of the lower melting form is theoretically recorded for the polymorphs.
The metastable zone width (MSZW) can be defined as the difference between the maximum solution concentration in the supersaturated state before crystallization occurs and the solution concentration at equilibrium. The melting temperature of a metastable phase of a material is always lower than that of the stable phase. When a substance with a metastable crystalline structure is heated continuously, the liquid phase is produced at the metastable crystalline phase’s melting temperature. Therefore the polymorph having a lower melting point of about 158.0 degrees celsius is the one that is metastable.
References
Aykol, M., Dwaraknath, S. S., Sun, W., & Persson, K. A. (2018). Thermodynamic limit for synthesis of metastable inorganic materials. Science advances, 4(4), eaaq0148.
Jordan Jr, E. F., Feldeisen, D. W., & Wrigley, A. N. (1971). Side‐chain crystallinity. I. Heats of fusion and melting transitions on selected homopolymers having long side chains. Journal of Polymer Science Part A‐1: Polymer Chemistry, 9(7), 1835-1851.
Plato, C., & Glasgow, A. R. (1969). Differential scanning calorimetry as a general method for determining the purity and heat of fusion of high-purity organic chemicals. Application to 95 compounds. Analytical chemistry, 41(2), 330-336.