Melting Point and Boiling Point of Organic Compounds

Topics: Van der Waals force, Melting point, Carboxylic acid Pages: 10 (3227 words) Published: September 20, 2012
Melting Point and Boiling Point of Organic Compounds

Bongo, Sayre, J1

1Student, Organic Chemistry 1 Laboratory / B11, School of Chemical Engineering, Chemistry and Biotechnology, Mapúa Institute of Technology

The melting point of a substance is the temperature at which the material changes from a solid to a liquid state while the boiling point is the temperature at which it changes from liquid to solid. In this experiment, the main objectives were to determine the effects of the following either on the melting or boiling point of organic compounds: 1) intermolecular forces of attraction and geometric isomerism on melting point, 2) purity on melting point range, and 3) intermolecular forces of attraction and branching on boiling point. The experiment was composed of two parts: determination of melting points and determination of boiling points. The Thomas-Hoover melting point apparatus was used for the first part of the experiment, while the micro method for boiling point determination was performed for the second. Both the melting and boiling point of compounds are affected by intermolecular forces. The three kinds of intermolecular forces that can operate on covalent molecules listed by increasing strength are van der Waals forces, dipole-dipole attractions, and hydrogen bonds. Also, purity and isomerism also affect melting point and branching for boiling point. Keywords: melting point, boiling point, intermolecular forces, chemical structure, Structural Theory INTRODUCTION

The structural theory states that properties of organic compounds, both chemical and physical, are largely influenced by their chemical structures. Two of these physical properties studied in the experiment were boiling point and melting point. The melting point of a compound can simply be defined as the temperature at which a substance undergoes a phase change from solid to liquid. More specifically however, we define melting point as the temperature at a specific pressure at which the solid and liquid phases are in equilibrium with each other. To melt a compound, energy must be supplied to break up the intermolecular forces holding the molecules of the compound together. The melting point of an organic solid is probably the most widely used physical constant. If you want a quick, easy, and cheap way to characterize an unknown solid, determining the melting point is a good way. A melting point of a solid is the temperature at which the first crystal just starts to melt until the temperature at which the last crystal just disappears. Thus, the melting point is actually a melting range. Which is why two temperatures (T1°C and T2°C) per compound were recorded in the experiment. For a pure compound the melting point is quite sharp (occurs over a 0.5 -1°C temperature range). A melting point range of greater than 5° C usually indicates an impure compound or poor technique. The melting points are characteristic of a compound but the melting point itself is not a unique characteristic of the compound. That is, two different compounds could have the same melting point, but two substances of differing melting point are unlikely to be the same compounds. Melting points are determined for three reasons. If the compound is known, the melting point will help to characterize the sample in hand. For new compounds, the melting point allows for future characterization by others. Lastly, the melting point range can be used to tell the purity of the compound because an impure compound will melt over a wide range of temperatures. The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it. In terms of intermolecular interactions, the boiling point represents the energy required to overcome the various intermolecular attractions binding the molecules as liquid and therefore undergo a phase change into the gaseous phase. Consequently the boiling point of a liquid is also an indicator of...
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