Abstract:

The purpose of this experiment is to determine how intermolecular interactions affect physical properties. Physical properties of substances and solutions are determined by the nature of the interactions between molecules. There are three main types of interactions, which are collectively known as "van der Waals" interactions: dipole-dipole (DD), ion-dipole (ID), and London-type dispersion forces (LD). The strength of the bonds between molecules determines the melting point, boiling point, and vapor pressure of a substance at a given temperature. Greater bond strength causes higher melting and boiling points, because more thermal energy is required to break the bonds. Greater bond strength also results in lower vapor pressure at a given temperature, because fewer molecules will possess the necessary kinetic energy to escape into the gas phase. Also, larger molecules have lower vapor pressure and higher melting and boiling points, because because less thermal energy is needed to break the bonds. The heat of vaporization reflects the greater amount of energy required for molecules to move from liquid phase to gas phase. The Clausius-Clapeyron equation illustrates the relationship between the heat of vaporization and temperature: In order to perform this lab, an Erlenmeyer flask filled with ethanol will be submerged in a beaker of 80° C water.  After the ethanol has begun to boil, the Erlenmeyer flask will be removed from the water and a stopper containing a pressure sensor will be inserted into the flask. Once the ethanol has cooled, the change in pressure over time and the heat of vaporization can be calculated, by using the Clausius-Clapeyron equation and analyzing the graphs produced by the data collection system.  The same experiment, will then be repeated with acetone. As stated, the results of this lab will inevitably allow the observation of the affects of intermolecular interactions on physical properties in both ethanol and acetone.