The purpose of this experiment is to determine how intermolecular interactions affect physical properties. The physical properties of substances and solutions are determined by the nature of the interaction between the molecules. Collectively, the three interactions are known as “van der Waals” interactions and they include dipole-dipole, ion-dipole, and London-type dispersion forces. Dipole-dipole interactions occur between polar molecules, like water. An ion-dipole interaction occurs between an ion and a polar molecule. An example of this is sodium chloride when dissolved in water. London-type dispersion forces are the weakest of the interactions and create a temporary polarity between non-polar molecules.
The varied strengths of these interactions determine the boiling point, melting point, and vapor pressure of a substance. As bond strength increases, melting and boiling points also increase because the amount of thermal energy necessary to break the bonds is higher. Additionally, the greater the bond strength, the lower the vapor pressure. This is because fewer molecules have the kinetic energy needed to escape the gas phase. Lastly, the size of the molecules plays a factor in molecular interactions because larger molecules need more thermal energy to escape and therefore the vapor pressure of larger molecules is less.
A crucial concept to comprehend in the lab is heat of vaporization. It is the energy required for a known amount of substance to escape from the liquid to gas phase. The Clausius-Clapeyron equation illustrates the relationship between the heat of vaporization and temperature:
When performing this lab, an Erlenmeyer flask filled with ethanol is submerged in a beaker of 80° C water. Once the ethanol begins to boil, a stopper holding a pressure and temperature sensor in it is tightly inserted into the Erlenmeyer flask. The pressure is monitored while the temperature decreases. The process is then repeated with a 60 degrees C bath and 50-mL of acetone. When the ethanol has cooled, the change in pressure over time and the heat of vaporizations are calculated using the Clausius-Clapeyron equation and analyzing the graphs. The expected results are for ethanol to have a lower vapor pressure compared to acetone because ethanol has greater bond strength and large molecules when compared with the weak bond strength and smaller molecules of acetone. Ethanol will also have a higher heat of vaporization than acetone. The results of this lab will inevitably allow the observation of the affects of intermolecular interactions on physical properties in both ethanol and acetone.