Lab 15: Evaporation and Intermolecular Attractions

Calculations and Results:

2. Explain the difference in temperature (ΔT) of these substances as they evaporated. Explain your results in terms of intermolecular force.

The differences in temperature varied for the five substances. Methanol's temperature went down 13.4, while glycerin actually went up 2.5. This was caused by the difference in intermolecular forces. Methanol, whose temperature went down the most, had the weakest bonds; it had just one hydrogen bond and three London Dispersion bonds. (Hydrocarbons form only dispersion bonds, and any N-H, F-H, or H-O compounds have hydrogen bonds). On the other hand, glycerin, the only substance whose temperature increased, had three hydrogen bonds and five London Dispersion bonds. (Further explained in the next two questions).

3. Explain the difference in evaporation of any two compounds that have similar molar masses. Explain your results in terms of interolecular forces.

It is possible for compounds of similar molar masses to have different changes of temperature. What we were testing was the change of temperature as a result of evaporation from the room temperature; evaporation is liquid becoming gas through energy overcoming intermolecular forces to break apart the bonds holding molecules together. Stronger intermolecular forces means slower evaporating rates because the energy used must be greater in order to break the bonds. Methanol and Ethanol had the closest molar masses, but ethanol's temperature decreased less than methanol's, and had stronger intermolecular forces.

4.  Explain how the number of -OH groups in the substances affects the ability of the tested compounds to evaporate. Explain your results in terms of interolecular forces.

OH forms hydrogen bonds. These are the strongest type of intermolecular forces, excluding ionic bonds. Hydrogen bonds are stronger than london dispersion bonds, which are what hydrocarbons (HC) forms. As a result the number of OH groups results in more evaporation and greater decrease in temperature. In fact, glycerin, which had three OH groups, actually went up in terms of its temperature. Methanol, on the other hand, had just one OH group, and evaporated the most, causing its temperature to decrease. As stated before, stronger intermolecular forces means slower evaporating rates because the energy used to break the bonds must be larger.