Lab 10: Evaporation and Intermolecular Attractions

Pre-Lab Table:

Data Table:

Questions #2-4:

2.  Explain the differences in the difference in temperature of these substances as they evaporated.  Explain your results in terms of intermolecular forces.

The differences in temperature change of the five substances varied due to the difference in the strengths of intermolecular forces.  Methanol had the highest change and highest vapor pressure because it had the weakest intermolecular force, so it evaporated and cooled faster.  On the other hand, glycerin had the smallest temperature change (the initial temperature actually increased) and the slowest evaporation rate because its intermolecular force was the strongest --- it had three -OH groups (three hydrogen bonds).

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

Two compounds that had similar molar masses were methanol (32.042 g) and ethanol (46.068 g).  The change in temperature for methanol (15.9°C) was higher than that of ethanol (10.1°C).  This is because even though they both have one hydrogen bond, the molar mass of methanol is smaller, so the bond is weaker.  A weaker bond causes faster evaporation and also lower vapor pressure.  Ethanol's molar mass is larger, so its bond is stronger, causing slower evaporation and thus lower temperature change.

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

Methanol and ethanol both have one -OH group (hydrogen bond), but because methanol's molar mass is smaller, it's temperature change is greater and its evaporation rate is quicker.  Ethanol had the second highest temperature change.  Water has two hydrogen bonds, but it has the smallest molar mass, so it had the third quickest evaporation rate and third highest temperature change.  n-Butanol has one hydrogen bond, but because its molar mass is so high, the bonds are stronger and thus the temperature change is smaller and the evaporation rate is slower.  Lastly, glycerin has three -OH groups, or the strongest intermolecular force, causing an increase in temperature (smallest difference in temperature) and the slowest evaporation rate.  This is because the rate glycerin was absorbing heat from the condensation in the room was faster than the evaporation rate.