Lab 10: Evaporation and Intermolecular Attractions

Data

Substance	Maximum or Initial Temp (°C)	Minimum or Final Temp. (°C)	∆ Temp. (°C) (- and + indicates whether temps. increased or decreased)
Methanol	18.9°C	3.2°C	-15.7°C
Ethanol	19.6°C	10.6°C	-9.0°C
n-Butanol	20.3°C	18.2°C	-2.1°C
Glycerin	20.3°C	23.3°C	+3.0°C
Water	19.9°C	14.8°C	-5.1

Pre-Lab Questions:

2. The substances that we tested had a vast range of the difference in temperature, in order from the substances that had the biggest decrease in temperature to the ones which had the smallest decrease in temperature it goes Methanol with a decrease of 15.7°C,  Ethanol with a decrease of 9.0°C, water with a decrease of 5.1°C, n-Butanol with a decrease of  2.1°C, and Glycerin which actually gained 3.0°C. The temperature increase or decrease were due to intermolecular forces. Each substance posses hydrogen bonds, which are the attraction between the partially positive positive hydrogen and partially negative atoms, which in this case is the oxygen in all these substances. More hydrogen bonds the more energy it takes to evaporate, and a less acute drop in temperature. So, the substance with the most hydrogen bonds was Glycerin with 3 potential places for hydrogen bonds per molecule, and Glycerin was the only substance to increase in temperature during the lab.

3. Hydrogen bonds only explain part of the difference in temperature seeing as three substances, Methanol, Ethanol, and n-Butanol, had the same amount of potential places for hydrogen bonds and yet had differences in the decreases in temperature. This is due to London Dispersion Forces. London Dispersion Forces create a temporary poles on molecules called a momentary dipole-dipole. This is due to the fact that electrons move around the nucleus and when more electrons move to one side that side becomes more negative and the other becomes more positive, this also forces the molecules around the molecule with the momentary dipole-dipole also to become momentary dipole-dipole. It takes more energy (and therefore a smaller decrease in temperature during evaporation) in molecules that have more molar mass. This is due to the fact more molar mass means more electrons, which mean a stronger momentary dipole-dipole. That explain why n-Butanol, which has the biggest mass had the smallest decrease in temperature.

4. The number of -OH’s correlated to evaporation because of that is where hydrogen bonds will form. This is expected because hydrogen bonds usually form between atoms with higher electronegativity such as oxygen, which is often partially negative in many molecules, and hydrogen. 1 -OH means one potential hydrogen bond could be formed, because there is one partially negative atom and one hydrogen which will for the hydrogen bond. This is why water has two places for bonding because it has two hydrogens which will form bonds, and more bonds means more energy needed for evaporation, and a less severe temperature decrease.