Temperature Change Due to Evaporation (James Rice)

Research Question and Hypothesis

Of six volatile organic compounds, which has the strongest intermolecular forces?

Standards

HS-PS1-3 (Evidence Statements)

Experimental Design

Six different liquids were used for this experiment: methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, octanol, and acetone. This experiment was performed by evaporating approximately equal volumes of each of the six sample liquids off the end of a thermometer that has had its temperature bulb wrapped in a strip of filter paper. Then, the temperature was recorded on video over time

Independent variable

The independent variable for this experiment is time, measured in seconds. It can be measured either with a timer, cell phone video, or automatically as part of digital temperature collection.

Dependent variables

The dependent variable for this experiment is the temperature of each of the measured liquids. It is measured with a thermometer, although it can also be measured with a digital temperature probe.

Series

The series to be tested will be temperature vs. time data for six different volatile small-molecule organic solvents: methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, octanol, and acetone.

Constants and Controls

Constants are the starting temperature of all the liquids (ideally, room temperature), the amount of liquid that is to be evaporated (controlled by the size and wrap of filter paper that is tied to the bulb of the thermometer), identical thermometers (or the same thermometer cleaned and allowed to return to room temperature between trial runs).

Materials

• Thermometer(s) and timer/video camera OR temperature sensor w/ data acquisition

• filter paper strips (cut identically)

• rubber bands or lengths of string

• six liquids (listed earlier)

• Test tubes with rubber stoppers (for containing liquids before use)

• Test tube rack (optional) or holding container

• Disposable volumetric pipettes

Procedures

1. Fill each of six test tubes with 1.0 mL of each solvent, respectively. Stopper them until they are to be used.

2. Cut six identical strips of filter paper to wrap around the end(s) of the thermometer(s) or temperature probe(s). Wrap the end(s) of the thermometer(s)/probe(s) and affix them with the rubber bands or strings.

3. Make sure that the thermometers/temperature probes and liquids are all equilibrated to room temperature after any handling.

4. Prepare either your data capturing software for the temperature sensor or the timer/video setup for capturing time data. Unstopper the first test tube and dip the thermometer/probe in the liquid for a few seconds until the filter paper appears visually to be saturated with solvent. As soon as you remove the thermometer/probe, being recording temperature and time. No part of the filter paper should be touching any surface, only room air. If using a thermometer, make sure to keep it oriented vertically to prevent gravity from affecting the reading. Also make sure to put the stopper back in the test tube in order to prevent excessive evaporation of solvent into the laboratory.

5. Continue to capture temperature data for up to five minutes. If you notice the temperature has stopped decreasing for more than about thirty seconds, end data collection. Your sample has most likely ceased evaporating and is now re-warming to room temperature.

Sample data and graphs

Include raw data and graphs. Make certain to include units in all measurements, titles on all graphs, and labels on all axes.

Analysis & Conclusions

From the collected data, the liquids cooled in the following order from slowest to fastest: octanol, n-propyl alcohol, isopropyl, ethyl alcohol, acetone, and then methyl alcohol. This speed was determined visually by inspecting the slopes of the best-fit curves for each substance. The best fit curves were based on second degree polynomials and each has an R2 value of 0.999 or greater, indicating very strong fit. However, because there was no actual data at the time = 0 s data points, it is possible that some of these substances might be re-ordered if the experiment were repeated multiple times. For this experiment, multiple trials were not an option.

Based on the analysis of the evidence from this experiment, it seems that octanol has the strongest intermolecular forces, with n-propyl alcohol, isopropyl, ethyl alcohol, acetone, and then methyl alcohol following in order of decreasing strength. This can be seen from their rates of evaporation. As mentioned in the introduction to this report, substances that are allowed to evaporate without any outside heating will actually decrease in temperature. However, the stronger the intermolecular forces for a given liquid, the slower this evaporation will occur and vice versa, thus pointing out octanol as the substance with strongest intermolecular forces. Other groups working on this experiment did not necessarily obtain the same results for all substances, but there was strong agreement that octanol did indeed have the slowest rate of evaporation, and therefore the highest intermolecular forces.

From the structural formulas listed in the table below, it would seem that five of the six compounds contained hydrogen-oxygen groups that could have contributed to making hydrogen bonding, a strong intermolecular force of the dipole-dipole type. On top of that, the larger a molecule was, the stronger its intermolecular force, which agrees with the idea of London dispersion forces.

References

Sampson, V., Carafano, P., Enderle, P., Fannin, S., Grooms, J., Southerland, S. A., . . . Williams, K. (2015). Argument-driven inquiry in chemistry: lab investigations for grades 9-12. Arlington, VA: NSTA Press, National Science Teachers Association.