Evaporation of Alcohols
Lesson 7 of 9
Objective: SWBAT infer the strength of intermolecular forces in four different alcohols from their evaporation rates.
In this lesson, students will use Vernier probeware to measure the change in temperature resulting from the evaporation of different alcohols. To see the experiment on their website, and download a sample preview, please visit this link. A similar lab is contained in the Pasco chemistry materials. Without these technologies available, the investigation can be done with digital thermometers and a stopwatch.
Each lab station requires:
- 4 test tubes containing methanol, ethanol, 1-propanol, and 1-butanol.
- Large piece of filter paper
- Small rubber bands
- Temperature probe and software
- Stopwatch(if using digital thermometers)
The difference in the four alcohols is in the length of their carbon chain, as seen in the handouts. Due to the mass differences, and the placement of the -OH group, this provides a neat difference in their evaporation rate, with the light methanol evaporating fastest and causing the quickest decrease in temperature. It is followed by the ethanol, 1-propanol and 1-butanol. It is important to have the correct form of the propanol and butanol, as having the -OH group on a different carbon atom can slightly affect the intermolecular forces, and therefore the results of the lab.
This lab addresses one Performance Expectation and three Science Practices.
- HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. Students will compare the change in temperature as each alcohol evaporates to assess the strength of attraction between the molecules within the alcohol.
- SP3: Planning and carrying out investigations. Students will carry out the lab investigation.
- SP4: Analyzing and interpreting data. Students will analyze their data, finding the change in temperature as well as creating bar graphs to compare their results.
- SP6: Constructing explanations. Students will use their data to explain the differences in attractions between the molecules in each sample of alcohol.
Introduction- Lab Safety
As students come in, I ask them to get out their lab handout and pre lab filmstrip from the day before. Once everyone has it out, I demonstrate for the students how to roll the filter paper around the temperature probe and secure it with the rubber band.
We discuss the safety concerns of the lab: inhaling the vapors of the various alcohols. I remind students to keep the rubber stoppers on the test tubes when they are not in use, and to dispose of the filter paper as soon as students are done with it. These two actions, along with keeping the class exhaust fan on, will minimize the amount of vapors in the air.
I ask students if they have questions regarding the procedure for the lab. This time there are not any, so I invite them back to the lab, with a stop to gather goggles.
When students come back to the lab, they drop into their roles, with one student logging into the computer and bringing up the Logger Pro software and the other preparing the temperature probes with the filter paper.
When both partners are ready, they submerge the probes into the first two alcohols. While groups are getting started, I am circulating the room to make sure they have enough materials, mainly the rubber bands. Some students are uncertain about which test tube contains which alcohol, so I demonstrate how the initials on the tubes match the alcohols.
When students are ready, they remove the probes from the alcohols and tape them to the edge of the table so they can evaporate. This video shows some students interacting in the lab as they work.
Once the temperature stabilizes, or the 20 seconds of evaporation time has ended, the computer expert clicks stop on the data collection, and students record the highest and lowest temperatures recorded. Many students are unfamiliar with the delta sign meaning change, and miss the instructions about it in the procedure. The most common question I got during the lab was "What does the triangle T mean?" I refer the students back to their procedure step first to find an answer, and if they are still struggling I tell them that it means the change in temperature. They will then subtract the numbers while their chemical partner is prepping for the 1-propanol trial as seen in this picture.
Students do much better with the propanol and butanol trials, having the first round under their belt. The most common error is forgetting which probe went in which chemical. In later periods, I encouraged students to label a piece of masking tape and put it on the probe to avoid confusion. Some students run the butanol and propanol trials simultaneously, removing the chance to make a prediction regarding the butanol.
When students finish, they create a bar graph of their delta-T's. Depending on the data collected, the graphs may or may not show a perfect pattern. While the data above is mostly good, the methanol and ethanol seem to have been swapped accidentally. In the graph below, the butanol is slightly higher than the propanol, which is inconsistent with the ideal results.
Students are then asked to infer the attractions based on the information in the lab introduction and their data. Students struggled with this tremendously. After reviewing their data and conclusions, I decided to spend additional time on the relationships the following day in a lab debrief. Below is an example of student conclusions that were inconsistent with their data.
Some students chose to work on their graphs and conclusions at the lab tables, while some return to the front student tables to finish. As I scanned the student work, I realized there were a number of issues with the data that was different from when I did a practice run of the lab.
I asked students to submit the work they had at the end of the period, and assessed it that evening. Upon reviewing it more closely, I could see where students had mixed up which probe was measuring which alcohol, and where they did not understand the relationship between change in temperature and the strength of attractions.
See my reflection for how I used this assessment data to adjust my instruction.