Intermolecular Forces and the Boiling Point
Project Overview
This RWLO is an inquiry-based learning exercise about intermolecular forces and their relationship to the boiling point. This exercise is flexible enough to be used as an in class activity, a dry-lab or home-work assignment. (Hand-outs based on the dry lab model have been provided.) Because the activity is inquiry-based, the activity works well for courses for both majors and non-majors. In addition to developing a body of knowledge about intermolecular forces, this RWLO will allow students to review and/or apply knowledge of molecular mass and Lewis structures.
This RWLO requires the students to use National Institutes for Standards and Technology’s (NIST) Chemistry Web-Book. Despite being called a webbook, no paper copy exists because the amount of information in it would fill libraries. Therefore, it is something not available off-line. It is the official source of chemistry data used by the National Institute of Standards and Technology. It defines what is correct in terms of chemical constants, names, and structures, which makes it unique. No other source on the web defines what is right in terms of these areas. Additionally, it is searchable by chemical name and formula, which separates from Google or other search engines. The data on the web-site is also updated constantly. It is compelling because it allows students to use a source that true chemical professionals would use. This database is so important to the field of Chemistry that before it developed a user-friendly interface, Chemistry majors studied it in their Chemical Literature course. By using the database, students will be using a source used daily by chemists in the real world.
The activity is inquiry-based because the students are asked to draw their own conclusions about a larger principle based on a set of data. They are then asked to provide further data to support their conclusions. The NIST Chemistry Web-Book will be used to provide the students with data on10 different organic compounds, but the students will determine on their own the relationship between the data and intermolecular forces. One unique inquiry-based aspect of this lab is students support their conclusions using examples they develop on their own from the NIST database. This RWOL does assume that students can already calculate molecular mass and draw Lewis structures.
Student Learning Objectives
· Students will be able to look up compounds in the NIST Chemistry Webbook.
· Students will be able to draw Lewis structures.
· Students will be able explain the intermolecular forces of dipole-dipole interaction, H-bonding, and London Dispersion.
· Students will be able to explain the comparative strengths of the intermolecular forces based on data obtained from the NIST Chemistry Web-Book.
· Students will be able to calculate molar mass and explain its effect on the boiling point.
· Students apply their conclusions to predict the relative boiling points for a new set of compounds.
· Students will validate their conclusions using compounds of their choosing from the NIST Chemistry Webbook.
Procedure
Time: Approximately 45 minutes
Materials: A computer with internet access, paper and a printer.
A class text is useful.
Prerequisites: Students will need to know the fundamentals of Lewis structures, molecular mass, VESPR and chemical nomenclature. They will also need to know the definitions of H-bonding, dipole-dipole interaction and London Dispersion.
Implementation: This RWLO can be used either as a group activity in the classroom, as a homework assignment or as an excellent dry lab for online courses.
Steps:
1. Define H-bonding, dipole-dipole interaction and London dispersion for the students.
2. Ask the students to calculate the molecular mass and draw the Lewis structures for methanol, ethanol, isopropyl alcohol, 1-butanol, methane, ethane, propane, fluoromethane, fluoroethane, and 1-fluoropropane. The students will need to be provided with the chemical formula for each of the compounds.
3. Ask the students to identity the type of intermolecular force that would exist between molecules of each of the compounds based on the Lewis structure. (To clarify, what intermolecular force would be present between two molecules of methane; not between methane and ethane.) I recommend providing the students with a data table similar to the one attached.
4. Provide students with the following web address: http://webbook.nist.gov. Once they reach the website tell them to click “NIST Chemistry Web-Book” link at the top of the page. On the next page click “Name” under General Search. This will take the students to the search function. On the search page students should check the box next to “phase change” and enter the name of one of the compounds from step 2. Then the students should hit enter. The search results will include the Lewis structure and boiling point for the compound at 1 atm. They should use the results to grade their Lewis structures. If the compound has more than one reported value for the boiling point have the students take an average. Have the students search by name all the compounds in step 2 and record the boiling points.
5. Ask students to draw conclusions based on their data chart. What is the strongest intermolecular force of the three discussed? Why? What is the relationship between molecular mass and boiling point? What intermolecular force would account for the relationship between molecular mass and the boiling point?
6. Have the students defend their answers to the questions in part 5 using data other than the data from the compounds in part 2.
Content Material
Student Directions:
· Please, note the best student directions are those found in the student handouts at the end of this RWLO. (It contains the referenced data table.)
· Define H-bonding, dipole-dipole interaction and London dispersion.
· Calculate the molecular mass and draw the Lewis structures for methanol, CH3OH; ethanol, CH3CH2OH; isopropyl alcohol, CH3CHOH CH3; 1-butanol, CH3CH2CH2CH2OH; methane CH4; ethane CH3CH3; propane, CH3CH2CH3; fluoromethane, CH3F; fluoroethane, CH3CH2F and 1-fluoropropane, CH3CH2CH2F. Beside each compound write the type of intermolecular force that would be found between molecules of the same compound.
· Goto web address: http://webbook.nist.gov. Click the“NIST Chemistry WebBook” link at the top of the page. On the next page click “Name” under General Search, this will take you to the search page. On the search page, check the box next to “phase change” and enter the name of one of the compounds. Then hit enter. The search results will include the Lewis Structure and boiling point for the compound at 1 atm. Use the Lewis Structure to grade the one you drew. If the compound has more than one reported value for the boiling point, take the average. Repeat this step for all the compounds name in the previous step.
· Answer the following questions based on the data you collected. What is the strongest intermolecular force of the three discussed? Why? What is the relationship between molecular mass and boiling point? What intermolecular force would account for the relationship between molecular mass and the boiling point? Which of the following would have the highest boiling point: 1-bromoethane, CH3CH2Br; acetic acid, C2H4O2; or ethane, CH3CH3?
· Defend your answers to the previous questions using data from the NIST Chemistry Web-Book not currently in the data table to support your conclusions.
Referenced URLs:
· http://webbook.nist.gov
Assessment
I would grade every square on the data table as 1 point meaning the student would receive 1 point for every correct Lewis structure, molecular and intermolecular force for a total of 15 points.
Then I would grade the questions as follows:
1. What is the strongest intermolecular force of the three discussed? Why? (5 points)
The answer must include H-bonding being stronger than dipole-dipole being stronger than London dispersion.(2 out of the 5 points) The answer should also justify this response by comparing analogous compounds-methane’s, methanol’s, and fluoromethane’s boiling points. (3 out of the 5 points)
2. What is the relationship between molecular mass and boiling point?(3 points)
The boiling point increases with increasing molecular mass. This trend was observed among all the compounds with the same functional groups.
3. What intermolecular force would account for the relationship between
molecular mass and the boiling point? (2 points)
The compounds such as methanol, CH3OH; ethanol, CH3CH2OH; isopropyl alcohol, CH3CHOH CH3; and 1-butanol, CH3CH2CH2CH2OH all are capable of H-bonding but they are also capable of London dispersion which is affected by molecular mass; therefore, as molecular mass increases so does the boiling point.
4. Which of the following would have the highest boiling point: 1-bromoethane, CH3CH2Br; acetic acid, C2H4O2; and ethane, CH3CH3?(2 points)
acetic acid> 1-bromoethane > ethane
Optional
Defend your answers to the previous questions by using data from the NIST Webbook not currently in the data table to support your conclusions. (5 points)
Did the compound they chose actually represent the intermolecular force that they claimed they did? (2 points.)
Did the compounds support their conclusions and sufficient in quantity. ( 2 points.)
Was the argument well presented? (1 point.)
Links to Course Competencies
This RWLO could be applied in the following courses: General, Organic, and Bio Chemistry, and College Chemistry. Specifically, this RWLO meets the following course competencies:
· Students will be able to define H-bonding, Dipole-dipole interaction, and London Dispersion.
· Students will be able tell the relative strengths of the different intermolecular forces.
· Students will be able explain the relationship between intermolecular forces and the boiling point temperature.
· Students will be able to analyze data and draw conclusions.
· Students will be able to draw Lewis structures.
· Students will be able to calculate molar mass.
Supplementary Resources
· Any good ACS Chemistry Text.
· The Cartoon Guide to Chemistry by Larry Gonick and Craig Criddle
Recommendations
Recommendations for Integration:
This RWLO is best suited for General, Organic and Bio Chemistry Courses as well as College Chemistry Classes. It should be done shortly after the discussion of polarity and Lewis structures, which in most texts is followed by a discussion of intermolecular forces. It would make an excellent virtual lab for those teaching online course. It would also serve as a great inquiry based learning activity for small lecture sections.
Back-up:
The NIST Chemistry Web-Book is a government run web-site if by some unforeseen event the web-site is down reschedule the activity.
Pre-lab
In today’s lab, the strength of 3 different intermolecular forces will be compared. All the information needed to compare the relative strengths of the different intermolecular forces can be found by completing the data table. After forming your conclusion on the relative strengths of the different intermolecular forces, defend your conclusions using data not included in the data table from the “NIST Chemistry Web-Book.”
1. Define H-bonding.
2. Define dipole-dipole interaction.
3. Define London dispersion.
Procedure
1. In the data sheet, calculate the molecular mass and draw the Lewis structures for methanol, CH3OH; ethanol, CH3CH2OH; isopropyl alcohol, CH3CHOH CH3; 1-butanol, CH3CH2CH2CH2OH; methane CH4; ethane CH3CH3; propane, CH3CH2CH3; fluoromethane, CH3F; fluoroethane, CH3CH2F and 1-fluoropropane, CH3CH2CH2F. Beside each compound write the type of intermolecular force that would be found between molecules of the same compound.
2. Goto the web address: http://webbook.nist.gov. Click the NIST Chemistry Web-Book link at the top of the page. On the next page click “Name” under General Search, this will take you to the search page. On the search page, check the box next to “phase change” and enter the name of one of the compounds from step 1. Then hit enter. The search results will include the Lewis Structure and boiling point for the compound at 1 atm. Use the Lewis Structure to grade the one you drew. If the compound has more than one reported value for the boiling point, take the average. Repeat this step for all the compounds named in the previous step.
Data Table
Post-Lab
1. What is the strongest intermolecular force of the three discussed? Why?
2. What is the relationship between molecular mass and boiling point? Why?
3. What intermolecular force would account for the relationship between molecular mass and the boiling point? Why?
4. Which of the following would have the highest boiling point: 1-bromoethane, CH3CH2Br; acetic acid, C2H4O2; or ethane, CH3CH3?
5. Defend your answers to the previous questions by using data from the NIST Chemistry Web-Book not in the current data table to support your conclusions.
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