In this lesson students will explore intermolecular forces, and their associated effect on physical and chemical properties. Students will experiment with volatile liquids to investigate their predictions about intermolecular strength.
High School (AP Chemistry)
This lesson will help prepare your students to meet the performance expectations in the following standards:
- 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.
- Scientific and Engineering Practices:
- Developing and Using Models
- Analyzing and Interpreting Data
- Planning and Carrying Out Investigations
- Engaging in Argument from Evidence
AP Chemistry Curriculum Framework
- Big Idea 2: Chemical and physical properties of materials can be explained by the structure and the arrangement of atoms, ions, or molecules and the forces between them.
- 2.1 Students can predict properties of substances based on their chemical formulas, and provide explanations of their properties based on particle views.
- 2.11 The student is able to explain the trends in properties and/or predict properties of samples consisting of particles with no permanent dipole on the basis of London dispersion forces.
- 2.16 The student is able to explain the properties (phase, vapor pressure, viscosity, etc.) of small and large molecular compounds in terms of the strengths and types of intermolecular forces.
By the end of this lesson, students should be able to
- Draw molecular shapes and identify intermolecular forces present.
- Predict physical and chemical properties of substances and justify based on IMF’s.
- Design and perform an experiment to test predictions of boiling points of volatile liquids.
This lesson supports students’ understanding of
- Intermolecular forces
- Physical Properties
- Physical Change
- Chemical Properties
- Molecular Geometry
- Molecular Structure
- Phases of matter
- Organic chemistry
Teacher Preparation: 15-20 minutes
Lesson: 60-75 minutes
- Introduction/Engagement: 5-10 minutes
- Practice & Review: 15-20 minutes
- Lab: 45-50 minutes
- Small white/dry-erase boards
- Assorted dry-erase markers
- Permanent marker (Sharpie, etc.)
- Water squeeze bottles 1-2/lab group
Lab activity (items are per lab group):
- 6 Stir rods
- 6 Filter paper sheets
- 6 Rubber bands
- Water (Distilled/D.I. if available)
- Approximately 15-20 mL of each of the following:
- Thermometer/temperature probe
- Stopwatch/timing device
- Optional: Molecular Modeling Kits (Molymods work great, or the newer Snatoms magnetic kits)
- Always wear safety goggles when handling chemicals in the lab.
- Students should wash their hands thoroughly before leaving the lab.
- When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
- Keep ethanol, methanol, isopropyl alcohol and acetone away from open flames as they are highly flammable.
- Students should have some prior exposure to the types and strengths of intermolecular forces, in order to predict volatility of liquids prior to conducting the lab. Alternatively, this lesson could be used as an introduction to IMF’s, and students could conduct the lab and then draw conclusions about the relative strengths of intermolecular forces based on their observations and data.
- Lesson Outline:
- Introduction/Engagement: Floating Inks
- Students should be grouped into groups of 3-4. Pass out whiteboards, white board markers, and permanent markers to each group.
- Ask students to review what they know about intermolecular forces and the attractive forces between molecules within solutions. Review should include types of intermolecular forces (to include ion-ion, ion-dipole, dipole-dipole, hydrogen bonding, London dispersion forces) and their relative strengths. Students may be asked to rank them, quiz each other on the types and examples, etc. Anticipate statements such as “like dissolves like” or polar/nonpolar interaction and elicit students to dig deeper by asking students the kinds of electron dispersion that causes polarity.
- Once students have discussed, ask students to make a stick figure drawing on their white boards, first with the dry erase marker, then with the permanent marker. Once complete, ask students to use the water squeeze bottles on the white board and observe the results.
- The dry erase marker ink *should* detach from the white board, but stay together, floating on the water due to its lower density, while the permanent marker will remain affixed to the board (this can sometimes take a few tries to get to work, depending on the cleanliness of the board and “freshness” of the marker ink).
- The big idea to get across to students here is that the ink from the dry erase marker will remain cohesive due to the solvent’s interaction, but not be attracted and therefore dissolved by the water.
- Ask students to discuss what they observed and some ideas as to why the dry erase marker is no longer affixed to the board, while the permanent marker is.
- Then ask students to use the acetone on their boards to observe the effects on the marker. The acetone will dissolve the inks, spreading them across the board, rather than floating on top like the water. Possible questions for discussion: “Is it the same effect as the water? Why or why not? What’s different about the two inks? What is different about water vs. acetone?”
- Pre-lab Questions: Molecular Structure of Liquids and Intermolecular Forces
- Using the lab handout, students complete the prelab tasks and questions. Students will draw the structures of the molecules, build a model to determine molecular geometry and polarity, and identify the intermolecular forces present. Then, using this information, rank these substances in order of likely boiling points, from lowest to highest. Have students discuss their results and analyze sources of confusion (examples include the difference between molar mass and polarizability with London Dispersion Forces, relative strengths of intermolecular forces, etc.) with regards to types of IMF’s and whether they occur in the different substances.
- This lab activity is relatively straightforward and students should be able to complete it within about 45-50 minutes. Students will need to construct their “evaporation apparatus”, using stir rods, filter paper, and rubber bands. Teachers can construct a sample apparatus for students to consider, or students can be challenged to design it themselves.
- Considerations and suggested methods:
- Students will dispense approximately 15-20mL of each liquid into a beaker and then dip each filter paper into the respective liquids and observe how long each takes to evaporate.
- Students could use temperature probes or thermometers instead of stir rods on which to wrap their filter paper to collect evaporative temperature data, to prove the endothermic nature of this process.
- Students may want to time the rate of evaporation, or measure temperatures.
- Alternate activities could include dispensing liquids onto lab tabletops and allowing each to evaporate and measuring rate of evaporation.
- *Extra care should be taken to ensure that the liquids are far away from any possible ignition source, as several of the liquids are highly volatile .*
- Post-lab assessment questions are provided in a format similar to that of AP free-response questions involving intermolecular forces. Additionally, students could be assessed based on their justification of the boiling point continuum, connecting this to the intermolecular forces present in each liquid. Students can also be assessed on their design and development of a testable procedure and ability to predict/justify boiling points based on IMF’s.