Intermolecular Forces

1. Concept Presentation By: Amarinder Sawhney Jeffrey Ip Intermolecular Forces

2. Diagnostic assessment • Why does salt dissolve in water?

3. Salt (NaCl) & Water (H2O) • Water is a polar molecule: one end has a slightly positive charge, another has a slightly negative charge

4. Salt (NaCl) & Water (H2O) • When the NaCl crystals get closer to the positively and negatively charged poles of the H2O molecule, the Na+ and Cl- ions are pulled apart • You can think of the H2O molecule as a strong magnet • Click here for more details • Attraction between (polar) water molecules and ions is an example of intermolecular forces at work (specifically ion-dipole forces)

5. Intermolecular Forces • Intermolecular forces are the forces of attraction and repulsion between molecules. Intermolecular forces should not be confused with intramolecular forces, which are the electrostatic forces that hold the atoms of a molecule together (e.g., covalent and ionic bonds) • Intermolecular forces are typically weaker than intramolecular forces, and account for the bulk properties of matter (e.g., boiling point, melting point, etc.) • Intermolecular forces are also known as van der Waals forces, named after Johannes van der Waals who first postulated them

6. Types of Intermolecular Forces • Dispersion (London) • Dipole-induced dipole • Ion-induced dipole • Dipole-dipole • Hydrogen bond • Ion-dipole

7. Summary of Intermolecular Forces http://www.chem.ufl.edu/~itl/4411/lectures/lec_g.html

8. Lesson Sequence Leading to Intermolecular Forces Periodic Trends – students will learn about the periodic table, the different groups of elements, and the how atomic radius, electronegativity, etc., change across rows and columns. (B2.1, B2.2, B3.3) Electronegativity – students will learn to predict the nature of a bond (e.g., nonpolar covalent, polar covalent, ionic), using electronegativity values of atoms. (B2.1, B2.5) Intramolecular Forces – Students will explore the difference between ionic and covalent bonds and their formation. (B2.1, B3.4) Lewis Structures and Molecular Models – Students will learn to draw Lewis structures to represent bonds in ionic and molecular compounds. Students will also build molecular models and write structural formulae to familiarize themselves with the shape and structure of molecules. (B2.1, B2.4, B2.6). Intermolecular Forces – Students will learn about intermolecular forces through lecture, a jigsaw activity, an inquiry based online module, and a lab

9. Video demonstration • Video demonstration can be viewed here • http://workbench.concord.org/database/activities/227.html • This is a two day module that allows students to explore intermolecular forces concepts in greater detail using an interactive inquiry based online module • Once completed students will “Create a Report of [their] Work” and hand it in for marking

10. Differentiated assessments ! Jigsaw activity • This jigsaw activity is a cooperative learning activity where students collaborate to explore concepts related to intermolecular forces • Instructions: Students form groups of (number of people per group depends on class size). Members of the group are numbered off. Each number is assigned a type of intermolecular force (e.g. Hydrogen bonding, dipole-dipole, etc.). Students with the same number form a new group (an “expert group”) and research on their assigned intermolecular force. Once research is complete, students reform their original groups and take turns teaching each other about their assigned intermolecular force.

11. Differentiated assessment2. Graphic organiser • Students will complete a graphic organizer (e.g. Flow chart, concept map, mind map, etc.) to summarize and demonstrate the knowledge they have gained about intermolecular forces so far • This will be used as a formative assessment • Click here for an example

12. Lab activity for students Penny drop experiment • Found here on pg. 35 – 37 • NOTE: a selection of 6 labs are found in the same document for teachers to choose from (pg. 27 and following) • This lab addresses the relationship between surface tension and strength of intermolecular forces, so prior to lab the concept of surface tension should be reviewed • Students place droplets of water on a penny until it spills over and they note the number of droplets • Students place droplets of rubbing alcohol on a penny until it spills over and they note the number of droplets • They record their observations and answer discussion questions related to intermolecular forces

13. Penny Drop Lab: Safety and Assessment Safety • Rubbing alcohol is used in this lab and it is both toxic and flammable, so students should be warned not to ingest it or let fire near it Assessment • Students write up a lab report in standard format including observation chart, answers to discussion questions, discussion of sources of error, and a conclusion; they are assessed based on KICA criteria

14. Practical application 1 Role of intermolecular forces • • The granulated sugar falls essentially grain by grain off of the black contact paper into • the sugar container and all of the sugar falls off (a few grains may remain). This is due • to the force of gravity on the sugar particles. • • The powdered sugar falls off in one clump. There will be a trail left by the powdered • sugar as well as clumps of powdered sugar remaining on the card. Even if the card is • tipped upside down, these clumps will remain on the card. All of this is due to • electrostatic forces, specifically Van der Waals forces, and not gravity. • • The size of the object affects the dominant forces acting upon it. • • IMPORTANT: The powdered sugar is not nanosized! The average particle size is 1-10 • μm, which is 1,000-10,000 times bigger than a nanometer.

15. Application 2 Role of intermolecular forces in illustrating behaviour of ZnO • • Zinc oxide forms weakly agglomerated nanoparticles. • • An aqueous dispersion of zinc oxide is very viscous due to Van der Waals interactions • which in turn makes the liquid very difficult to stir. • • Addition of Darvan C, a polyelectrolyte, coats each nanoparticle with negative charge. • This causes the particles to repel each other significantly lowering the viscosity. • • This repulsion causes the particles no longer want to stick together as much and the • solution becomes easy to stir. • • The behavior of the ZnO illustrates how important electrostatic forces are in • determining the behavior of the ZnO solution. • • The ZnO particles range from 200-800 nm which is much closer to the nanometer size

16. Additional Application Questions • 1. Why oil and water don’t mix? • 2. What is role of hydrogen bonding in DNA model? • 3. What role is played by intermolecular forces in human immune system? • http://workbench.concord.org/database/activities/227.html • All application answers are in this link

17. Safety considerations • Maintaining appropriate internet conduct • Students will be given explicit instructions to perform jigsaw activity and role play effectively • Proper use of PPE in penny drop lab

18. Evaluation and Assessment • Diagnostic assessment (after demonstration): Exit ticket, and/or KWL chart [K/U, C] • Jigsaw activity: Students fill out an information chart covering all concepts that expert groups research; informal observations by teacher [K/U, T/I, C] • Interactive online module: Students hand a printed copy of their work by clicking the “generate report” button on the module and hand it in to be marked [K/U, T/I, C, A] • Graphic organizer: Students hand in their completed graphic organizer for marks [K/U, T/I, C] • Students complete problem sets for homework [K/U, T/I] • Students write a test/ quiz about the material [K/U, T/I, C, A] NOTE: K/U = knowledge and understanding, T/I = thinking and investigation, C = communication, and A = application

19. Possible Student Difficulties & Solutions • Visual – Spatial and Kinesthetic learners may have difficulties understanding the 3-D shapes of the atoms, valence electrons and orbitals as well as the shapes of the molecules. Hence, the shape and the bonding properties would be difficult to determine. This could be addressed by using hands-on laboratories and role playing of bonds and atoms. • Students will have difficulties with abstract ideas and therefore need to view videos, demonstrations. • Special needs and students with exceptionalities must be accommodated to fit their varying learning styles and modified according to their individual IEPs. (lab activities) • A handout for ELL learners will be given. A sample handout with important terms is given in the next slide.

20. Curriculum Expectations Addressed • SCH3U • B3. demonstrate an understanding of periodic trends in the periodic table and how elements combine to form chemical bonds • B2.1 use appropriate terminology related to chemical trends and chemical bonding, including, but not limited to: atomic radius, effective nuclear charge, electronegativity, ionization energy, and electron affinity • B2.2 draw Lewis structures to represent the bonds in ionic and molecular compounds • B2.5 predict the nature of a bond (e.g., non-polar covalent, polar covalent, ionic), using electronegativity values of atoms • B2.6 build molecular models, and write structural formulae, for molecular compounds, including those with multiple valences, and name the compounds using the IUPAC nomenclature system • B3.3 state the periodic law, and explain how patterns in the electron arrangement and forces in atoms result in periodic trends (e.g., in atomic radius, ionization energy, electron affinity, electronegativity) in the periodic table • B3.5 compare and contrase the physical properties of ionic and molecular compounds (e.g., NaCl and CH4; NaOH and H2O) • E3.1 describe the properties of water (e.g., polarity, hydrogen bonding), and explain why these properties make water such a good solvent • E3.2 explain the process of formation for solutions that are produced by dissolving ionic and molecular compounds (e.g. Salt, oxygen) in water, and for solutions that are produced by dissolving non-polar solutes in non-polar solvents (e.g., grease in vegetable oil)

21. Websites • Salt and Water demonstration: • http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf • Good overview of intermolecular forces concepts: • http://www.chem.ufl.edu/~itl/2045/lectures/lec_g.html • Inquiry based online module for intermolecualr forces: • http://workbench.concord.org/database/activities/227.html • Sample graphic organizer: • http://www.concord.org/~btinker/molo/molo_concept_maps/atom_forces_attractions.html • Intermolecular forces labs (including the Penny Drop Lab plus other activities and a concept overview): • http://www.haspi.org/curriculum-library/MedicalChemistry/03%20Standard%202%20Chemical%20Bonds/Labs%20and%20Activities/IntermolecularForces.pdf