Intermolecular Forces Concept Presentation By: Amarinder Sawhney Jeffrey Ip

Salt (NaCl) & Water (H2O) Why does salt dissolve in water?

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

Salt (NaCl) & Water (H2O) Salt (NaCl) is an ionic compound consisting of a positively charged sodium ion (Na+) and a negatively charged chlorine ion (Cl-)

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)

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

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

Dispersion (London) Forces Dispersion (aka London) Forces are the weakest intermolecular force They occur between all molecules, and are the primary intermolecular force between nonpolar molecules Electrons are always moving and at any instant can be distributed as to create an instantaneous dipole, which momentarily attracts or repels a neighbouring molecule

Charge-Induced Dipole Forces Dipole-induced dipoles and Ion-induced dipoles are charge-induced dipoles Dipole-induced dipoles occur between a polar molecule and a nonpolar molecule Ion-induced dipoles occur betwen an ion and a nonpolar molecule Electrons can be pictured as clouds and the polar molecule or ion distorts the electron cloud of a nonpolar molecule by pulling it towards a positive charge or pushing it away from a negative charge creating a temporary, induced dipole moment

Dipole-dipole Forces Dipole-dipole forces occur between two molecules that each have a permanent dipole moment (e.g., polar molecules) Ends of the dipoles possess partial charges represented by Greek letter delta (δ) The positive pole (δ+) of one molecule attracts the negative pole (δ-) of another

Hydrogen Bonds Hydrogen bonds are a special type of dipole- dipole force occuring between molecules with an H atom bound to a small highly electronegative atom with lone electron pairs Typically, hydrogen bonds are found in molecules in which highly polar H–N, H–O or H–F bonds are present Hydrogen bonds are found in water and DNA

Ion-Dipole Forces Ion-dipole forces occur between an ion and a polar molecule Example: Salt and Water

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

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

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

Teaching Strategies (in Sequence) Diagnostic Demonstration Lecture Presentation Jigsaw Activity Inquiry based online module Graphic Organizers (formative) Penny Drop Lab

Diagnostic Demonstration Teacher will ask students to differentiate solids, liquids and gases and the arrangement of their molecules, and illustrate these states of matter and their phase changes using melting ice and boiling water Teacher asks verbally what is happening at each phase transition and reviews the types of intra-molecular bonding that occurs in each phase As the ice melts, teacher will ask students to make educated guesses about how INTERmolecular bonds of the ice molecules are broken and liquid water molecules are formed. As the water is boiled, teacher discusses the energy that is needed to break the INTERmolecular bonds of the liquid water so the molecules can change into the gaseous phase from the liquid phase

Lecture Presentation Using a slideshow and a lecture similar to the content portion of this slideshow, teacher will explain the key concepts of intermolecular forces to students Slideshow and lecture would benefit ELL students as they become introduced to the concepts and definitions verbally and with visual aids

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.

Inquiry Based Online Module Click here for the online module 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

Graphic Organizer 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

Penny Drop Lab: Overview 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

Penny Drop Lab: Safety and Assessment 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

Application and Societal Implications Following the above activites, teacher can challenge students by having them apply their new knowledge of intermolecular forces to new contexts. Two new contexts involve inquiry into Sugar or Zinc

Notes for an Application Lab w/ Sugar • 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.

Notes for an Application Lab w/ Zinc • 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

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?

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 compelted graphic organizer for marks [K/U, T/I, C] Penny Drop Lab: Students hand in a formal lab report that includes observation chart, answers to discussion questions, discussion of sources of error, and a conclusion [K/U, T/I, C, A] 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

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.

Sample ELL Definition Sheet Electrostatic forces : Forces between electric charges. The magnitude of the force depends on the magnitude of the charges and the distance between them. Dipole moment : The product of partial charge on either side of a dipole multiplied by the distance between them. It is a measure of the polarity of a molecule. Instantaneous dipole : A temporary dipole, which lasts for a very short duration of the order of a fraction of a second. Temporary dipoles : Dipole set up in a neutral molecule or an atom under the influence of some electrical charge nearby. It vanishes when the source is removed. Polarisability : The ease of distortion of electron cloud of an atom/ molecule or ion is called its polarisability. Partial charges : A charge found on either side of end of the dipole that is less than full +1 or – 1 charges. Polar molecules : A molecule having partial positive and negative charges on opposite ends. Dipole : Two equal and opposite charges or magnetized poles separated by a distance. Induced dipole : The separation of positive and negative charges in an atom (or a non polar molecule) caused by proximity of an ion or a polar molecule.

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)

Websites Salt and Water demonstration: http://www.mhhe.com/physsci/chemistry/essentia lchemistry/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_conc ept_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%20Acti vities/IntermolecularForces.pdf