Concept Presentation
By:
Amarinder Sawhney
Jeffrey Ip

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Why does salt dissolve in water?

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Water is a polar molecule: one end has a slightly positive charge, another has a slightly negative charge

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

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When the NaCl crystals get closer to the positively and negatively charged poles of the
H
2
O molecule, the Na + and Cl ions are pulled apart
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You can think of the H
2 magnet
O molecule as a strong
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Click here for more details
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Attraction between (polar) water molecules and ions is an example of intermolecular forces at work (specifically ion-dipole forces)

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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)
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Intermolecular forces are typically weaker than intramolecular forces, and account for the bulk properties of matter (e.g., boiling point, melting point, etc.)
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Intermolecular forces are also known as van der Waals forces , named after Johannes van der Waals who first postulated them

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Dispersion (London)
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Dipole-induced dipole
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Ion-induced dipole
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Dipole-dipole
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Hydrogen bond
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Ion-dipole

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Dispersion (aka London) Forces are the weakest intermolecular force
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They occur between all molecules, and are the primary intermolecular force between nonpolar molecules
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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

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Dipole-induced dipoles and Ion-induced dipoles are charge-induced dipoles
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Dipole-induced dipoles occur between a polar molecule and a nonpolar molecule
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Ion-induced dipoles occur betwen an ion and a nonpolar molecule
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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

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Dipole-dipole forces occur between two molecules that each have a permanent dipole moment (e.g., polar molecules)
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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

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

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

http://www.chem.ufl.edu/~itl/44
11/lectures/lec_g.html

http://www.chem.ufl.edu/~itl/44
11/lectures/lec_g.html

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

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

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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
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Teacher asks verbally what is happening at each phase transition and reviews the types of intra-molecular bonding that occurs in each phase
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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.
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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

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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
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Slideshow and lecture would benefit ELL students as they become introduced to the concepts and definitions verbally and with visual aids

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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.

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Click here for the online module
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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

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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
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This will be used as a formative assessment
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Click here for an example

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Found here on pg. 35 – 37
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NOTE: a selection of 6 labs are found in the same document for teachers to choose from
(pg. 27 and following)
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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
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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

Safety
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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
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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

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

• 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

• 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

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?

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

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.

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

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

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: