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 H

2O molecule, the Na+ and Cl- ions are pulled

apart You can think of the H

2O 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

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

Summary of Intermolecular Forces

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

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

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 CH 4; 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/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