page background

Intermolecular Forces

Bishop State Community College CHM 104/105

IntermolecularForcesBishop State Community College CHM 104/105

Why do substances exist as gases or liquids or solids at room temperature?

The answer is the forces of attraction between particles determines whether a substance will be a solid, liquid or gas AT room temperature

It really comes down to polar vs nonpolar...remember that?

https://www.youtube.com/watch?v=PVL24HAesnc

The attractions between molecules are not nearly as strong as the intramolecular "force" such as the covalent bond in the example below.

zoom_out_map

These intermolecular attractions are, however, strong enough to control physical properties, such as boiling and melting points, vapor pressures, and viscosities.

An important exception to intra-vs-inter is in ionic compounds. The interparticle force is the same as the intermolecular force: the ionic bond.

3 Main IMF's explained below

zoom_out_map

Dipole-dipole (including H-bonding)

Think polar bonds and molecules. The polar bond is a dipole and if there is a polar bond that ends up making a molecule overall polar, then dipole-dipole is the IMF and this one can be pretty strong.
zoom_out_map

Hydrogen Bonding

A special case of dipole-dipole is H-bonding and that just means a polar bond with H at one end and either N, O, or F at the other end (only these 3 highly electronegative atoms bonded to H will give H-bonding).
zoom_out_map

London Dispersion (van der waals, induced dipole)

This type of IMF is present in almost every substance. The key to understanding London dispersion forces is to think of nonpolar molecules in particular as like water balloons that can be "squishy". The squishiness increases with size and the same is true for nonpolar substances. The larger the nonpolar molecule, the "squishier" the electron density (science word = polarizability) and the stronger the attraction between molecules. Another way to think about it is this IMF is like velcro. Small molecule = small piece of velcro = weaker London dispersion force. Large molecule = larger piece of velcro = larger London dispersion force.
zoom_out_map

Factors affecting London Dispersion forces

  • The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane) tend to have stronger dispersion forces than short, fat ones (like neopentane).This is due to the increased surface area in n-pentane.

IMF Paper Lab for CHM 105 or Extra Credit project for CHM 104

As part of a good review and to learn about this important topic, read the sections above and follow the links below. Make sure you answer any questions indicated in the directions below in your comp book.


1. Compare and contrast ionic and molecular substances in terms of their physical properties and the types of forces that govern their behavior.


2. Compare and contrast the 3 types of intermolecular forces (IMFʼs) discussed above. Be sure to compare their relative strength and the types of substances that exhibit these different kinds of forces.


3. Next, here are two tutorials to help you (do them in this order):


http://www.ausetute.com.au/intermof.html


http://employees.csbsju.edu/hjakubowski/classes/Chem%20and%20Society/IMF_Solutions/olIMF_solutions.htm


4. Add the two “additional” intermolecular forces discussed in the second tutorial to your list from #2 above. For each type of IMF, provide two different examples of substances that exhibit that force and why each example exhibits that particular IMF.


6. View the animation: "Like Dissolves Like" and answer the questions that are part of the tutorial:


http://employees.oneonta.edu/viningwj/modules/


In order to better understand hydrogen bonding, re-visit this website and do the tutorial on "Hydrogen Bonding" in water (Chapter 10). Play with the controls and explain the differences between the structure of liquid water and ice and how hydrogen bonding contributes to these different structures:


http://www.wwnorton.com/college/chemistry/chemistry3/ch/10/chemtours.aspx


8. Your final task is to construct physical models that would clearly show each of the IMFʼs. Be creative!


The models should show the relative strength of each type of IMF (Do the 3 main types identified in #2 above). Have a real physical example that would “fit” each model. For example, carbon dioxide is made of nonpolar molecules and, therefore, London dispersion forces exist between carbon dioxide molecules. If this nonpolar molecule is one of your examples, the model should show a weaker attraction as compared to a polar example. (Sorry, think of another example for London forces.) You will turn in your models in class on March 26. Be ready to discuss your models with your teacher.

Chemistry Tutorial 7.03a: Intermolecular Attractive Forces

Extra Practice

Practice with answers

What can you do to help your understanding?

Chemistry is a subject that must be practiced everyday if possible. Take advantage of the practice in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Blackboard everyday. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!