CH301 - Exam 3 Review (Chapter 16)

CH301 - Exam 3 Review (Chapter 16) - Dr. McCord CHAPTER 16...

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Chapter 16 – Which Sections for the Exam? Chapter 16: You need to know only the first part of the chapter on intermolecular forces - this is sections 1 and 2. We are not covering all the crystal structure material which are sections 3-9. We also covered bits of section 10. In particular, we covered the concept of vapor pressure and how intermolecular forces affect it. Plus, some of the section on changes of state. Refer to your notes. You should note that the material for sections 1 and 2 was covered more thoroughly in class (and on this review sheet). Carry over from previous chapters In order to understand the discussion about dipoles and partial charges, you MUST know what those are and what polarity is. You should also still know your periodic table trends especially size and electronegativity. Why molecules stick If molecules had no sticking power, all substances would be gases. There would be no condensed phases of solid or liquid. Well thank goodness there ARE forces of attraction (the sticky) between molecules. These forces are called intermolecular forces. They are also known as Van der Waal’s forces. These are the forces that we are most concerned with in chapter 15. These are also the forces that govern various physical properties such as boiling point, melting point, surface tension, viscosity, etc… I’ll come back to this after I remind you of the forces of attraction that we have all ready covered quite extensively. Intramolecular forces are the forces of attraction within molecules and are simply the bonds that hold the atoms together to make the molecule in the first place. Realize that intermolecular forces are much much less than intramolecular forces. On average, intermolecular forces are 100 times less depending on what you are comparing. Looking back to Chapter 13 you can see how strong an intramolecular force (covalent bonding) really is. Check out Table 13.6 on page 608 to see numerous bond dissociation energies. Those are the amounts of energy that would be required to pull those bonds apart. The lowest value on that table is 149 kJ/mol and goes up to 1072 kJ/ mol for a carbon-oxygen triple bond. Realize that is just a sampling of all the possible covalent bond strengths, but you need to have an idea of where all those strengths are on the energy scale and what their average is. Let’s put the average at around 400 kJ/mol. Now let’s get back to the main topic here. Intermolecular forces: All intermolecular forces are governed by charges attracting or repulsing one another. Heck, pretty much everything chemically speaking can be traced back to this fundamental interaction of charges. Bearing that in mind, it is still not just a simple case of positive attracts negative. We want to quantify the amount of attraction/repulsion so that we can better understand and predict the properties that are a result of these intermolecular interactions. What’s in a name?
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This note was uploaded on 11/02/2008 for the course CH 301 taught by Professor Fakhreddine/lyon during the Fall '07 term at University of Texas.

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CH301 - Exam 3 Review (Chapter 16) - Dr. McCord CHAPTER 16...

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