Major Exam, Wednesday, 1/25/2012

Your first major exam of the semester will be on Wednesday, January 25th, 2012. I have not finished writing the exam yet, but I can tell you that the test will be between 30 and 40 questions. It will be mostly multiple choice with a few free response questions. Below I have written out the key concepts that you need to be aware of to do well on the exam.

• Be able to complete a Lewis dot structure for a compound given.



• Based upon a Lewis dot structure, you must be able to determine the parent geometry, number of ligands around the central atom, number of bonding and non-bonding ligands around the central atom, and the molecular geometry.



• Know that non-bonding ligands on the central atom repel more than bonding ligands. This has an influence on the bond angles. Example: tetrahedral molecular geometry (4 bonding ligands) has a bond angle of 109.5^o; trigonal pyramidal molecular geometry (3 bonding ligands and one non-bonding ligand) has a bond angle of 107^o; and bent molecular geometry (2 bonding ligands and 2 non-bonding ligands) has a bond angle of 104.5^o. You will be supplied a table of parent and molecular geometry names and the angles associated with each molecular geometry. An image of the table is at the bottom of this post.



• Based upon molecular geometry and the ability to visualize a 3-D model of the molecule with dipoles (based on electronegativities), determine if the molecule is polar or non-polar. This is based upon the symmetrical or asymmetrical distribution of charge around the outside of the molecule.



• Based upon the polarity of the molecule, determine the type of intermolecular bonds that would occur between molecules of the same type.



• Non-polar molecules will experience only London dispersion forces.



• London dispersion forces are the weakest of the types of intermolecular bonds. They are created by a temporary dipole randomly occurring across a molecule do to movement of electrons within the molecule. London dispersion forces strength difference comes from the different sizes of molecules. Larger molecules have more surface area for bonding and more electrons to create a greater temporary dipole.



• Polar molecules can experience dipole-dipole forces or hydrogen bonding intermolecular forces. Dipole-dipole forces are stronger than London dispersion forces. Hydrogen bonding is a special type of dipole-dipole force because it occurs between polar molecules that contain hydrogen covalently bonded to fluorine, oxygen, or nitrogen. Hydrogen bonding is the strongest of the intermolecular bond types.



• Know the reasons why hydrogen bonding is the strongest type of intermolecular bond. (1) The atoms (H, F, O, and N) are all small atoms which allows for the molecules to fit in close together for stronger bonding. (2) The large electronegativities of fluorine, oxygen, and nitrogen create a large charge separation when covalently bonded to hydrogen. (3) Hydrogen has no core electrons. When hydrogen's one electron is closer to the other atom in a covalent bond due to the other element being more electronegative, the proton of hydrogen is completely exposed. (A.k.a. - hydrogen is a mooner)



• **Please note: The strength of the intermolecular (between molecules) bonds are only to be compared to one another. Both ionic bonds and covalent bonds between atoms (intramolecular) are much stronger than any of the intermolecular bond types (London dispersion, dipole-dipole, hydrogen bonding).



• **Please note: All molecules experience London dispersion forces. This is why the size of the molecule (more surface area / more electrons for temporary dipole formation) is the indicator of intermolecular bond strength when the type of intermolecular bonds are the same when comparing more than one molecule.



• Know how the concepts of melting/freezing point, boiling/condensation point, heat of fusion, and heat of vaporization are based upon intermolecular bond strength. (1) low values of all of the concepts listed will have weaker intermolecular bonds. (2) high values of all of the concepts listed will have stronger intermolecular bonds.



• Know how to compare intermolecular bond strengths on the values of joules or kilojoules per mole. Know that the mole concept is needed to accurately assess bond strength do to the amount of energy used in comparison with the number of bonds broken. This comparison cannot be made with energy value per gram of substance.



• Know the graphical concepts of the Intermolecular Bond Strength / Vaporization lab. Know how the graphs of the lab were interpreted. The weakest intermolecular bonding type belonged to acetone, so it finished evaporating first. This was indicated by the temperature on the graph for acetone beginning to rise first. Once all the acetone had evaporated, energy was no longer being taken from the temperature probe to evaporate the liquid acetone (break bonds between the molecules). Energy from the room then went back into the temperature probe to raise the temperature of the probe. Intermolecular bond strength was determined by when the temperature increase occurred. The longer it took for the temperature to rise again, the stronger the intermolecular bonds.



• Be able to look at a phase change (Temperature vs Time) graph and determine the state of matter and (melting/freezing) / (boiling/condensation) points of a substance.



• Know that heat of fusion and heat of vaporization calculations would take place at the plateau's of the graphs.



• Know that any sloped part on a phase change graph would require (q=s x m x delta T) to determine the amount of energy associated with the change in molecular motion of the substance.



• Know how to solve for any variable associated with the heat of fusion or heat of vaporization equations.



• Be able to calculate the total amount of energy associated with the heating or cooling of a substance. This would involve multiple calculations using the heat of fusion and/or heat of vaporization with (q=s x m x delta T).



• THERE WILL BE NO QUESTIONS ABOUT THE ENERGY NEEDED TO MELT ICE LAB.



• Look at all material used for the unit of study. Watch the VODCast of the first work packet done in class. A key (pdf file) for the second work packet is posted on the Events Calendar under the date, Monday, January 23rd, 2012.



• You will get to use your periodic table. I will supply you with an electronegativity table and a copy of the diagram below. You will also be supplied with all equation for the exam.