11/11/13 Weekly Reflection

This week we started by delving back into IMF's, but this week we looked more at how they can affect and how they explain phenomena, such as surface tension, boiling point, evaporation rate, and viscosity of a liquid. We learned about how cohesive and adhesive forces affect all of these things and how they are affected by IMF's. We also looked at solids, including the two main groups of solid - crystalline and amorphous- as well as a couple of groups contained in those two - Ionic crystals, Covalent-network solids, and Molecular solids. We then looked at vapor pressure. Zach had a really good definition of vapor pressure, saying that it was about how many gas particles were moving at a given time and with what force. Thus, as vapor pressure increased, more molecules were moving in a gaseous state at a given temperature, so the higher the vapor pressure, the closer the substance is to boiling. Basically, the higher the temperature, the more particles have enough energy to escape the liquid and hang about as a vapor. Then we worked with lattice energy, which is the energy required to separate a mole of a solid ionic compound into its gaseous ions. It explains why the reaction forming an ionic compound is exothermic, and it is related to the charge and size of the ions. On Friday, we used everything we had learned about vapor pressure to try to figure out which compound was which given five chemical formulas and five liquids in dropper bottles. We used surface tension, viscosity, and evaporation rate to determine which compound was in which bottle.

I had a couple of questions. Why is it that you can pump something up as far as necessary but it can't be pulled up without a state change? What does enthalpy mean and why is it relevant? There was a question on the task chain for the test that had a phase graph - what was up with that graph? Which line were we supposed to be focusing on?  I tried hard to participate in class this week. My group has changed, so my style of explanation has to shift as well, which has forced me to reexamine how I think about concepts. I understood everything fairly well in this section, although keeping the various relationships in order in my head was probably the most difficult part. I still need to work on my method of remembering which relationships correspond in which ways. This section has been fascinating, and I'm now trying to figure out why various substances have higher or lower surface tensions than others.

11/4/13 Weekly Reflection

Ionic compounds and metals were the first topic of the week. Specifically, we worked on metallic solids, alloys, and the electron sea model. Next, we dealt with van der Waals intermolecular forces, including H-bonding, dipole-dipole interactions, induced dipole-dipole interactions, and London dispersion forces. H- bonding is when hydrogen is in an extremely polar bond, causing it to be nearly entirely positive and attract other, negatively charged ends of molecules in which H-bonding is present. This is the most powerful of van der Waals forces. Dipole-dipole interactions are when molecules with dipole moments are attracted to the oppositely charged ends of other molecules with dipole moments. This is the next strongest. Next is dipole-induced dipole interactions, where the dipole moment in one molecule will cause the electron cloud of another molecule to shift, inducing a dipole moment in that molecule and causing some attraction. London dispersion forces (LDF's) are the weakest intermolecular forces and are present in every substance. Because electrons are constantly moving, there will occasionally be an instant when the electrons are shifted to one side of the molecule, causing a temporary dipole moment which is then passed around the substance. We spent the rest of the week white boarding and working with water models.  

I tried very hard to participate in the learning process this week because I understood all of the material very well. I especially liked the idea that the electron cloud around any given molecule is not fixed, and that they can all be on one side temporarily, spreading an induced dipole throughout the substance. I need to work more on interacting with my group - everyone in my new group is kind of quiet, and we don't know each other that well, so it's a little awkward, and I need to work to iron that out. I was curious about surface tension. Specifically, I was wondering why, if water has a stronger adhesive force with glass than a cohesive force with itself, things can rest on the surface tension of the water. Is it just because there is a different and weaker adhesive force between water and, for example, water striders than between water and glass? Also, in the PowerPoint on cohesive and adhesive forces you said that trees could only pull water up thirty-three feet. How are trees taller than this if they can only bring water up thirty-three feet? This section was fun because now I get to consider the possible cohesive, adhesive, and intermolecular forces operating all around me.