9/30/13-10/4/13

This week in AP Chemistry, I continued to learn about Lewis structures. Over the course of the week I was introduced to the topics of bond order, resonance, hypervalency, and covalency. Bond order is the number of chemical bonds between a pair of atom. A single bond has a bond order equal to one, a double bond has a bond order equal to two, and so on and so forth. To help learn the concept of bond order, I completed the Lewis Structures Part 2 lecture at home and completed a POGIL on Lewis structures in class.

Resonance is a term used to describe a situation in which there are more than one valid Lewis structures for a given molecule. Initially I was unsure of the exact definition of resonance, but this website gave a nice concise definition. The Lewis Structures II POGIL and the Lewis Structures Part 2 lecture dealt with this topic.

Possible resonance structures of NO3. The double headed arrows indicate interchangeability (all three structures are valid). 

If there are multiple valid Lewis structures, the bond order is affected. This was a point of confusion for me. Initially, after watching the Lewis Structures Part 2 lecture, I came under the impression that the bond order was equivalent to the number of bonds divided by the number of possible structures for a molecule. However, after starting the POGIL on Monday I discovered that this was certainly not the case. I learned that bond order between two atoms was simply equivalent to the number of bonds present. I am still confused as to how resonance affects bond order. I have searched online to find an answer, but I have not found one that makes it clear to me.

Hypervalency is a situation in which there is more than eight electrons around an atom in a molecule. When making a Lewis structure, if you satisfy the octet rule for all of the outer and central atoms and still have extra valence electrons, distribute them about the central atom. This is only possible if the central atom is in period 3 or greater.

In the PCl5 molecule, the Phosphorus atom is hypervalent. It exceeds the octet rule and has 10 valence electrons.  

When considering hypervalency, size is important. The larger the central atom, the larger the number of electrons that can surround it. Expanded octet most often occurs when the central atom is bound to atoms with high electronegativity such as Fluorine, Chlorine, or Oxygen. To learn the concept of hypervalency, I watched the Lewis Structures Part 3 lecture. The concept was also part of the VSPER Theory POGIL we started in class on Friday. I found most of this topic easy to understand. With that being said, I thought the portion in the lecture that explained how the expanded octet is possible was very confusing.

Covalency was the final new topic we covered this week. In the lecture on Covalency, I learned that molecules with covalent bonds stay together because the force of repulsion between protons is weaker than the force of attraction between protons and electrons. Electrons are said to be "paired" when they have opposite spins and enter the domain of the other. Additionally I learned that as bond order increases, strength increases.

Overall I though I understood most of the concepts covered this week well, with the exception of the relationship between resonance and bond order as well as the part of hypervalency that I explained earlier.

In addition to learning these concepts, we did a stoichiometry-related lab in class on Wednesday and Thursday. The purpose of the lab was to find the percentage mass of copper in a brass screw. In the lab, we took a brass screw and dissolved it in nitric acid under the fume hood, which produced both toxic NO2 gas and a liquid, Cu(NO3)2.

The brass screws react with nitric acid to produce a blue liquid, Cu(NO3)2, and a brown gas, NO2.

Then, half of the groups made dilutions with the Cu(NO3)2 liquid, measured their absorbance, calculated concentration, and made a calibration curve. The other groups (mine included) tested the visual comparison method of calculating concentration. In this process, you filled two beakers with solution. One beaker had stock solution and the other had the product of the brass screw/nitric acid reaction, known as the "mystery solution". The beaker with mystery solution was filled until its color matched that of the stock solution. The depth of each solution was then measured and then the concentration of the mystery solution was calculated with the equation:

(Molarity1)(Depth1) = (Molarity2)(Depth2)    

The lab was fairly straight forward. I think my work on it is an improvement over my work on the previous lab. Now that I have a little bit of experience doing labs, I am more comfortable and the work in my lab notebook is much tidier. I am unsure when the lab is due and would like to know so I know when to start working on the post-lab questions.