10.28.13 - Test on Covalent Bonding, Hydrogen Bonding, Ionic bonding, and Metals

Testing on Covalent Bonding

This Tuesday, we had a test on Covalent bonding.

                     

Overall, I felt better about this test than the last test on Stoichiometry. I haven't looked back at my test yet so I don't know what I got wrong.

Hydrogen Bonding

We also got introduced to hydrogen bonding this week. We read an article about the osmosis of paintballs and the reason behind it. It turns out that there are small attractions called hydrogen bonding between the hydrogen and the oxygen atoms because of the positive polarity on hydrogen and negative polarity on oxygen.

To help understand the material, we wrote an essay on a Paintball essay by Brian Rohrig called "Paintballs"

Ionic bonding

We started our next unit on ionic bonds. In an ionic bond, the electrons are freely moving within the material, unlike covalent bonds where they are sharing electrons. The ionic bond is between a Cation (positive) and an Anion (negative). In NaCl, the sodium ion is smaller than the chlorine atoms because it looses one row of electrons to the Cl.

To help understand this, we did a pogil on Ionic Bonding. 

We also learned that those with an anion or cation with a higher charge has more force between their attraction. The smaller the ionic compound, the larger the melting point. Melting point depends on how much energy it takes to break or spread the bonds between the ions.

Metals

There is a lecture quiz due this Sunday on Metals.

Characteristics of metals include:

• conductivity
• malleable (able to be pressed into different shapes)
• ductile (able to be drawn into wire)
• metals aren't as brittle as other solids

They are not covalently bonded, but they have extremely strong bonds between the ions.

Electron-Sea Model is a representation of how electrons on the outermost valance swims around freely inside the sample. The rest of the ions are structured like this

This explains its conductivity.

Alloys are mixtures of elements that have property characteristics of metals. This can be a metal within a metal, or can be a non-metal within a metal (they are located in the staircase on the periodic table). Solution alloys are solutions such as Steel (Carbon and Iron). They can be substitutional alloys where solute particles take the place of the solvent metal atom, or an interstitial alloy where the solute particles are so small they can fit into the holes between the solvent metal ions. Therefore they are usually more dense than substitutional alloys.

Main Ideas

The main idea that was new this week was the introduction to ionic bonds and metals. I feel relatively comfortable with these ideas because a lot of this material has been gone over last year in SG Chem 1. The only think that I'm wondering about is what is the charge on these atoms measured in? (what is the unit of charge? Coulomb?)

10.21.13 - WebMO, hybridization, sigma/pi bonds, test next week

WebMO

This week, we've mostly worked out building our VSEPR balloon models on WebMO. To build a model on WebMO, we first placed out elements on the program and submitted it to clean up the shape of the structure. Then, by creating the structure, we were able to determine the angle measures, the dipole moment, the charges of the elements, and we were able to build the space filling models.

Here are some examples:

By building these models, we were able to understand the dipole moments, polarity, electron domain geometry and the molecular domain geometry. This activity really helped me with determining the different domain geometries and the angles that correspond to each molecule.

Hybridization

I believe we were introduced hybrid orbitals in one of the lecture quizzes, however this week we went over some examples for homework. sp corresponds with two hybrid orbitals, with angle measures of 180 (linear). sp2 corresponds with three hybrid orbitals with angle measures of 120 (trigonal planar). sp3 has four hybrid orbitals with angle measures of 109.45 (tetrahedral). Although there use to be a theory that the d orbitals also exist, we no longer learn this in class because there is doubt in the theory. So, anything above four hybrid orbitals does not have a hybrid orbital.

sigma/pi bonds

On Friday, we went over a little about sigma and pi bonds to clear up some confusion we had since out lecture quiz. A single bond is always a sigma bond. When there is a double bond, there is a sigma and pi bond.

Here is a picture of a diagram of CO

In this way, there are one sigma bond and two pi bonds that make the CO molecule covalent.

Test Next Week

We have a test next week on Tuesday. We will be tested on drawing Lewis structures, calculating formal charges, resonance structures, bond length and strength, VSEPR models, bond order, bond angles and their reasons, shapes of the Electron Domain Geometry and Molecular Domain Geometry, hybridization, polarity, and other material we have covered in this unit.

Main Ideas

This week, the main idea was to become comfortable with the uncertainties we had since our lecture quizzes. Practicing the electron domain geometry and the molecular domain geometry on the WebMO really helped me because I got more practice and understanding to why the molecules take the shapes they take. I'm still unsure about fully understanding the material for this unit. For example, what is the difference between a sigma bond and a pi bond? I understand that there are two different types of bonds in a double bond or triple bond, but I don't know what the differences are. I would like to relearn the unit with a deeper understanding if I ever have time before the AP exam. I still need to work on understanding the general ideas of all the topics (under Test Next Week) because I mix up a lot of the concepts we've learned over the past few weeks, like hybridization and polarity. However, my ideas have changed about atoms: I know they are much more complicated than what I assumed they were like!

10.14.13 - VSEPR, Lab Data, Formal Charges, and Resonance Structures.

VSEPR Theory Lab

This week, we started off by finishing the VSEPR theory lab. In the worksheet, VSEPR Theory Lab (1), we went through 13 examples of molecules and found it's Electron Domain Geometry and Molecular Domain Geometry.

In order to determine the Electron Domain Geometry, you must...
1. Draw a Lewis Structure for the molecule.
2. Count the number of bonded atoms around the central atom.
3. Count the number of lone pair electrons around the central atom.
4. Figure out its molecular class.
5. Find out its Geometric shape.

• Total of two bonded atoms and lone pair electrons: linear.
• Total of three bonded atoms and lone pair electrons: Trigonal Planar.
• Total of four bonded atoms and lone pair electrons: Tetrahedral.
• Total of five bonded atoms and lone pair electrons: Trigonal Bipyramidal.
• Total of six bonded atoms and lone pair electrons: Octahedral.

In order to determine the Molecular Domain Geometry, you must...

1. Find the Electron Domain Geometry.

2. Think of what the shape that the Bonded atoms would take (the red balloons, in context of our lab)

• They would be either linear, bent, trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal, seesaw, T-shape, octahedral, or square pyramidal.

To follow this main idea, we did a lab using balloons and gum drops. Balloons helped us figure out the Electron Domain Geometry by tying the red balloons (bonded electrons) and the white balloons (lone pair electrons) together. The gum drops represent the Molecular Domain geometry (Orange was the central atom, black was the bonded atoms). The gum drops were tied with toothpicks that depicted the shape of the molecules relatively accurately.

Lab Data

We also finished figuring out the answer to our lab questions from last week (What is the mass percent of copper in a brass sample?). This is our class data:

By using the information given, we figured out the mass percent of copper.

After doing some stoichiometry, we found that the mass percent of copper was 58.7%

Later next week, Dr.J will (try to) give us our lab reports from the last lab back in order to finish our lab this week of brass.

Formal Charges

We had a lecture quiz on formal charges, as well as a Pogil (Lewis Structures (III)) to further become comfortable with the material. To calculate formal charge, you must...

1. Make a Lewis structure for the molecule.

2. Count the number of electrons corresponding to the atom you are trying to figure out.

3. Subtract the number of original electrons (from the periodic table) by the number of assigned electrons.

Here's an example.

Resonance Structures

We also did a Pogil for Resonance Structures (Lewis Structures (III)) to understand which formal charge is the best for each molecule. By figuring out the resonance structures of the molecules, we were able to decide which structure was the best for that molecule (the one with the least number of formal charges).

Main Ideas

I understood most of the material that we learned this week, such as formal charges, resonance structures, bond order, and hypervalancy, however I still have trouble finding the Electron Domain Geometry and Molecular Domain Geometry for molecules. Why can't H2O be linear, instead of bent? Does the structure look like this?

All of the material we learned this week helps us understand the deeper facts of Lewis structures, beyond their models. VSEPR theory is a really interesting idea to me, however I wish I could become more comfortable with it. Next week, I will probably have more questions about the lab as well.

10.7.13 - Bond Order and Bond Strength, Brass Lab, and VSEPR Theory Lab

Bond Order and Bond Strength

First, before the worksheet, we went through some Lecture Quizzes that gave us an understanding of how to draw Lewis Structure Diagrams. Here are the steps:

1. Calculate the total number of valance electrons for the molecule.
2. Draw the skeleton structure for the molecule. The least electronegative element should be the central atom.
3. Make bonds between the surrounding atoms and the central atom.
4. Distribute as many remaining valence electrons to the surrounding atoms until their octet (or duet) are filled.
5. Distribute the rest to the central atom. If there is not enough, borrow lone pairs of electrons from one or more surrounding atoms and create multiple bonds until the Octet Rule is satisfied.

This week, we started off with a Pogil on Bond Order and Bond Strength. Bond Order is the number or pairs of electrons that are shared between two elements. Bond Energy (kJ/mole) is the energy required to sever the bond that holds two adjacent atoms together in a molecule. We found that the bigger the bond order, the bigger the bond energy. Bond length is the distance between the nuclei of two bonding atoms. As bond length increases, bond energy decreases.

We went through lots of practice to become comfortable with this material. I feel that I understood this pretty well, because it is basically a little bit more detail added to the Lewis Structure Diagram.

Brass Lab

Before we started our lab, we received a sheet of productive and non-productive beginning questions.

This week, we started on our Brass lab. The main goal of this lab is to find the percentage by mass (mass percent) of copper in brass screws. As a class, we decided to investigate these question: what is the relationship between the absorbance of the solution and the concentration of copper, and how does deducting concentration visually compare to the measurements of the colorimeter?

The procedure includes reacting the brass solution in HNO₃ to produce a light blue solution of Copper.

Then, we took the stock solution of 0.393M and made a serial dilution of 0.200M, 0.100M, 0.050M, and 0.025M. We then measured the absorbance using the colorimeter.

Next week, we will interpret the class data and write our lab report.

VSEPR Theory Lab

We were introduced to the VSEPR Theory on Friday. VSEPR Theory predicts the shape an individual molecule will take depending on their bonding and non-bonding electron pairs.

Electron domains are the number of bonds there are between the elements. For example, carbon dioxide has an electron domain of two, because there is one bond between O and C, and another between C and O.

The shapes of the pairs are not perfect circles or lines. To show this, we tied small red balloons to represent the bonded pairs and larger white balloons to represent the non-bonding pairs. For example, ClF₃ would look like this:

To go along with the new material, we did VSEPR Theory Lab (I), as well as the balloon models.

Main Ideas

This week, I feel like we went deeper into the bonds that tie together the elements and the structure of the molecules. However, I still have trouble understanding the shape of the compounds according the the VSEPR theory. The balloon structures representing the region in which the electrons are most likely found make sense to me, but I don't understand how the actual molecules look like. I think I will come to an understanding once we go over this worksheet and see the other models of other molecules. Other than that, I feel I understand the material we went over this week. I think there will be more problems when we enter the next week, where we discuss the results of the Brass Lab and finish up the VSEPR theory lab. Because I was not in class on Tuesday, I don't think I've participated very well this week. My ideas about the structure of a molecule has definitely changed after the VSEPR theory lab. I also feel really excited about doing more labs in this class now that we did a lab with real chemicals and fairly strict safety precautions. I just need to remember to use more drawings in the lab procedures.