Blog Entry 12/15/13

This week heralds the return of the AP Chem blogs! Much time has past since we have had to revisit these writing assignments. We have been exploring the topic of enthalpy and entropy (the dreaded “E” word!) and applying them to given scenarios. We started off with discussing our precipitation lab we performed last week. We were given a collection of mystery solutions, and we were to determine the identity of the solutions. We mixed on drop of one solution with a drop of another and observed the reaction; many of them formed precipitates with varying colors. Using our knowledge of net ionic equations and properties of precipitates, we slogged through the reactions and eventually arrived at the correct answers.

Also early in the week, we were introduced to the concept of entropy. The elusive yet dreaded “E-word” had appeared a few times in Tri 1, but this week is when we strove to understand the concept. Using laptops, we completed a lesson online measuring the order of solutions and solutes.

I have confused enthalpy with entropy many times. However, I’ve learned that enthalpy is the measure of energy in the system, while entropy is the measure of randomness or disorder. Admittedly, entropy is still somewhat cloudy of a concept to me; I’m still trying to understand how to connect the disorder of a system with the available microstates of each molecule.

In the middle of the week, we worked out the equation for dissolving sodium acetate trihydrate through the use of standard enthalpies. It takes 36 kJ to perform the reaction, and it is exothermic. We witnessed this reaction when we were given heat packs; after activating them, the pack released 36 kJ of heat, which is what we felt in our hands.

We also started a thermodynamics worksheet involving the calculation of reaction enthalpy values using standard bond enthalpies. We whiteboarded some problems which, as usual, proved very useful to me. The boards did a great job of outlining the steps for finding enthalpy, and many of my classmates asked some good questions, which helped my understanding of the whole process.

We were also introduced to our new lab next week. We aren’t given a procedure, just an objective and a few scraps of information. I’m actually kind of excited for this lab; I enjoy figuring things out even though it may be difficult for me to follow. On Friday, in fact, we worked out the reactions for determining the heat of formation of MgO and the reaction between Mg and HCL. Truthfully, I was having some trouble following the process, but I understood the experiment towards the end through the help of my classmates who were leading the process on the board. I enjoy this challenge, and I’m quite sure 3^rd hour will prevail and win the much sought-after prize: the famed Einstein bagel!

Julien Griffith summons a ball of flame (just kidding, it's just the HCL and Mg reaction)

We also went through questions in the Entropy HotPot. We were to read the question on the screen, and then we held up our fingers indicating which answer we thought was correct. This proved to be inexplicably helpful to me. I really enjoy class discussion, because many people ask questions I myself didn’t know I had. Dr. J explained the process through which we could answer these questions, sometimes by eliminating answers immediately and then using logic to attack a question when we didn’t know the exact answer. I am somewhat apprehensive towards the upcoming exams, but I’m sure the HotPots will prove to be invaluable as a study aid.

Blog Entry 11/10/13

This week in AP Chemistry, we focused on state changes. We started off with some discussion on liquids and solids, and also what it looked like when potassium chloride was dissolved in water. Water is a polar molecule, so when the individual K and Cl ions are broken off, they are surrounded by water molecules and are said to be hydrated. K is a cation, so the water molecules orient themselves so that the more electronegative O atom is pointing towards the ion. For chloride, it’s just the opposite. The partially positive areas of the water molecule, the H atoms, orient themselves so they are pointing towards the chloride anion.

Soap is an everyday necessity. It’s common knowledge that soap is able to clean the grease off of a dish But how does it remove those oils? We discussed this question in class. Soaps are fatty acid chains with hydrocarbon chains. It is a polar molecule, and the hydrophilic part of the chain interacts with water through ion-dipole interactions and hydrogen bonding. The hydrophobic parts curl up into themselves. The chains are attached by dispersion forces and form a spherical surface, which attract the oils and fats on a surface and store them within a sphere.

We also held a brief discussion on why humans aren’t just a mass of liquid or gas by discussing the composition of cell membranes. Cell membranes are made up of phospholipids, a form of fats. These phospholipids form the lipid bilayer, and are composed of a hydrophilic head and hydrophobic tails. Only water and gases can easily pass through this membrane, and large molecules and small polar molecules cannot without the assistance of proteins.

On Tuesday, we whiteboarded what was in the bubbles that come out of boiling water. The bubbles contain water vapor, which is water in its gaseous form. We also went over the IMF and Solids POGIL.

On Friday, we performed a minilab where we were given mystery substances, and based on their properties, we were to decipher which substance it was. The first part of the lab involved testing the electrical conductance of given substances (water, ethanol, acetone, nonane, sodium chloride, steel, and sucrose). We used conductance testers and immersed them in the liquid states of the substances, and also tested the conductance of the dissolved and solid states of the substances. For the substances that weren’t able to be simulated in a certain state (such as molten steel) , we used a QR code to view videos that tested the conductance of these substances. We found that the ionic substance NaCl was an insulator in its solid state, but became a good conductor of electricity when it was dissolved in water.

The second part of the minilab involved observing the properties of different substances and trying to name them. We swirled the liquids to test their viscosity. Glycerin was quite easy to find; it was easily the most viscous due to its three hydroxyl groups. We also tested the evaporation rate and surface tension of the substances, and tried to see if one substance was soluble in another. Acetone evaporated extremely quickly due to its weak intermolecular London dispersion forces.

This lab was very helpful in observing how a molecule’s composition affects its properties. It all depends on the intermolecular forces. The stronger the intermolecular force, the slower a substance will evaporate and the more viscous it will be.

Blog Entry 11/3/13

This week in AP Chemistry, we started off by pondering the effects of the charge and size of an ion on melting and boiling points. We were given a question on the projector screen along with a few choices. Without any discussion with our tablemates, we voted on the choice we found most correct. It was quite similar to the Socrative website we used earlier in the tri. After voting individually, we discussed the question with our tablemates and came up with an answer we all agreed on. Many times, there was a wide variety of answer preference. Other times, however, there was dispute between two choices. I particularly remember having a very engaging discussion about one of the questions, and our table ended up being split in half on what answer we thought was correct. After our table discussion, we saw the correct answer and discussed why the other choices would not have been correct. Coulomb’s law was especially important when thinking of the solution.

The ConcepTest was quite helpful for me, and I hope we do more in the future. The discussions I had with my tablemates really helped me view different ways of approaching a problem, and based on their logic, I often changed my answer when I felt their explanation was more reasonable.

On Tuesday, we started a new POGIL on intermolecular forces. My experience in Latin class already gave me some prerequisite knowledge on the subject, the Latin word “inter” means between, so I assumed that intermolecular forces were forces between molecules. “Intra” means within, and those are the forces within a molecule. We had dealt mostly with intramolecular forces before this week during our covalent bonding unit.

In the middle of the week, we explored why ice is slippery. We all know that it’s difficult to walk on an icy surface without our feet slipping out from underneath us, but we were baffled when asked why this is so. We explored the question at the molecular level by using models of water molecules and simulating the structures of ice and water. When we created the molecular structure of ice, which is a ring-like structure filled with many gaps, it took up a lot of space. This explains why when water is frozen, it expands in the container it is in.

We used our hands to represent the pressure of an ice skater’s blade on the surface of ice. When we pushed down on the ice structure with our hands, the structure collapsed and assumed the structure of water again. Basically, when a skater’s blade makes contact with the ice, the molecules directly under the blade turn into water and cause the ice to be slippery.

Due to catching the nasty cold that has been going around the school, I was unable to attend class on Thursday and Friday. However, I heard we whiteboarded much of the intermolecular forces POGIL and I also stayed caught up by watching the lectures and completing the lecture quizzes. We discussed liquids and solids, and also further discussed intermolecular forces, the strongest of which is hydrogen bonding.

Week 7 Blog Entry

This week in AP Chemistry, we started off the week by going over some more review problems in preparation for our covalent bonding unit test. The night before, we were to complete a task chain which helped us review for our test. When I answered a question wrong—which happened quite often—there was valuable feedback which I added to my notes. On Tuesday, we took our test and I was actually quite happy with my results, although my most prominent emotion was relief.

Wednesday was perhaps the most important day of all, however—it was Mole Day! Mole Day (10/23) was a great time to relax in AP Chem and enjoy some mole cookies and hot chocolate. We had to crawl into class through the mole cave, a not-so-easy task with an unreasonably heavy backpack to carry. We listened to a mole day song, a tune that ended up getting stuck in my head for the rest of the day. We also received an essay assignment discussing the chemistry of paintballs. Truthfully, I have always wanted to go paintballing but I’m hesitant due to the pain of an exploding paintball on skin. The article discussed some familiar concepts such as polarity and hydrogen bonding, and also introduced some new ones such as solubility.

On Thursday, we took a grueling AP Chem pretest and filled in the multiple-choice answer we thought was best. I flipped through the packet of test questions and answered the ones I knew first—these usually involved VSEPR and stoichiometry. However, I would say that 90% of the test contained questions of which I had no idea how to answer. It gave me a good idea on what the AP Chem test at the end of the year will be like. Truthfully, I ended up guessing on most of the answers but I was pleasantly surprised when found out I scored around 40% (I was expecting something in the 10-20% range)! That just goes to show that my blind guessing skills can’t be all bad.

On Friday, our class began our new unit on metals and ionic bonding. We completed a POGIL on ionic bonding and started a new metals POGIL. We explored the relationship between ion size and melting point, and determined the most common ion of an element. Metals were usually cations, and nonmetals were anions. Dr. J displayed how the melting point increased along with the energy by using a sodium model with magnets. It took force to pull the atoms apart, and there was energy when the atoms snapped back together.

This weekend, we also completed a lecture quiz on metals. The lecture involved taking an excerpt from a movie that went into depth on the process of making an alloy for a bell. The video was actually very interesting to me because at one point, a sample of brass was analyzed in a machine and superimposed on a computer. The actual atoms could be seen! That was really neat for me; it makes me wonder if eventually, we’ll be able to capture the exact shape of an atom so we won’t need to rely completely on VSEPR theory.

Week 6 Blog Entry

This week in AP Chem, we began compiling a table of the thirteen different molecules we used back in our first VSEPR POGIL. We used software called WebMO. This is a neat computer program that lets you create any molecule and then submit it as a job, where it can then be optimized for different forms. For example, we used the geometry and molecular orbital optimization. From this software we could record the bond angles of the molecule, find and display the dipole moment, and also create electrostatic potential models that showed the polarity of the molecule. We then took all of this information and compiled it in a table, which we then used to create a full lab report on each of the 13 molecules.

I was kind of confused at first when making the molecules; I had to scrutinize the instructional packet in order to reassure myself that I was performing the steps correctly. However, after two or three molecules, I had gotten the hang of creating new molecules and soon enough, I whipped out nearly ten molecules by myself (molecular and geometry optimization) within the first period! It became quite methodical for me, and I was really impressed by the growing ease with which I could identify the molecular shapes. I remember looking at the molecular optimization of BF₃ and unconsciously identifying it as a trigonal planar molecule. I was quite surprised how fast I arrived to that conclusion, when just a week earlier I was struggling with the VSEPR balloon molecules and scratching my head over how to determine the shape and bond angles. The WebMO models really helped me gain a lot of practice on determine VSEPR shape.

On day two of model making, we optimized the electrostatic potential models for each molecule. This was incredibly helpful for clarifying the concept of polarity for me. When completing the polarity lecture quiz, I really couldn’t wrap my head around the concept of how electrons would be more dense in certain parts of the molecule. The electrostatic maps showed the electron density through the use of color, where the more red areas would represent more electrons and the blue areas would signify fewer electrons. In every case, the more electronegative atoms in the molecule were surrounded by red. This is because these atoms attract more electrons, resulting in a more electron-dense area.

I could also see why some molecules were overall nonpolar even if they had polar bonds. I could visualize the surrounding atoms as horses pulling outwards from the central atom. If they were symmetrically spaced from each other about the central atom and pulled with the same strength (magnitude), then they would all cancel out and result in a net dipole moment of 0.

At the end of the week, we did some reviewing of our upcoming test. We discussed polarity and whiteboarded some questions from Lectures 9 & 10. As usual, the whiteboarding really helped me gain some extra tips on how to go about solving a chemistry problem. Each of my classmates has a different way of approaching a problem, and that became visible when each group demonstrated their answer for the whiteboard questions. Whenever a group did not correctly answer a question, it was very helpful when Dr. Finnan went through the reason why it was incorrect because I would often make the same mistake the group did.

Overall, I feel much more confident in my knowledge of Lewis structures and VSEPR models. I have become stronger in polarity, but I still have some reviewing to do to feel completely sure of myself. I also have some touching up to do on concepts like sigma and pi bonding, and bond orders/lengths.