9/23/13-9/27/13

This week in AP Chemistry I took the second exam of the school year. The test covered stoichiometry, which I have been learning since the beginning of the year, and was held on Wednesday. To prepare for the test, we reviewed many of the concepts of stoichiometry in class using socrative.com. Additionally, to prepare for the test I watched a lecture on no-calculator math and completed three no-calculator math hotpots. I used the hot potato quizzes to study, doing each quiz two times. After starting to do the hotpots I became worried about the exam. I was able to complete the hotpots with most of the correct answers, but it was taking me too much time. Upon looking at a problem, it would take me a while to delineate what calculations to use to find the solution. However, I resolved this problem by simply practicing the quizzes again and by figuring out the shortest way to achieve the correct solution. I felt comfortable with the majority of the concepts of stoichiometry, so I primarily studied using the no-calculator math quizzes. On the test I got a B on the multiple choice portion. I have not seen the corrected copy of my test, however, I assume that I lost points on the questions where I had to chose an appropriate diagram of a given reaction. These were the problems on the hot potato quizzes that I had the most trouble with. On the free response I got a perfect score. Despite the fact that I got an 86 on the multiple choice, overall, I was satisfied with my performance on the stoichiometry exam. I was anticipating getting a far worse score prior to taking the exam. Nevertheless, I still would like to come in after school to see what problems I got incorrect.

After the test on Wednesday, we moved on to learning about Lewis dot diagrams. Lewis dot diagrams are renderings of substances that also show the number of valence electrons per atom and covalent bonds in the substance. To make one of the diagrams, you first write the element symbols. Then, you draw dots around the elements where there are valence electrons and dashes where there are covalent bonds. The total number of valence electrons in the substance should be accurately represented in the diagram. The diagrams should also follow the octet rule (atoms tend to have eight electrons in their outer shell). Sometimes, Lewis dot diagrams do not include the dots that represent valence electrons. However, for AP Chemistry it is always necessary to include dots in the diagrams.

Examples of Lewis Dot Diagrams. Valence electrons are represented by dots, while covalent bonds are represented by dashes.

To help learn about the Lewis dot diagrams, I completed a POGIL in class with my group and watched two lectures on the diagrams at home this weekend. I also found this website which offers a good summary on how to properly draw the diagrams. At first, I was unclear as to how to determine how many bonds to place in the Lewis dot structure, but the lecture helped to clear this up for me. Other than that, I believe I understood the Lewis dot diagrams well.

9/16/13-9/20/13

This week in AP Chemistry, I continued to learn about stoichiometry. However, this week it became more complicated as limiting reactants, excess reactants, and yield were added into the mix. To help learn the concept of limiting reactants, I completed several worksheets, including Stoichiometry 6 & 7. I also viewed a lecture on limiting reactants. Limiting reactants are reactants that limit the amount of substance that is produced in a chemical reaction. To determine the limiting reactant, you calculate the amount of product produced by each reactant. Whichever reactant produces the least amount of product is the the limiting reactant.

An example of how to find the limiting reactant from the worksheet Stoichiometry 6. 153.5 g of carbon monoxide produces in 175.6 g of methanol. 24.50 g of hydrogen produces 194.0 g of methanol. Since carbon monoxide produces a lesser amount of methanol, it is the limiting reactant.  

I found this concept straightforward and easy to understand. The concept behind excess reactants is similar. When the limiting reactant is used up there is excess of the other reactant. This is the excess reactant. Initially, I did not fully understand how to determine the amount excess reactant. However, after searching online, I found this website which helped me to understand the how to calculate excess reactant. To help learn the concept of excess reactants, I completed the Reaction Particle Diagrams worksheet in class.

I found the concept of yield straightforward as well. There are two kinds of yield: theoretical yield and actual yield. Theoretical yield is the calculated maximum amount of product possible from a given amount of reactant. Actual yield is the measured amount of product experimentally produced from a given amount of reactant. Percentage yield can be determined by the following equation:

In order to understand the concept of yield, I had to complete the Stoichiometry 8 worksheet, which is comprised of problems in which one must calculate the percentage yield of reactions.

Additionally, this week I was introduced to the concept of empirical formulas. An empirical formula is a formula that represents the simplest ratio among the elements of a compound. (for example, the empirical formula of glucose, C6H12O6 is CH2O). Ionic compounds are always empirical formulas. Molecular formulas are sometimes empirical formulas. I was required to complete the Empirical Formulas 1 worksheet, in which I had to determine the empirical formula of compounds when given the mass composition of each compound.

I found in class this week that we completed many of the stoichiometry worksheets by having different groups writing the answers on whiteboards. While this is one way to learn the concepts, I found myself mindlessly copying down the answers. I think that I learn more effectively when there is a lecture quiz that offers guidance on the worksheet and I must complete most of the worksheet on my own.

With that being said, I feel as though I have a relatively strong understanding of all of these concepts. However, at the same time I think that some more practice would certainly not hurt. I will review the worksheets I did this week before the test this upcoming week.

9/9/13-9/13/13

This week in AP chemistry, I learned more about the mole and was introduced to the concept of molarity. A mole of anything equals 6.022 x 10^23 of that thing (For example, a mole of atoms is 6.022 x 10^23 atoms, a mole of molecules is 6.022 x 10^23 molecules, etc. There could even be a mole of moles, 6.022 x 10^23 moles). The mole is not a measure of mass, but is rather a quantity. The mole's mass depends on the mass of the substance. The mass of a mole of a substance equals the atomic mass of the substance, but in grams instead of atomic mass units. For example, an atom of Hydrogen has a mass of 1.01 amu while a mole of Hydrogen has a mass of 1.01 g. To help learn the concept I had to complete a lecture quiz on the mole as well as the Stoichiometry 1 & 2 worksheet. Additionally, we worked on an in class worksheet that included calculations involving the mole.

The Stoichiometry 1 & 2 tied into another concept that I learned this week, dimensional analysis. Dimensional analysis is a method of calculating that involves units. By using dimensional analysis, you can make sure that you are doing the correct kind of calculations and ensure that you are ending with the correct unit. In addition to the Stoichiometry 1 & 2 worksheet, I also had to complete a lecture quiz on dimensional analysis.

In addition to the mole, I learned about molarity. As one can tell from the name, molarity is related to the mole. Molarity (M) equals the moles of the solute divided by the volume of the solution in liters. It is one way to measure the concentration of a solution. Molarity was a central part of a lab that I completed over the course of the latter half of the week. In the lab, I measured absorbance of solutions composed of stock solution (water with blue #1 dye) and water, as well as the absorbance of blue Gatorade. I accomplished this with a partner using a device called a Spectrophotometer. As a class, we calculated the concentration of the solutions using the equation M1 x V1 = M2 x V2. I was initially unclear as to how the concentration was calculated. However, as I finished up the lab this weekend I figured out how this was accomplished. Additionally, I was not entirely sure I did some of the post-lab questions correctly. I struggled with the question that asked you to determine the molar concentration of the blue #1 in the sports drink. I know that we covered how to do that using Beer's law in class, but I still do not fully understand Beer's law. From the notes I took in class I was able to calculate the concentration of these substances, but I am unsure if my calculations are correct. This also made me unsure of the subsequent question (which read: "Determine the mass of the blue dye #1 found in 500 mL of the drink.") I understood how to calculate this because we went over it in class of Friday, however because I was unsure of my calculations in the previous question I was unsure my solution for this question was correct. The numbers I got seemed to be larger than the ones that we calculated in class. I also feel I did a insufficient job on the lab. I think my procedure was not in depth enough and in general I did a sloppy job in formatting the entire lab. That is certainly something I need to improve on next time.

Of the concepts that we covered this week I feel as though I have a sufficient understanding of the mole and dimensional analysis. I don't have an as strong understanding of Molarity and I think that I need to go over that more. With that being said, this week I got a better understanding of the mole and I have learned to think of it more as a quantity rather than a unit with mass.