Week 3

This past week in AP Chemistry, we began with some extended applications of stoichiometry to real world situations. We were asked to calculate the empirical formula of a compound consisting of only carbon, hydrogen and oxygen with a known mass, given only that it had reacted with an unknown amount of oxygen gas in a combustion reaction forming certain masses of carbon dioxide and water. Up to this point we had only learned how to calculate empirical formulas given the mass percentage of each element directly, through the use of molar masses and mole ratios.

This new challenge required no additional knowledge of stoichiometry, but significantly more thought and work. One had to retrieve previous information from other units such as the law of chemical equations stating that the number of atoms of each element as well as the total mass must be equivalent on the reactant and product sides of every balanced chemical reaction. Using what we had learned in this lesson, we could solve for the mass of each of the three elements through the mass percentages of the elements in the products. Subtracting the total mass of the reactants by the given compound gave the mass of the oxygen gas that had been burned. You were left with the mass of each element in the compound being examined; the same setup as all other problems this unit.

Challenging problems such as these are greatly beneficial to our learning process in chemistry. They help students connect multiple ideas, often from different content areas together to answer a more complex problem than those they have encountered in the past. These are the best way to assess and strengthen a students true understanding of the material at hand because they must pull the information together by feel, navigating on their own, rather than simply following a step-by-step process they were given for one specific set of problems.

After being assessed on stoichiometry, we moved on to the Lewis structures of atoms (as depicted in the images below). Developed by scientist G. N. Lewis, this notation is commonly used to show the structure of molecules through the covalent bonds they form. Covalent bonds form between atoms that share electrons in order to complete their octet and become more stable like the noble gases nearest them, aside from hydrogen which only forms a duet with a single bonding pair. The octet rule is mainly a result of the eight electron spaces in their valence electron shell. The valence electrons of any atom are those in the s and p orbitals of their highest energy level. These particular electrons are the only reason that the nonmetal elements form compounds, because nonmetal elements will not form positive ions (aside from ammonium). For the best practice drawing Lewis structures, see the link below.

Lewis structure practice:
http://www.chem.purdue.edu/vsepr/practice.html

Lewis structure of HCN (left) and other common
molecules (right)

Week 2

This past week in AP Chemistry we focused on stoichiometry, the area of chemistry dealing with the ratios of atoms in chemical reactions, and the many applications it has in the real world. To help the class better understand the topic of stoichiometry, we "white-boarded" problems from various worksheets in class. This consists of each table group being assigned a single problem to work through quickly, but thoroughly and presenting their work to the class.

This approach to students doing practice problems offers multiple advantages over the standard way of having each student work through every problem on their own. First, a significant amount of time is saved by doing this simply because each student does less work. One might jump to the conclusion that saving this time would only sacrifice the students' understanding of the material, but this is not the case. This is because of the presentation of work. While students present their work, others are encouraged to double-check their classmates work, which requires thinking through the process of the problem and getting the required practice, while excluding the manual calculation of numbers which students gain little from. Working in groups also increases the efficiency of getting students through parts they may get stuck on or make a mistake. Instead of spending time trying to remember something that is not there, or looking for a mistake, students are quickly aided by their peers to get them through while reminding them what area they may need to review. Lastly, students have an added positive motivation to focus on their own problem as to not make a mistake for the rest of class to see.

This week has also changed the way that I think about the numbers and units given to me in any particular problem. Previously, I had considered all given information to be finite, single pieces of information that can help to find other information. I now know that given information used to find the solution to a problem is part of a large web of values relating to the problem that can always be traversed with the right conversion factors and aided by units (see figure below)(see URL below for stoichiometry summary and practice problems). The idea that units should be used as a guide was also reinforced as we did many multiple-step problems in which you could easily get lost in.

I have observed many connections between the material we covered during this last week and the weeks before that. During the summer we learned how to verify that a chemical equation is balanced and change coefficients if necessary to follow the law of conservation of mass and the idea that atoms themselves are not changes in chemical reactions. These same principles are present in the stoichiometry we covered this week. Without following the law of conservation of mass, many of our calculations regarding limiting and excess reagents would be impossible.

Stoichiometry summary and practice:
http://s-owl.cengage.com/ebooks/vining_owlbook_prototype/ebook/ch3/Sect3-3-c.html

The general molar web connecting each value

Week 1

This past week in AP Chemistry we learned about molarity, or concentration of solutions, being the number of moles of a substance in a liter of given solution. This number can be used to calculate the mass of solute in a solution. Given the equation: M1(V1)=M2(V2), we learned how to calculate either the molarity (M) or volume (V) of a solution before (1) or after (2) dilution with the solvent, given the other three values. We used this equation when calculating the concentration of Blue #1 in each solution we made of a known ratio of stock solution and distilled water. Knowing this new concept of molarity and how to calculate it allowed us to develop a linear regression equation that related molar concentration to the absorbance of light by the solution. Absorbance is a measure of the percent of light of a particular wavelength impeded by the solution, related to transmittance. Transmittance is the percent of light of a particular wavelength allowed through a solution.

We were detailed on all of the aspects involved in a lab experiment. I learned that you must always follow the step-by-step process in order to succeed in the testing. This includes completing pre-lab and thinking through all aspects of the experiment before arriving in class on the specified day. During the lab, it was very apparent who had prepared before class and who had not, as seen by the many puzzled faces of those who watched others starting before beginning their own experiments. You must also stay organized and follow the rubric formatting in order for your lab to look professional and presentable. Neglecting this seems to run a higher risk of error. Your responses to the post lab questions

We also explored the ways in which you can identify the molar concentration of a solute in distilled water if there is only one solute, through the application of light. This requires the use of a colorimeter (see figure below) which can accurately measure the absorbance of light of a particular wavelength by the solution being tested. With this data you could find many important details about the solution, which can have many practical details outside of this controlled school experiment. These applications could include the measurement of dissolved minerals in lake or ocean waters of certain areas in which fluctuations could result in catastrophic changes to the life in that area.

We came to know most of the information in class from guides this week. To understand both the principles of light transmittance and absorbance and the instructions for the colorimeter we read through the Vernier Colorimeter manual, which detailed these by describing the units of each term, the use for each and how they relate to each other physically and mathematically. In class we also had discussions on how to calculate the concentration of the stock solution using beer's law and known absorbance and molar extinction coefficient. These calculations were carried throughout the entire experiment and were therefor very important to the success of the experiment as a whole.

Here is a reference for the use of notebooks throughout experimentation:
http://www.ruf.rice.edu/~bioslabs/tools/notebook/notebook.html

This is an overview of the mathematical relationships for light transmittance in solutions:
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm

Vernier Colorimeter for measuring absorbance (Left) and standard lab notebooks (Right)