AP Chemistry Stoichiometry and Reactions Lessons

The first unit in my sequence for AP Chemistry covers stoichiometry and reactions. The new AP Chemistry Course and Exam Description has identified Learning Objectives which need to be taught and practiced to ensure students perform well on the AP Chemistry Exam. I will identify and describe activities I use to teach students some of the Learning Objectives that I tie into this unit.

First, let me give a background of my classes, The majority of my students take a year of state level chemistry before AP Chemistry and my district requires me to give a summer assignment. I have decided to flip my classroom by showing videos at home.

Therefore, the summer assignment I give is a series of videos that I have made which reinforce stoichiometric calculations, naming simple compounds, and various types of reactions from the students’ first year of chemistry. So at the beginning of my year, my AP chemistry students get right into higher level stoichiometry calculations. I will outline and explain some of my activities. (I wasn’t always this lucky. For three years I taught a first year AP chemistry class to freshmen in high school in New York State where we start school after Labor Day. I understand the struggle of having limited time in class to complete the coursework. So pick and choose what you think will help your students the most.)

In the first week I have my students begin to memorize a selected sample of common ions and rules for naming similar ions (polyatomic ions ending in “-ite” have one less oxygen atom, “hypo- -ite” polyatomic ions have two less oxygen atoms, and “per- - ates” have one more oxygen atom than the “-ate” polyatomic ions; all with the same charge). Though some teachers find memorization archaic and, in this case, unnecessary, I disagree. I don’t need my students wasting time and working memory wondering what the carbonate ion is while performing a lab experiment or while taking an exam. Research shows that by memorizing certain facts and procedures, students can handle more cognitive demands and make more connections between concepts at a faster rate. By the end of the week the students need to be able to name compounds using the stock system and the following polyatomic ions using only the AP Chemistry periodic table. We also memorize the following solubility rules and identify if the compounds they have named are soluble or insoluble according to these selected solubility rules.

Polyatomic Ions: C₂H₃O₂^- acetate, NO₃^- nitrate, CO₃^2- carbonate, SO₄^2- sulfate, PO₄^3- phosphate.

Solubility Rules: Compounds containing group 1 ions, acetate, nitrate, and ammonium ions are always soluble. All other compounds may be considered only moderately, slightly, or not soluble.

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Figure 1: Representing particulate models of substances in beakers

While reviewing nomenclature, we also begin calculations using dimensional analysis. We start with simple one step mole calculations and progress toward multi step conversions (grams of one substance to atoms of another given a reaction). Each day we add on another fraction in dimensional analysis and practice at least one long problem until we have mastery. In year one, we discuss and calculate molarity, so I do a brief review and include molarity in my AP stoichiometry practice as well. We then move on to discussing limiting reactants. We begin limiting reactants with particle models representing what is happening on a molecular level (see figure 1). It is important for students to be able to visualize how substances run out at different rates depending on initial quantities and the reaction. After they have a visual, we construct our own particle diagrams (see figure 2) to show how the reactions progress. If students are struggling, they work with various colored Legos to identify how colored Legos run out similar to reactants. Finally, we calculate limiting reactant, excess reactant, and left over quantities using mole, mass, volume, and molecular quantities. Some teachers have not been convinced that limiting reactant questions are on the AP Chemistry Exam. However, the 2018 AP Chemistry Exam question 1c assessed limiting reactants which required Molar calculations. It is a topic that needs to be taught and practiced.

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Figure 2: Representation of changes at particulate level during a reaction

I start working with net ionic equations early in the year so my students are very comfortable with the equations by the time the exam comes. In the 2019 AP Chemistry Free Response, students were asked to interpret numerous reactions and construct two equations on their own. To aid this difficult objective, I once again have my students draw particle diagrams (see figure 3) to show before and after visuals of the species. They are required to show the particles in the correct phase, with the correct ratio of particles, and a key for their diagrams. This really helps tie together the ideas of what a net ionic reaction is, how the solids form, and review solubility rules.

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Figure 3: Students practice particulate level drawings

I love to use POGIL materials in class. They can be extremely effective if facilitated in the proper way (utilizing student roles, in teams of 3-4 students, with content and process skill goals, etc). If you are unsure of how to use POGIL activities effectively, there are numerous POGIL workshops you could attend, or you can search for and read blogs about POGIL on ChemEd X. The POGIL activities I found most useful include “Mole Ratios” from the book “POGIL Activities for High School Chemistry” for my struggling learners and “Combustion Analysis” from the book “POGIL Activities for AP Chemistry” for all learners. The combustion style questions can be difficult. It is nice to ease students into these types of calculations by including only compounds with carbon and hydrogen atoms at first and then move on to compounds that may have three atoms. I avoid questions with any more than three atoms (unless a student wants a challenge!). Calculations of combustion analysis help practice stoichiometry and information processing.

Some teachers claim that the AP Chemistry Exam can be mastered without having the students perform any labs in class. Not only would that class sound incredibly boring, I think that those students are missing the hands on experience, critical thinking, and problem solving skills that the students should be developing in high school. We are trying to make the next generation of scientists that may move on to be doctors and researchers. Therefore, I find it not only necessary to provide the lab experience to my students for their long term goals, but also I require lab experiences because many labs are mentioned on the AP Chemistry Exam and I would imagine it is easier to answer those questions if the students can visualize the actual lab because they have tried a version of that lab. In this unit I assigned three major labs. My students have worked with hydrates before so we perform a lab experiment where I give them an unknown hydrated salt and they have to create and describe their own procedure and then conduct the experiment to determine the formula of the hydrate. Another idea for hydrate labs is to create a challenge lab in which the students have to get as close to the actual answer as possible, with closest teams winning a “valuable” prize of your choice. In our second lab I have students determine the amount of calcium in a calcium supplement (I have also seen this done with antacids). Students use their solubility rules to dissolve the calcium tablet, extract the calcium ions, and then precipitate the calcium ions. They use stoichiometric calculations to determine the amount and percent of calcium in their supplement. Both labs end with a post lab handout that helps students practice the calculations and makes them think critically about the process and how error could affect their results. I find this to be difficult for my students to do but also very important for the AP Chemistry Exam. You will find an example of post lab questions I have used for the hydrate lab in the Supporting Information below. At the end of the unit we complete the “Baking Soda Challenge” to determine the correct decomposition reaction of baking soda. Challenges help engage students in scientific reasoning and justifications (see figure 4).

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Figure 4: Baking Soda Challenge

Since I have not done atomic structure at this point, I do not use the “Personal Progress Checks” yet from the my AP website, but I do give an exam to assess their skills. The exam uses modified old AP exam questions. If you would like some of my materials, you can access them on my personal website www.chemisme.com. What do you have your students do in your Stoichiometry unit? Please log into your ChemEd X account and share by commenting at the conclusion of this post! We would love to see your ideas and sequencing!