Using Visual BCA Tables to Teach Limiting Reactants

A few months ago I was searching the internet, looking for a better way to teach stoichiometry to my pre-AP chemistry students. While my methods of dimensional analysis “got the job done” for most students, I would still always lose students and many lacked true understanding of what was happening in the reaction. I wanted to try something new that would promote a better chemical understanding. In my search for this elusive stoichiometry method, I came across Dena Leggett’s ChemEd X blog post entitled “Doc Save Everyone”, as well as other posts about BCA tables from Lauren Stewart, Lowell Thomson, and Larry Dukerich. These posts highlighted the method of using a “Before, Change, After” (BCA) table to organize stoichiometry problems, turning stoichiometry problems into a chemical “Sudoku puzzle”.

I agree with other bloggers that teaching BCA tables takes commitment and time. However, when you see your students solve complex limiting reactant problems without breaking a sweat, it is worth it. I have been converted to BCA tables!  I want to share my experience teaching BCA tables to my pre-AP Chemistry students for the first time. I hope you can use my experience as a springboard for new lessons in your classroom.

From reading other’s blog posts about their BCA experiences, I learned the BCA method would not be the quickest or the easiest for students to grasp as it requires a deeper understanding of concepts. I decided that I needed a “carrot” to keep my students motivated in this process. I teamed up with an engineer at a local manufacturing business (through the AACT Science Coaches program) to help me develop a real-life problem for the students to solve with stoichiometry. We decided to have students solve a rusting problem at a manufacturing plant. Students needed to develop a process that would use a strong acid to remove unwanted rust from steel parts and use a strong base to neutralize the waste acid to meet EPA guidelines. This project gave me an acid-base titration context to work with for stoichiometry and limiting reactants. I believe this real-life context kept my students motivated to learn stoichiometry and use of theBCA table, even when the work got challenging.

The acid-base context helped me formulate questions that would assess student understanding of limiting and excess reactants in chemistry. Students only needed to know basic information about acids and bases (acids have a pH < 7, bases have a pH > 7, etc.) to apply it to limiting reactants. Through BCA and acid-base titration questions, I could figure out if students were just crunching the numbers or if they were correctly thinking about the chemistry of the reaction. This is where BCA got tough. I now had a group of students that did not understand the chemistry and I could not just rely on them memorizing the dimensional analysis process to get through like before. The “change” or “C” row was the most challenging feature of BCA tables for students to grasp. I showed my students how to use proportions to figure out the numbers or how to solve for “x” or 2x” to complete this row. With practice, most students mastered it, but I still had some students struggling.

To students, much of chemistry is “invisible” as molecules are too small to see. I realized that my struggling students were not “seeing” the molecules in their head and therefore had no clue what the numbers represented in the BCA table. A few days before the test, I decided to make concrete connections between lab observations, molecules, and the math. I guided my students through the “Visual BCA Chart” worksheet (available as supporting information below the post). From numbers of moles given, we drew in the acid and base molecules in the reactant beakers and then predicted the products and leftovers (excess) in the product beaker after mixing. We went back to the BCA chart to make connections between the drawings and the numbers in the chart. After we answered the questions, we performed the reaction in lab using 1 M strong acid, 1 M strong base, and phenolphthalein indicator solution. Since the concentrations were equal and 1M, I had students use 1 mL for each mole in the chart. Through the indicator change and pH paper tests, students confirmed their answers to the problems and made connections between lab observations, BCA math, and molecules reacting. This was powerful and I had many “aha” moments. Eventually, students were able to do the rest of the problems on their own and help explain ideas (like problem #3 with a 1:2 ratio) to their peers.

My hope is you can take this idea I generated from reading ChemEd X blogs, run with it, and make it better. BCA has transformed my stoichiometry unit into one focused on understanding and real-life applications. Once my students figured out the BCA chart, limiting and excess reactants problems were a breeze. By the end of the stoichiometry unit, I felt I had fewer students overwhelmed by the chemical concepts than in previous years. I feel that next year will be even better as I plan to start the unit off with a visual BCA chart worksheet like the one I shared. Connecting lab observations, particulate diagrams and the BCA math were key in helping my students understand the chemistry of limiting reactants.

NGSS standards addressed: (HS PS 1 - 7) Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

ACKNOWLEDGEMENTS: ​

Special thanks for Science Coach: Dan Mack, Paint Process Engineer, John Deere​

AACT Science Coaches: https://teachchemistry.org/about-us/science-coaches

ChemEd X Blog Posts

Doc Saves Everyone: Applying BCA Tables To Titration Calculations: https://www.chemedx.org/blog/doc-saves-everyone-%E2%80%93-applying-bca-tables-titration-calculations

Rethinking Stoichiometry: https://www.chemedx.org/blog/rethinking-stoichiometry

Conceptual Chemistry: https://www.chemedx.org/blog/conceptual-chemistry

One Teacher's Attempt at Using BCA Table for Stoichiometry: https://www.chemedx.org/blog/one-teachers-attempt-use-bca-tables-stoichiometry