9. How can chemical changes be controlled?

particulate level model of example of chemical change

“How can chemical changes be controlled?” is a question that involves understanding how changes in conditions affect the relative stability of the species involved in a chemical process. Control can be achieved by selecting reactants with structural features that change their energetic stability, varying the concentrations of reactants or capturing and removing products, adding substances which react with intermediates to facilitate or inhibit different mechanistic steps, changing temperature to activate chemical species,, or choosing solvents that facilitate or inhibit certain interactions. For example, controlling the replication of a virus may involve tuning conformations of a substance involved in the replication to block one pathway in the process. This chemical thinking question is often central to chemical process design and analysis activities, such as improving solar cell operation, analysis of battery efficiency, or characterizing the degradation of a dye.


The Pringles design challenge formative assessment was developed by the ACCT team (indicated by the star) and the Exploring Concentration and Airbag challenge formative assessments were designed by past ACCT cohort members.


 Pringles design challenge

The Pringles design challenge is a laboratory-based formative assessment tool for learning about how students think about controlling chemical reactions. It has been tested in high school chemistry classrooms in the US, and in university chemistry classes in the US, Mexico, and Costa Rica. The tool is available in English and Spanish.


 Exploring Concentration

The Exploring Concentration formative assessment asks students to make their “ideal” Kool-aid solution and to make variations of their recipe to explore the different variables of concentration. This formative assessment targets chemical control by allowing students to experience the processes and effects of changing concentration. This guides students to learn about chemical reactions and how to produce more or less of a product by adjusting concentration. The formative assessment focuses on introducing molarity so that students are able to have a real world example of each variable (molarity, moles, and volume) to use while doing stoichiometry in subsequent lessons.


 Airbag Challenge

In the “Airbag challenge” the students are tasked with developing a safe airbag for a car company. This formative assessment explores students’ thinking about the question “How can chemical changes be controlled?” The central concept in this challenge is the application is stoichiometry. Students are expected to use the numbers of moles of reactant consumed or product formed in a balanced chemical equation and to determine the change in the number of moles of any other reactant and product. Students need to use molar mass to convert mass of a reactant or product to moles for use in stoichiometric calculations or to convert moles from stoichiometric calculations to mass. Students use the ideal gas law equation to determine the numbers of moles in a sample of gas not at standard conditions.