Lesson 3: 4.3 Representations of Reactions

How can we visually prove that atoms are neither created nor destroyed during a chemical process?

How do we represent a limiting reactant versus an excess reactant in a closed particulate system?

Particulate Model
Conservation of Matter
Excess Reactant
Limiting Reactant
Closed System

Learning Objective 4.3.A; Suggested Skill 3.B (Representing Data and Phenomena)

This topic translates symbolic balanced chemical equations into physical particulate models. Students must ensure that the total number of circles representing each element matches exactly before and after the reaction arrow. They must also account for any leftover excess reactant molecules that did not find a pairing partner.

Support: Provide students with physical snap-together blocks. Have them build the reactant molecules, snap them apart completely, and rebuild the product molecules to visualize that no blocks are left unaccounted for or magically created.

Extension: Provide a particulate drawing of a reaction vessel that contains a mixture of elements after a reaction has gone to completion. Ask students to deduce the initial ratio of reactants and write the balanced chemical equation based solely on the image.

Consider the chemical reaction for the synthesis of ammonia gas:

The box labeled "Reactants" below contains 3 molecules of Nitrogen gas (represented as pairs of dark circles) and 6 molecules of Hydrogen gas (represented as pairs of open circles).

Draw the contents of the container in the "Products" box after the reaction has gone to completion. Ensure your drawing satisfies the Law of Conservation of Mass.