AP® Chemistry · Unit 4 · Topic 4.3
Representations of Reactions
A particulate drawing shows every individual molecule before and after a reaction, forcing an exact atom count that exposes leftover reactant and mistakes a balanced equation can hide.
A balanced equation like 2 H2 + O2 → 2 H_2O tells you ratios: two molecules of hydrogen react with one of oxygen to make two of water. But ratios can hide a messy reality. If you actually have 3 H2 and 1 O2, the equation alone does not show you what is left over. A particle drawing forces you to draw every single molecule, before and after, so you have to account for each atom individually. That bookkeeping catches two kinds of mistakes: atom-count errors, where a drawing shows a nitrogen appearing from nowhere, and limiting-reactant errors, where a student forgets that one reactant runs out while the other has molecules stranded with no partner left to react with.
To draw it correctly, sketch the starting molecules as separate clusters of atoms, then sketch the ending molecules the same way, using only atoms that were already there. If 3 H2 and 1 O2 start out, only 2 H2 and the 1 O2 can react completely; after the reaction you draw 2 H_2O plus 1 leftover H2 molecule, untouched, because it had no oxygen to pair with. The total count of H and O atoms must match exactly between the before and after pictures, since conservation of mass means atoms rearrange into new molecules but are never created or destroyed.
This connects directly to limiting reactant and percent yield problems later in the unit, and also previews equilibrium, where a drawing of a reaction that only goes partway shows both reactant and product molecules coexisting in the final picture rather than reactants disappearing completely.
Key terms
- Particulate drawing
- A sketch showing individual molecules of reactants and products rather than just chemical formulas.
- Conservation of mass
- The principle that atoms are rearranged in a reaction but never created or destroyed, so total atom counts stay equal.
- Limiting reactant
- The reactant that runs out first, stopping the reaction and leaving excess of the other reactant unreacted.
- Excess reactant
- The reactant left over, undrawn as changed, after the limiting reactant is fully consumed.
Watch it explained
- How it worksDraw the before and after particle pictures for 2 H2 plus O2 forming 2 H2O.
- How it worksHow does a particle drawing show which reactant runs out first?
- Common mistakeWhy isn't a particle drawing allowed to show more atoms after the reaction than before?
- Going furtherWhat does the drawing look like for a reaction that only goes part of the way to products?
Check yourself
End of topic.