4.3 Representations of Reactions

Representations of Reactions

Pick a balanced equation and see the matching particulate diagram. Build it molecule by molecule with live coefficient narration, or view the full representation.

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Topic 4.3

Representations of Reactions

Represent a given chemical reaction or physical process with a consistent particulate model.

Balanced chemical equations in their various forms can be translated into symbolic particulate representations — and back again. The AP Exam moves between three levels constantly: macroscopic (what you observe), particulate (what the atoms do), and symbolic (the equation).

Rules for a correct particulate drawing:

  • Conserve atoms. Count each element in the "before" box; the "after" box must match exactly.
  • Respect the mole ratio. If the equation is N₂ + 3 H₂ → 2 NH₃, three H₂ molecules must disappear for every N₂.
  • Show leftover excess reactant. If one reactant is in excess, the unreacted particles belong in the "after" box.
  • Be consistent with identity. Use the same color/size for the same element throughout.
  • Show the right state. Ions in solution are drawn separated; solids are drawn as ordered clusters; gases are sparse.

Reading in the other direction is just as common: given a before-and-after diagram, write the balanced equation by counting particles and reducing to the smallest whole-number ratio.

Key points

  • Atoms are conserved in every particulate drawing; molecules are not.
  • Excess reactant must still be shown after the reaction.
  • A particulate diagram can be converted to a balanced equation by counting and reducing.
  • Aqueous strong electrolytes are drawn as separated ions, not as intact formula units.

Common mistakes

  • Drawing product without leftover excess reactant. Very commonly penalized.
  • Losing or inventing atoms. Always recount before submitting.
  • Drawing dissolved salts as neutral units.

Worked example

A container initially holds 6 molecules of NO and 4 molecules of O₂. They react by 2 NO + O₂ → 2 NO₂. Describe the contents of the container after the reaction is complete.

Find the limiting reactant. The ratio required is 2 NO : 1 O₂.
6 NO would need 3 O₂ — available: 4 ✓
4 O₂ would need 8 NO — available: 6 ✗

NO is limiting.

Products and leftovers: 6 NO consumes 3 O₂ and produces 6 NO₂.

After the reaction the container holds 6 NO₂ molecules and 1 leftover O₂ molecule.

Atom check: before — 6 N, 6 + 8 = 14 O. After — 6 N, 12 + 2 = 14 O ✓

Full notes for topic 4.3 →