AP Chemistry Unit 5 Study Notes

AP Chemistry 5.3: Elementary Reactions

Connect single-step reactions, molecularity, and elementary rate laws.

Aligned to Kinetics from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

Study these notes

Start with each main idea, then follow the indented explanations and worked examples. Try the next calculation before reading its answer.

Organized from the provided Unit 5 study document. Further study: Khan Academy.

Elementary Reactions

  • An elementary reaction occurs in a single molecular-level step.

  • Because an elementary reaction represents an actual collision or rearrangement event, its rate law can be determined directly from its reactant coefficients.

    • For example:
    • A → products
    • If elementary:
    • Rate = k[A]

  • For:

    • A + B → products
    • if elementary:
    • Rate = k[A][B]

  • For:

    • 2A → products
    • if elementary:
    • Rate = k[A]²
    • This is the major exception to the rule that you cannot obtain a rate law from the balanced equation.

  • For an overall reaction, you normally need experimental data.

  • For an elementary step, the coefficients can be used as rate-law exponents.

Molecularity

  • Elementary steps can also be described according to the number of reacting particles.

  • Unimolecular: one reactant particle participates.

  • Bimolecular: two reacting particles participate.

  • Reactions requiring simultaneous collisions among many particles are extremely unlikely, which is one reason complicated overall reactions usually occur through multiple simpler steps.