5.4 Elementary Reactions

Elementary Reactions

Particles collide across uni-, bi-, and rare termolecular steps. Elementary rate laws come from coefficients, while overall exponents must be measured.

MolecularityRate = k[A]ᵃ[B]ᵇTermolecular Rare4 Steps
Topic 5.4

Elementary Reactions

Represent an elementary reaction as a rate law expression using stoichiometry.

An elementary reaction is a single molecular event — one collision, one rearrangement — not a summary of several steps. For an elementary reaction, and only for an elementary reaction, the rate law can be inferred directly from the stoichiometry of the particles participating in the collision.

Molecularity is the number of particles that must collide:

  • Unimolecular: A → products, rate = k[A]
  • Bimolecular: A + B → products, rate = k[A][B]
    or 2 A → products, rate = k[A]²
  • Termolecular: A + B + C → products, rate = k[A][B][C]

Termolecular steps are extremely rare, because three particles colliding simultaneously with the right energy and orientation is statistically improbable. If a proposed mechanism contains one, that is a red flag.

The essential distinction: for an overall reaction, orders must be determined experimentally and generally differ from the coefficients. For an elementary step, the coefficients are the orders. The reason is physical: the rate law of an elementary step describes an actual collision, so the concentration of each colliding species enters directly.

You cannot tell from a balanced equation alone whether it is elementary. The problem must tell you, or the mechanism must present it as a single step.

Key points

  • Only for an elementary step do coefficients equal orders.
  • Molecularity counts the colliding particles: uni-, bi-, or termolecular.
  • Termolecular steps are rare and should make you suspicious of a proposed mechanism.
  • You cannot assume a reaction is elementary — you must be told.

Equations

  • not on the sheetValid ONLY for an elementary step.

Common mistakes

  • Applying this rule to an overall reaction. The single most common kinetics error.
  • Forgetting the square for 2 A → products. Two A particles must collide, so rate = k[A]².
  • Proposing a termolecular step when a two-step alternative exists.

Worked example

Write the rate law for each elementary step, and state its molecularity. (a) O₃ → O₂ + O (b) O + O₃ → 2 O₂ (c) 2 NO₂ → NO₃ + NO

(a) One particle decomposes: unimolecular, rate = k[O₃]

(b) Two different particles collide: bimolecular, rate = k[O][O₃]

(c) Two identical particles collide: bimolecular, rate = k[NO₂]²

Each rate law comes straight from the coefficients — legitimate here because each is stated to be an elementary step.

Full notes for topic 5.4 →