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Elementary reactions

For most reactions you cannot read the rate law from the equation. The exception is an elementary step — a single molecular event — where the reactants and their counts do give the rate law directly.

§1

Single molecular events.

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An elementary reaction is a single molecular event — the reaction happens in one step, exactly as written. Its molecularity is the number of particles that come together: unimolecular (one), bimolecular (two colliding), or the rare termolecular (three).

For an elementary step only, the rate law follows directly from the molecularity: the exponents are the coefficients of the reactants in that step. A bimolecular step A + B → products has rate = k[A][B].

This is the exception, not the rule. An overall equation usually represents several steps, so its coefficients are not the rate-law orders. Only a genuine elementary step lets you write the rate law from the equation.

UNIT 5 TOPIC 5.4 • ELEMENTARY REACTIONS ELEMENTARY STEPS An elementary reaction occurs in a single step, so its molecularity gives its rate law directly. UNIMOLECULAR BIMOLECULAR TERMOLECULAR A A → products rate = k[A] one molecule reacts on its own A B A + B → products rate = k[A][B] or 2A → products, rate = k[A]2 A B C new A + B + C → products rate = k[A][B][C] three-body collision — rare CED ANCHOR · reading a rate law from molecularity You may read the rate law directly from the molecularity ONLY for an elementary step. NEVER do this for an overall (non-elementary) reaction — its rate law must be found by experiment. Molecularity (uni / bi / ter) counts the particles that collide in that single step. AP Chemistry · Unit 5 · Kinetics
Fig. 5.4.1 An elementary reaction is a single molecular event, classified by molecularity: unimolecular (one particle), bimolecular (two colliding), or the rare termolecular (three). For an elementary step only, the rate law comes directly from the molecularity.
§2

Working with elementary steps.

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Confirm it is elementary, then read the rate law from molecularity.

  1. Confirm the step is elementary. Only a single-step molecular event qualifies; an overall reaction usually is not elementary.
  2. Count the molecularity. How many particles collide or react in the step: one, two, or three.
  3. Write the rate law from the reactants. For an elementary step, the reactant coefficients are the orders.
  4. Do not do this for overall equations. For a multistep overall reaction, orders must come from experiment, not the coefficients.
§3

The pieces you'll meet.

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Molecularity governs an elementary step.

elementary
Elementary reaction
A single molecular event; one step as written.
molecularity
Molecularity
Number of particles in the step (1, 2, or 3).
unimolecular
Unimolecular
One particle: rate = k[A].
bimolecular
Bimolecular
Two particles: rate = k[A][B] or k[A]².
termolecular
Termolecular
Three particles; rare, because triple collisions are unlikely.
overall
Overall reaction
Usually multistep; its coefficients are not the orders.
§4

Worked example: rate law of an elementary step.

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Elementary step. 2NO₂ → NO₃ + NO, known to be a single bimolecular event.

Molecularity. Two NO₂ molecules collide, so it is bimolecular.

Rate law. Because it is elementary, the rate law is rate = k[NO₂]² — the coefficient 2 becomes the order.

Caution. This works only because the step is elementary. If 2NO₂ → NO₃ + NO were merely an overall equation, we could not assume the order is 2; that would require experimental data.

§5

Mistakes that cost real points.

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Pitfall · 01

"You can write the rate law from any balanced equation's coefficients."

Only for an elementary step do the coefficients give the orders. Most balanced equations are overall reactions made of several steps, so their coefficients are not the rate-law exponents. Reading orders off an overall equation is a frequent error.

Fix. Use coefficients as orders only when the step is explicitly elementary; otherwise get orders from experiment.

Pitfall · 02

"Molecularity can be a fraction or come from an overall equation."

Molecularity is a whole-number count of particles in a single elementary step (1, 2, or 3). It is not a fraction, and it applies only to elementary steps, not to overall reactions.

Fix. Treat molecularity as an integer particle count for an elementary step; do not assign it to an overall reaction.

Pitfall · 03

"Termolecular steps are common."

Termolecular (three-particle) steps are rare, because three particles colliding simultaneously with the right energy and orientation is very unlikely. Most elementary steps are unimolecular or bimolecular.

Fix. Expect unimolecular or bimolecular steps; treat termolecular steps as uncommon.

§6

Skill Check.

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