Elementary Reactions

Learning Objective 5.4.A Represent an elementary reaction as a rate law expression using stoichiometry.

Quick Notes

An elementary reaction is a single-step process where reactants form products in one collision event.
For elementary steps only, the rate law can be written directly from the stoichiometric coefficients.
Common types:
- Unimolecular: A → products rate = k[A]
- Bimolecular: A + B → products rate = k[A][B]
- Termolecular reactions (3 particles collide) are rare due to low probability of simultaneous collisions.

Full Notes

What is an Elementary Reaction?

An elementary reaction is a simple, one-step process in a reaction mechanism. It shows a single collision between particles that leads directly to product formation.

Since the step is not a combination of smaller steps, we assume:

The reaction occurs exactly as written.
The molecules in the equation are the ones physically colliding.

Rate Law from Stoichiometry (for Elementary Steps Only)

For elementary reactions, the rate law can be written directly using the stoichiometric coefficients from the balanced equation.

Matt’s exam tip

If you’re told a step is elementary, then you’re allowed to use the coefficients in the rate law. But never do this for an overall reaction — always rely on experimental data unless told otherwise.

There are three types of elementary reactions - unimolecular, bimolecular and termolecular.

Unimolecular Reaction

AP Chemistry Unimolecular elementary step: one particle changes to products.

A → B
One molecule decomposes
Rate = k[A]
First order

Bimolecular Reaction

AP Chemistry Bimolecular elementary step: two particles collide to form product.

A + B → C
Two molecules collide
Rate = k[A][B]
Second order (1st order in A, 1st in B)

Termolecular Reaction (rare)

AP Chemistry Termolecular elementary step: three particles must collide simultaneously.

A + B + C → D
Three particles collide simultaneously
Rate = k[A][B][C]
Third order overall

Why Are Termolecular Reactions Rare?

To have three particles collide at the same time in the correct orientation and with the required energy (see Collision Model) is highly unlikely. That’s why:

Most reactions proceed through a series of bimolecular steps.
Reaction mechanisms break complex reactions into a sequence of simpler elementary reactions.

Important Distinction

This direct use of stoichiometry only applies to elementary steps, not overall reactions. For overall reactions, the rate law must be determined experimentally – you cannot assume it from stoichiometry.

Summary

Elementary reactions are one-step processes where the stoichiometry directly gives the rate law. Unimolecular and bimolecular elementary steps are common and predictable. Termolecular steps are theoretically possible but rare due to the improbability of three-particle collisions. Understanding these fundamentals is key to interpreting reaction mechanisms and constructing correct rate laws.