Chemistry · Unit 5 Kinetics · 14 min read · Updated 2026-05-11
Elementary Reactions — AP Chemistry
AP Chemistry · Unit 5 Kinetics · 14 min read
1. Definition and Key Properties of Elementary Reactions★★☆☆☆⏱ 2 min
An elementary reaction (or elementary step, when part of a larger mechanism) is a single-step reaction that occurs exactly as written, with no intermediate sub-steps between reactants and products. Unlike overall balanced reactions, which only describe the net result of multiple reaction events, elementary reactions represent individual collision events between reactant particles.
The key distinguishing feature of elementary reactions is that the stoichiometric coefficient of each reactant directly equals its reaction order in the rate law. This rule does not hold for overall reactions, making elementary steps the foundation for all multi-step reaction mechanism problems on the AP exam.
2. Molecularity of Elementary Reactions★★☆☆☆⏱ 3 min
Molecularity is defined as the number of reactant particles that collide and react in a single elementary step. Because it counts discrete particles, molecularity can only be a small positive integer: 1 (unimolecular), 2 (bimolecular), or 3 (termolecular). Termolecular steps are very rare, because the probability of three particles colliding simultaneously with correct orientation and sufficient energy is extremely low.
A common point of confusion is distinguishing molecularity from reaction order. Reaction order describes how the rate depends on concentration, and can be zero, fractional, or negative for overall reactions. Molecularity is only defined for elementary steps, and is always 1, 2, or 3. To find molecularity, count the total number of reactant particles on the left-hand side of the balanced elementary step.
Exam tip: AP MCQ often includes distractors with non-integer molecularity. If an option lists molecularity as 0, 1.5, or any non-integer, eliminate it immediately.
3. Rate Laws for Elementary Reactions★★☆☆☆⏱ 4 min
For overall balanced reactions, reaction orders cannot be determined from the balanced equation — they must be measured experimentally. However, because an elementary reaction is a single collision event, the rate of the reaction is directly proportional to the concentration of each reacting particle raised to the power of its stoichiometric coefficient. This is because the probability of all required reactant particles colliding at the same time is proportional to the product of their individual concentrations.
\text{rate} = k [A]^a [B]^b \quad \text{for a general elementary reaction } aA + bB \rightarrow \text{products}
where $k$ is the rate constant for the elementary step, $a$ is the reaction order with respect to $A$, and $b$ is the reaction order with respect to $B$. The overall order of the elementary step is simply the sum of $a$ and $b$.
Exam tip: In AP FRQ questions asking for a rate law for a forward elementary step, never include product concentrations. Only reactants appear in the rate law for forward elementary steps, which is what you will be asked for 99% of the time on the exam.
4. Elementary Steps in Multi-Step Reaction Mechanisms★★★☆☆⏱ 5 min
Nearly all overall reactions are not single elementary steps — they proceed via a sequence of multiple elementary steps called a reaction mechanism. When you add all elementary steps in a mechanism together, you get the balanced overall reaction.
For a mechanism to be valid, two conditions must hold: 1) the sum of elementary steps matches the experimental overall reaction, and 2) the rate law derived from the mechanism matches the experimentally determined rate law for the overall reaction.
Exam tip: Always double-check the order of production/consumption to avoid confusing intermediates and catalysts on FRQ questions — this is one of the most commonly missed points on mechanism problems.
5. AP-Style Practice Problems★★★★☆⏱ 5 min
Common Pitfalls
Why: Students generalize the rule for elementary reactions to all reactions, forgetting that only elementary steps have orders matching coefficients.
Why: Students mix up the definitions of molecularity (count of particles) and reaction order (can be any value).
Why: Students forget intermediates are not stable species and must be substituted out using equilibrium expressions for fast pre-steps.
Why: Students count all particles in the equation instead of only reactants.
Why: Students mix up the order of production and consumption for the two species types.