\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.