The order of reaction is the power to which a reactant's concentration is raised in the rate law (rate = k[A]^m[B]^n); it tells you how the rate responds when that reactant's concentration changes, and it must be determined from experimental data, not from the balanced equation.
The order of reaction is the exponent attached to each reactant's concentration in the rate law. For a rate law like rate = k[A]^m[B]^n, the reaction is "order m" with respect to A and "order n" with respect to B. Per the CED (5.2.A.3), the power of each reactant in the rate law is the order with respect to that reactant, and adding all the powers gives the overall reaction order.
Here's the intuitive read: the order is a sensitivity dial. Zeroth order means the rate ignores that reactant entirely. First order means doubling the concentration doubles the rate. Second order means doubling the concentration quadruples the rate (2² = 4). The big AP rule is that orders come from experiments, usually the method of initial rates, where you compare trials that change one concentration at a time. You cannot read orders off the coefficients in the balanced chemical equation.
Order of reaction lives in Topic 5.2 (Introduction to Rate Law) in Unit 5: Kinetics, supporting learning objective 5.2.A: represent experimental data with a consistent rate law expression. The whole point of 5.2.A is matching a rate law to data, and the orders are the part you actually have to figure out. The rate constant k just falls out afterward.
Orders also set up everything that follows in Unit 5. Whether a reaction is zeroth, first, or second order determines which integrated rate law applies, which plot gives a straight line, and how half-life behaves. Later in the unit, orders become the bridge between experimental data and proposed reaction mechanisms. If you can't determine orders, the rest of kinetics is locked.
Keep studying AP Chemistry Unit 5
Rate Constant (k) (Unit 5)
The order and the rate constant are the two pieces you extract from initial-rates data, in that order. You need the orders first because the units of k depend on the overall order. For a first-order reaction k has units of s⁻¹, but for second order it's M⁻¹s⁻¹.
Overall Reaction Order (Unit 5)
Add up the individual orders and you get the overall order (5.2.A.3). A reaction that's first order in A and first order in B is second order overall. Exam questions love asking for one when the data gives you the other.
Integrated Rate Laws and Half-Life (Unit 5)
The order is the switch that picks your equation. Zeroth order gives a linear [A] vs. time plot, first order gives linear ln[A] vs. time, and second order gives linear 1/[A] vs. time. First order is the only one with a constant half-life, which is why it dominates half-life problems.
Reaction Mechanisms (Unit 5)
For a single elementary step, and only an elementary step, the orders do match the coefficients. That's the connection point: a valid mechanism's rate-determining step must produce a rate law whose orders agree with the experimentally determined ones.
Order of reaction shows up most often as a method-of-initial-rates problem. You get a table of trials with different starting concentrations and initial rates, and you compare two trials where only one concentration changes. The classic pattern: doubling [A] makes the rate quadruple, so the reaction is second order in A (because 2^m = 4 means m = 2). Multiple-choice stems also ask directly what the exponent in a rate law represents, or flip it and ask you to predict how the rate changes given an order.
FRQs push further. Expect to determine orders from data, write the full rate law, calculate k with correct units, and sometimes justify your reasoning in a sentence or two. Trickier versions add an experimental-error twist, like a wet beaker diluting a stock solution, and ask how that affects the determined order or rate. The core skill is always the same: let the data, not the balanced equation, tell you the exponents.
The single most-tested misconception in Unit 5. The coefficient in a balanced equation tells you mole ratios; the order tells you how rate depends on concentration, and it must come from experimental data. A reaction 2A → B can be zeroth, first, or second order in A. The only time coefficients equal orders is for an elementary step in a mechanism, and the exam will absolutely test whether you know that exception.
The order of reaction is the exponent on a reactant's concentration in the rate law, rate = k[A]^m[B]^n.
Orders are determined experimentally, usually by the method of initial rates, never by copying coefficients from the balanced equation.
If doubling a concentration doubles the rate, the reaction is first order in that reactant; if the rate quadruples, it's second order; if nothing changes, it's zeroth order.
The overall reaction order is the sum of the individual orders of every reactant in the rate law.
The order determines the units of the rate constant k, which integrated rate law applies, and whether the half-life is constant.
Orders only match coefficients for elementary steps, which is how experimental rate laws are used to test proposed mechanisms.
It's the power a reactant's concentration is raised to in the rate law. In rate = k[A]²[B], the reaction is second order in A, first order in B, and third order overall. It tells you how strongly the rate responds to concentration changes (CED 5.2.A.3).
No, and this is the most common kinetics mistake on the exam. Orders must come from experimental data like initial-rates tables. The one exception is an elementary step in a mechanism, where the molecularity does give you the order.
Compare two trials where only one reactant's concentration changes. If doubling [A] quadruples the rate, solve 2^m = 4 to get m = 2, so the reaction is second order in A. Repeat for each reactant, then write the full rate law.
Order of reaction usually refers to the order with respect to one specific reactant (the single exponent). Overall reaction order is the sum of all the exponents in the rate law. A rate law of k[A][B] is first order in each reactant but second order overall.
It doesn't change k's value, but it determines k's units. A first-order reaction has k in s⁻¹, a second-order reaction has k in M⁻¹s⁻¹, and a zeroth-order reaction has k in M/s. Getting k's units wrong is an easy point to lose on an FRQ.
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