Reaction order is the exponent on a reactant's concentration in the rate law (rate = k[A]^m[B]^n), determined experimentally, that tells you how strongly that reactant's concentration affects the rate; the sum of all the exponents is the overall reaction order (EK 5.2.A.3).
Reaction order is the power each reactant's concentration is raised to in the rate law. For a rate law like rate = k[A]^m[B]^n, the reaction is order m with respect to A, order n with respect to B, and the overall order is m + n (EK 5.2.A.2 and 5.2.A.3). The order is basically a sensitivity dial. Zero order means changing that reactant's concentration does nothing to the rate. First order means doubling the concentration doubles the rate. Second order means doubling the concentration quadruples the rate.
Here's the part AP loves to test. You cannot read reaction orders off the balanced equation's coefficients. Orders come from experimental data only, either by comparing initial rates across trials where one concentration changes at a time, or by checking which concentration-vs-time graph is linear ([A] vs. t for zero order, ln[A] vs. t for first order, 1/[A] vs. t for second order, per EK 5.3.A.1-5.3.A.3).
Reaction order lives in Unit 5: Kinetics, anchoring Topics 5.2 and 5.3. It supports LO 5.2.A (represent experimental data with a consistent rate law expression) and LO 5.3.A (identify the rate law from concentration-vs-time data). It's also the bridge concept for everything downstream in kinetics. You need the order to pick the right integrated rate law, to know whether half-life is constant, and to interpret what a mechanism's rate-determining step predicts. Order even shows up in Unit 9's kinetic control discussion (LO 9.4.A), since rate laws explain why a thermodynamically favored reaction can still crawl along at an unmeasurable rate. If you can find orders from a data table fast, you've unlocked a guaranteed chunk of Unit 5 points.
Keep studying AP Chemistry Unit 5
Integrated Rate Law (Unit 5)
The order tells you which integrated rate law to use. The graphical test works backwards too. If ln[A] vs. time is a straight line, the reaction is first order in A; if 1/[A] vs. time is linear, it's second order. The order and the linear plot are two views of the same fact.
First-Order Reaction (Unit 5)
First order is the special case where rate scales directly with concentration, and it's the only order with a constant half-life (t½ = 0.693/k). When AP gives you radioactive decay or a constant half-life, it's quietly telling you the order is one.
Zero-Order Reaction (Unit 5)
Zero order means the reactant's concentration is in the rate law with an exponent of zero, so [X]⁰ = 1 and the rate ignores it completely. This usually signals something else is the bottleneck, like a saturated catalyst surface.
Activation Energy and Kinetic Control (Units 5 & 9)
Reaction order tells you how concentration affects rate, while activation energy tells you how temperature affects the rate constant k. Topic 9.4 ties them together. A reaction can be thermodynamically favored but stuck under kinetic control because its rate, governed by k and the orders, is tiny.
Reaction order is one of the most reliably tested skills in Unit 5. The classic MCQ gives you a table of initial-rate experiments and asks for the order in each reactant or the full rate law. The move is mechanical. Find two trials where only one concentration changes, see how the rate responded, and match it to an exponent. For example, if doubling [NO] makes the rate go from 8.0 × 10⁻⁵ to 3.2 × 10⁻⁴ M/s (a factor of 4), the reaction is second order in NO. Other stems hand you a rate law like rate = k[X]⁰[Y]¹ and ask you to interpret it, meaning you should say changing [X] has no effect on rate while rate is directly proportional to [Y]. On FRQs, expect to determine orders from data, justify your reasoning with the trial comparison, then use the rate law to calculate k (with correct units) or predict a rate at new concentrations. The graphical version asks which plot is linear, so memorize the zero/first/second order plots from Topic 5.3.
The coefficients in a balanced equation tell you mole ratios, not reaction orders. For 2NO + O₂ → 2NO₂, you might guess the rate law is rate = k[NO]²[O₂] from the coefficients, and sometimes the experiment agrees, but you can never assume it. Orders must come from experimental rate data. The only time coefficients predict orders is for a single elementary step in a mechanism, and AP questions are built to punish anyone who skips that distinction.
Reaction order is the exponent on a reactant's concentration in the rate law, and the overall order is the sum of all the exponents.
Orders are determined experimentally from rate data, never by copying coefficients from the balanced equation.
Zero order means concentration changes don't affect rate, first order means rate doubles when concentration doubles, and second order means rate quadruples when concentration doubles.
You can identify order graphically by finding the linear plot, which is [A] vs. time for zero order, ln[A] vs. time for first order, and 1/[A] vs. time for second order.
The units of the rate constant k change with overall order, so checking k's units is a quick way to confirm or identify the order.
Only first-order reactions have a constant half-life, which is why nuclear decay problems always behave first order.
Reaction order is the power a reactant's concentration is raised to in the rate law rate = k[A]^m[B]^n. The order with respect to A is m, with respect to B is n, and the overall order is m + n. It's covered in Topic 5.2 under LO 5.2.A.
No, not for an overall reaction. Orders must be found from experimental data, like comparing initial rates across trials or testing which concentration plot is linear. The one exception is an elementary step in a mechanism, where the order does match that step's coefficients.
Pick two experiments where only one reactant's concentration changes and compare the rates. If doubling the concentration doubles the rate, that reactant is first order; if the rate quadruples, it's second order; if the rate doesn't change, it's zero order.
Reaction order is an experimentally measured exponent in the rate law for the overall reaction. Molecularity is the number of particles colliding in a single elementary step of a mechanism. They only match for an elementary step, never automatically for the overall equation.
It means that reactant appears in the rate law with an exponent of zero, so changing its concentration has no effect on the rate. For rate = k[X]⁰[Y]¹, you could double [X] and the rate wouldn't budge, but doubling [Y] would double the rate.
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