Kinetic order is the exponent on a reactant's concentration in the experimentally determined rate law (rate = k[A]^m[B]^n); the sum of all the exponents is the overall order of the reaction (EK 5.2.A.3).
Kinetic order tells you how sensitive a reaction's rate is to one reactant's concentration. In a rate law like rate = k[A]^m[B]^n, the exponent m is the order with respect to A, n is the order with respect to B, and m + n is the overall order. The CED states this directly. The rate of a reaction is proportional to each reactant's concentration raised to a power, and that power is the order with respect to that reactant (EK 5.2.A.2 and 5.2.A.3).
Here's the intuition. Order is the "doubling test." If you double [A] and the rate doesn't budge, the reaction is zeroth order in A. If the rate doubles, it's first order. If the rate quadruples, it's second order. The critical rule, and the thing AP Chem hammers, is that order comes from experimental data only. You cannot read it off the balanced equation. That's why EK 5.2.A.1 emphasizes monitoring reactant or product amounts over time. The data, not the coefficients, reveals the order.
Kinetic order lives in Topic 5.2 (Introduction to Rate Law) in Unit 5: Kinetics, supporting learning objective 5.2.A, which asks you to represent experimental data with a consistent rate law expression. That phrase "consistent with experimental data" is the whole game. Almost every kinetics problem in Unit 5 starts with determining order, whether from an initial-rates table, a concentration-time graph, or half-life behavior. Get the order wrong and everything downstream collapses, including the units of the rate constant k (EK 5.2.A.4), the choice of integrated rate law, and any mechanism check. Order is the hinge that connects Topics 5.2 through 5.8.
Keep studying AP® Chemistry Unit 5
Rate Constant (Unit 5)
The order and the rate constant are partners in the rate law, and the order dictates k's units. A zeroth-order k has units of M/s, first order is 1/s, and second order is 1/(M·s). If you can name the overall order, you can derive k's units instead of memorizing them.
Zeroth, First, and Second Order Reactions (Unit 5)
Each order has a signature straight-line graph. [A] vs. time is linear for zeroth order, ln[A] vs. time for first order, and 1/[A] vs. time for second order. AP questions love handing you a linear plot and making you work backward to the order and the rate law.
Reaction Mechanisms and the Rate-Determining Step (Unit 5)
Kinetic order is your lie detector for proposed mechanisms. A mechanism is only plausible if the rate law predicted by its slow step matches the experimentally determined orders. For an elementary step (and only an elementary step), the order does equal the coefficient.
Half-Life (Unit 5)
Only first-order reactions have a constant half-life, which is why first-order kinetics gets its own special treatment in the CED. If a problem says the half-life doesn't depend on starting concentration, that single clue tells you the order is one.
Multiple-choice questions test kinetic order through the doubling test and through graphs. A classic stem describes doubling a reactant's initial concentration and asks what happens to the rate. For a zeroth-order reactant, the answer is nothing, and that surprises a lot of people. Another common setup gives you a linear plot, like 1/[A] vs. time with a positive slope, and asks for the matching differential rate law (that one is second order, so rate = k[A]^2). On the free-response side, kinetics FRQs routinely provide an initial-rates data table and ask you to determine the order with respect to each reactant, write the full rate law, and calculate k with correct units. Always justify order using the data ("when [A] doubles and [B] is held constant, the rate doubles, so the reaction is first order in A"). Never justify it with stoichiometric coefficients; that earns zero points.
The coefficients in a balanced equation tell you mole ratios, not orders. Kinetic order must come from experimental rate data (EK 5.2.A.1-5.2.A.3). A reaction like 2A → B could be zeroth, first, or second order in A; you can't know until you test it. The one exception is an elementary step in a mechanism, where the order does match the coefficients because the step describes an actual molecular collision.
Kinetic order is the exponent on a reactant's concentration in the rate law, and the sum of all the exponents is the overall order (EK 5.2.A.3).
Order is determined only from experimental data, never from the coefficients of the balanced overall equation.
Use the doubling test on initial-rates data. If doubling a concentration leaves the rate unchanged, doubles it, or quadruples it, the order in that reactant is 0, 1, or 2.
The overall order sets the units of the rate constant k, so M/s means zeroth order, 1/s means first order, and 1/(M·s) means second order.
Each order produces a different linear plot, so a straight line in [A], ln[A], or 1/[A] vs. time identifies zeroth, first, or second order respectively.
For elementary steps in a mechanism, and only there, the order equals the coefficient, which is how you test whether a proposed mechanism matches the experimental rate law.
It's the exponent on a reactant's concentration in the rate law, rate = k[A]^m[B]^n. The order with respect to A is m, and the overall order is m + n. It comes from Topic 5.2 and learning objective 5.2.A.
No, and this is the most common kinetics mistake on the exam. Order must be determined experimentally, usually from initial-rates data or concentration-time graphs. The only time order matches coefficients is for an elementary step in a mechanism.
Kinetic order with respect to one reactant is that reactant's individual exponent in the rate law. Overall order is the sum of all the exponents. For rate = k[A]^1[B]^2, the reaction is first order in A, second order in B, and third order overall.
No. Zeroth order means the rate doesn't depend on that reactant's concentration, so doubling it leaves the initial rate unchanged. The reactant still gets consumed; its concentration just doesn't appear in the rate law.
Look for the linear plot. A straight line on [A] vs. time means zeroth order, on ln[A] vs. time means first order, and on 1/[A] vs. time means second order. AP questions often give you the linear plot and ask you to identify the rate law from it.
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