rate order
Summary
Rate order is the exponent of a reactant's concentration in the experimentally determined rate law. It shows how strongly the rate depends on that concentration, and the overall order is the sum of the exponents. Orders come from data, not from the balanced equation's coefficients, except for an elementary step.
Detail
Meaning: The order with respect to a reactant is the exponent of its concentration in the experimentally determined rate law. It shows how the rate responds to a concentration change. Order 0 means no effect, order 1 means the rate scales in direct proportion, and order 2 means doubling the concentration quadruples the rate. The overall order is the sum of the individual orders.
Equation: rate = k[A]ᵐ[B]ⁿ, with overall order = m + n. Rate is in M/s (mol L⁻¹ s⁻¹), k is the rate constant (M¹⁻ᵖ·s⁻¹ for overall order p), [A] and [B] are concentrations in M, and m and n are unitless orders.
On the MCAT: A passage gives a table of initial rates for several trials. Compare two trials in which only one concentration changes and see how the rate scales. Graphs also test it: a straight line for [A] vs time means zero order, ln[A] vs time means first order, and 1/[A] vs time means second order. The units of k reveal the overall order. A first-order reaction has a constant half-life.
Common confusion: Students read the orders from the coefficients in the balanced equation. Orders are set by experiment. They equal the coefficients only for an elementary step. In a multistep mechanism, the rate law follows the slow step, so use the data table, not the equation.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.