reaction order
Summary
Reaction order for a reactant is the exponent of its concentration in the experimentally determined rate law; the overall order is the sum of these exponents. It is found from data, such as initial rates, and matches the balanced-equation coefficients only for an elementary step.
Detail
Meaning: Order shows how strongly the rate depends on a reactant's concentration. Zero order: the rate does not change with concentration. First order: doubling the concentration doubles the rate. Second order: doubling the concentration quadruples the rate. The overall order is the sum of the individual orders.
Equation: rate = k[A]ᵐ[B]ⁿ, where rate is in M/s, k is the rate constant (units depend on overall order: s⁻¹ for first, M⁻¹s⁻¹ for second), [A] and [B] are concentrations in M, and m and n are the orders (unitless). Overall order = m + n.
On the MCAT: A table of initial rates for several trials is given. Choose two trials in which only one concentration changes, then use rate ratio = (concentration ratio)ᵐ to find that order. Graph questions: [A] vs time is linear for zero order, ln[A] vs time for first order, and 1/[A] vs time for second order. A constant half-life points to first order.
Common confusion: Students read the order from the coefficients of the balanced equation. That works only for an elementary step. Otherwise the orders come from experiment, and in a multistep mechanism the slow step determines the rate law.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- 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.