order reaction

Meaning: Order describes how the rate depends on concentration. In a rate law, the order with respect to a reactant is that reactant's exponent, and the overall order is the sum of the exponents. Orders come from experiment. They equal the coefficients only for an elementary step.

Equation: rate = k[A]ᵐ[B]ⁿ, where rate is in M/s, k is the rate constant (s⁻¹ for first order, M⁻¹s⁻¹ for second order, M/s for zero order), [A] and [B] are concentrations in M, and m and n are the orders (unitless). Overall order = m + n.

On the MCAT: A passage gives a table of initial rates for several starting concentrations. Compare two trials where only one concentration changes. If doubling [A] leaves the rate unchanged, the order is 0. If the rate doubles, the order is 1. If it quadruples, the order is 2. Graphs also identify order: [A] versus time is linear for zero order, ln[A] versus time for first order, and 1/[A] versus time for second order. A first-order half-life is constant, t½ = 0.693/k. You may also be asked for the units of k.

Common confusion: Students read the orders from the coefficients of the balanced equation. Only experiment, or a stated elementary step, gives the order. Also, zero order does not mean no reaction. It means the rate does not depend on that reactant's concentration.