Order of a reaction and units of k

Order is the sum of the powers in the experimental rate law. In rate = k[A]^x[B]^y the order is x + y; x alone is the order with respect to A.

Order can be 0, 1, 2, 3, or a fraction. Zero order means the rate does not change with concentration of that reactant.

An elementary reaction happens in one step. A complex reaction is a series of elementary steps (a mechanism), and its order comes from that mechanism, not from the overall equation.

Units of k follow from rate = k[A]ⁿ: k has units (mol L⁻¹)^(1−n) s⁻¹.

Zero order: mol L⁻¹ s⁻¹. First order: s⁻¹. Second order: L mol⁻¹ s⁻¹ (mol⁻¹ L s⁻¹).

Working backwards, the units of a stated k give its order: k = 3 × 10⁻⁴ s⁻¹ is first order; k = 2.3 × 10⁻⁵ L mol⁻¹ s⁻¹ is second order.

Order is always an experimental quantity. It can change with conditions, as when a reactant is present in large excess.