zeroth order

Meaning: A reaction is zeroth order in a reactant when its rate does not depend on that reactant's concentration. The rate stays constant until the reactant is used up. This occurs when something else limits the rate, such as an enzyme or a catalyst surface saturated with substrate.

Equation: rate = k, and [A] = [A]₀ − kt, with t½ = [A]₀/(2k). Here [A] is concentration at time t (mol/L), [A]₀ is initial concentration (mol/L), k is the rate constant (mol L⁻¹ s⁻¹), t is time (s), and t½ is half-life (s).

On the MCAT: In initial-rate tables, if changing [A] leaves the rate unchanged, the order in A is zero. In graphs, [A] versus time is a straight line with slope −k, and rate versus [A] is a horizontal line. Enzymes at high [S], where the rate approaches Vmax, behave as zero order in substrate.

Common confusion: Zeroth order does not mean the rate is zero; the rate is constant and equals k. Half-life is also not constant as it is in first order. It is proportional to [A]₀, so it gets shorter as the reaction proceeds.