zero order

Meaning: A reaction is zero order in a reactant when changing that reactant's concentration leaves the rate unchanged. The rate law is rate = k[A]⁰ = k. This typically happens when something other than the reactant's concentration limits the rate, such as an enzyme or a catalyst surface that is already saturated. The reactant is still consumed, at a constant rate.

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

On the MCAT: A table of initial rates shows the rate staying the same while [A] is doubled, so the reaction is zero order in A. Graphs also give it away. [A] versus time is a straight line with slope −k, and rate versus [A] is a flat horizontal line. Enzyme passages use it too: at high substrate concentration the rate approaches Vmax and the reaction is roughly zero order in substrate.

Common confusion: Zero order does not mean the rate or k is zero, or that the reaction has stopped. Order is found from experimental data, not from the balanced equation's coefficients. Half-life is not constant here, unlike in first-order reactions. It shrinks as the reaction proceeds because t½ is proportional to the remaining [A].