Intermediate Topic 426 of 1102 Kinetics & Equilibrium
Zero Order Reaction
Rate is independent of concentration, and concentration falls linearly with time.
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In-depth explanation of Zero Order Reaction
In a zero order reaction the rate is constant and does not depend on the reactant concentration, so the rate law is rate = k. Integrating gives [A] = [A]0 - kt, which means the concentration falls in a straight line when plotted against time, with slope minus k. The half-life is t1/2 = [A]0/2k, so it depends on the initial concentration, unlike first order reactions. Zero order behaviour arises when a catalyst surface or enzyme is saturated, so the rate is limited by the number of available sites rather than by the amount of reactant. Once the reactant is nearly exhausted the rate drops quickly.
Going deeper: Zero Order Reaction
Key ideas: Use [A] = [A]0 - kt with k in mol per L per s, and t1/2 = [A]0/(2k). The reaction finishes completely at t = [A]0/k, a finite time, unlike first order decay which only approaches zero. Typical cases are enzyme-saturated reactions, decomposition on a metal surface and some photochemical processes where light is limiting.
Common mistakes and connections: Students sometimes think zero order means the reaction does not occur or that the rate is zero. It means the rate does not depend on that concentration, while the rate itself is k. Another error is using the first order half-life formula for these runs. When saturation ends at low concentration, the order can switch to one, linking this topic to Michaelis-Menten kinetics.
Visualization of Zero Order Reaction
Concentration falls linearly at constant rate k, as on a saturated catalyst surface, until the reactant is gone; half-life depends on the initial concentration. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Decomposition of ammonia on a hot tungsten surface is zero order at high pressure, because the surface is saturated.
Frequently asked questions about Zero Order Reaction
What is Zero Order Reaction?
Rate is independent of concentration, and concentration falls linearly with time.
Is Zero Order Reaction a beginner, intermediate or advanced topic?
Zero Order Reaction is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Zero Order Reaction?
Study these earlier topics in Kinetics & Equilibrium first: Rate Law, Rate Constant, Order of Reaction.
What comes after Zero Order Reaction?
The next topics in Kinetics & Equilibrium are First Order Reaction and Second Order Reaction.
Can I practise Zero Order Reaction with quizzes?
Yes. The interactive lesson includes a self-check question on Zero Order Reaction, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Zero Order Reaction fits in Kinetics & Equilibrium
Zero Order Reaction is topic 7 of 91 in Kinetics & Equilibrium.