Zero Order Reaction - Definition, Rate Law, Integrated Rate Equation and Examples

Zero Order Reaction

Chemical kinetics deals with the study of reaction rates and the factors affecting them. One of the most important reaction types is the zero-order reaction, where the reaction rate remains constant and does not depend on the concentration of the reactant.

Definition

A reaction is called a zero-order reaction if its rate is independent of the concentration of the reactant.

General Rate Law

Rate = k[A]⁰ = k

Since A⁰ = 1, the reaction rate is simply equal to the rate constant.

Integrated Rate Equation

For a zero-order reaction,

[A] = [A]₀ − kt

where

• [A] = concentration after time t
• [A]₀ = initial concentration
• k = rate constant
• t = time

Half-Life

The half-life of a zero-order reaction is

t_1/2 = [A]₀ / 2k

Unlike first-order reactions, the half-life depends upon the initial concentration.

Characteristics

• Rate remains constant.
• Independent of reactant concentration.
• Half-life changes with initial concentration.
• Integrated equation is linear.
• Concentration decreases uniformly with time.

Graph

A graph of concentration versus time gives a straight line having a negative slope equal to –k.

Examples

• Photochemical reactions under constant light intensity.
• Catalytic decomposition when catalyst surface becomes saturated.
• Some enzyme-catalyzed reactions at high substrate concentration.

Applications

Zero-order kinetics is widely used in pharmaceuticals, enzyme chemistry, industrial catalysis and photochemical reactions.

Exam Tip: If the rate does not change when concentration changes, the reaction follows zero-order kinetics.

Summary

Property	Zero Order Reaction
Rate Law	Rate = k
Integrated Equation	[A]=[A]₀−kt
Half-life	[A]₀/2k
Depends on Concentration	No
Graph	Straight line