Zero-Order Reactions

9 min read Updated Mar 26, 2026

A zero-order reaction is the simplest type of kinetics: the rate does not depend on the concentration of reactants at all. No matter how much reactant you add, the reaction proceeds at the same constant speed.

Rate Law

For a zero-order reaction:

Integrated Rate Law

The integrated rate law lets you calculate the concentration of a reactant at any time t:

Graphical Analysis

Plotting [A] vs. time for a zero-order reaction gives a straight line with:

Slope = -k (negative because concentration decreases)
y-intercept = [A]₀

This is the simplest graph you will see in kinetics. If [A] vs. t is linear and decreasing, the reaction is zero order.

Graph of ammonia concentration in molarity versus time in seconds showing decomposition of NH3 on two different catalytic surfaces: tungsten (W) shows a nearly linear decline characteristic of zero-order kinetics, while silicon dioxide (SiO2) shows a curved decline — Decomposition of NH₃ on two different catalytic surfaces. On tungsten (W), the nearly linear decline in [NH₃] over time is characteristic of zero-order kinetics - the surface is saturated with reactant. On SiO₂, the curve shows non-zero-order behavior. *Credit: OpenStax Chemistry 2e, CC BY 4.0*

Half-Life

The half-life of a zero-order reaction is the time it takes for the concentration to drop to half its initial value:

What Can Change the Rate?

Since the rate equals k, the only ways to change the rate of a zero-order reaction are:

Change the temperature - this changes k via the Arrhenius equation
Add a catalyst - this lowers the activation energy, increasing k

Changing reactant concentrations has no effect. This is the defining feature of zero-order kinetics.

Enzyme kinetics is the classic place to see both orders in one curve. Slide the
substrate concentration: at low [S] the rate climbs with concentration, which is
first-order behavior, and once every active site is occupied the rate plateaus at
V_max, which is zero-order.

Animation Enzyme Kinetics (Michaelis–Menten)

[S]

Key idea

As substrate rises, rate climbs then plateaus at Vmax — every enzyme is busy. Km is the [S] giving half-Vmax (lower Km = tighter binding). Competitive inhibitors raise Km (more substrate beats them); noncompetitive inhibitors lower Vmax (substrate can't).

Slide [S] to move along the curve; switch the inhibitor to see Km and Vmax shift.

For a zero-order reaction, what happens to the rate when you double the concentration of the reactant?

Click to reveal answer

Nothing - the rate stays the same. In a zero-order reaction, rate = k. The rate is independent of reactant concentration. Only changing the temperature or adding a catalyst will change the rate.

Which graph gives a straight line for a zero-order reaction: [A] vs. t, ln[A] vs. t, or 1/[A] vs. t?

Click to reveal answer

[A] vs. t is linear for zero-order reactions. The integrated rate law [A]_t = [A]_0 - kt is in y = mx + b form. The slope is -k and the intercept is [A]_0.