Chemical Kinetics · Integrated Rate Laws

Reaction Order & Half-Life

How fast a reactant disappears depends on its order. Watch [A] fall for a zero-, first- or second-order reaction, and see the half-life markers: only for first order is each successive half-life the same length — the fingerprint that identifies the order. Switch to the linearized plot to see which transform straightens the data.

■ [A] vs time ┊ successive half-lives concentration view

Reaction order

First order: rate = k[A]. Half-life 0.693/k is independent of concentration — equal steps.

Parameters

Rate constant k 0.30
Initial conc [A]₀ 1.00 M
2.31
first half-life (t½)
constant
successive t½
s⁻¹
units of k
ln[A] vs t
straight-line plot
[A] = [A]₀·e−kt

Try this

Compare the half-lives. On first order the dashed markers are evenly spaced. Switch to zero order — each half-life gets shorter (the reactant runs out abruptly). On second order they stretch farther apart. Then hit Linearized: the plot is a straight line only for the order that matches.