First-Order Reaction
A reaction whose rate is directly proportional to the concentration of a single reactant.
Formula / equation
ln[A] = ln[A]₀ − kt
Unit
s⁻¹
In depth
A plot of the natural logarithm of concentration against time is linear, and the half-life is constant regardless of starting concentration — a distinctive signature. Radioactive decay, many decompositions and most drug eliminations follow first-order kinetics.
Examples in the real world
N₂O₅ decomposing in solution; the elimination of most pharmaceuticals from the bloodstream.