First-order kinetics governs processes, such as radioactive decay, whose rate depends only on the amount of material present, yielding an integrated rate law N = N₀e^(−kt) and a half-life t₁/₂ = 0.693/k that is independent of starting quantity; second-order kinetics governs processes whose rate depends on the square of concentration, yielding the integrated form 1/[A] = 1/[A]₀ + kt and a half-life that depends on the initial concentration. This belongs to chemical kinetics, the branch of physical chemistry concerned with reaction rates and rate laws, and connects to chemical equilibrium through the relation that the equilibrium constant equals the ratio of forward to reverse rate constants (K = k₁/k₋₁).
First-order kinetics governs processes, such as radioactive decay, whose rate depends only on the amount of material present, yielding an integrated rate law N = N₀e^(−kt) and a half-life t₁/₂ = 0.69…