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Half-life is the time for concentration to fall to half its initial value. Zero order: t_1/2 = [A]_0/(2k) — directly proportional to [A]_0. First order: t_1/2 = 0.693/k — independent of [A]_0 (most distinctive feature). Second order: t_1/2 = 1/(k[A]_0) — inversely proportional to [A]_0. General: t_1/2 proportional to [A]_0^(1-n) where n = order. For first order, key relationships: 50% complete = 1 t_1/2. 75% = 2 t_1/2. 87.5% = 3 t_1/2. 93.75% = 4 t_1/2. 96.875% = 5 t_1/2. 99.9% ≈ 10 t_1/2. Time for any fraction: t = (2.303/k)log([A]_0/[A]). For 90% completion: t = 2.303/k = 3.32 t_1/2. For 99%: t = 4.606/k = 6.64 t_1/2. For second order: successive half-lives form a geometric series (each 2x the previous). The half-life method is a powerful tool for determining reaction order from experimental data.