5.3 Concentration Changes Over Time

Plot data three ways. The straight line tells the order: [A] vs t → zero; ln[A] vs t → first; 1/[A] vs t → second.

First-order half-life is constant: t½ = 0.693/k. Radioactive decay is first order.

First order: ln[A] = −kt + ln[A]₀; slope = −k.

Worked example: t½ = 5 min. After 20 min → 4 half-lives → 1/16 remains.