Rate = k[A]. Integrated: [A] = [A]_0 * e^(-kt), or ln([A]_0/[A]) = kt, or k = $\frac{2.303}{t}$log([A]_0/[A]). Plot: ln[A] vs t is linear with slope = -k. Half-life: $t_1$/2 = $\frac{ln2}{k}$ = 0.693/k — independent of initial concentration. This is the hallmark of first order. After n half-lives: [A] = [A]_0/2^n, or fraction remaining = $\frac{1}{2}$^n. Examples: radioactive decay, N2O5 decomposition, H2O2 decomposition, first-order drug elimination. For reactions involving pressure (gases): k = $\frac{2.303}{t}$log($\frac{P_0}{2P_0 - P_t}$) where $P_0$ = initial pressure and $P_t$ = total pressure at time t. Most tested order in JEE.