$word_{count}$: 210

The rate of reaction measures how fast reactant concentrations decrease or product concentrations increase. For aA + bB -> cC + dD: rate = -$\frac{1}{a}$d[A]/dt = $\frac{1}{c}$d[C]/dt. The rate law, rate = k[A]^m[B]^n, is determined experimentally — orders m and n may differ from stoichiometric coefficients for non-elementary reactions. The rate constant k depends only on temperature and the specific reaction, not on concentration. Units of k for nth order: $\frac{mol}{L}$^(1-n).$s^{-1}$. Zero order: $\frac{mol}{L.s}$. First order: $s^{-1}$. Second order: $\frac{L}{mol.s}$. Overall order = m + n (can be zero, integer, or fractional). For elementary reactions (single-step), molecularity = order. Molecularity is always a positive integer (1, 2, or 3). Multi-step reactions: the slowest step (rate-determining step) determines the overall rate law. Intermediates must be eliminated from the rate expression using pre-equilibrium or steady-state assumptions. The initial rates method is the standard way to determine order: double one concentration at a time and observe the rate change. If rate doubles: order 1. Quadruples: order 2. Unchanged: order 0.