Rate Laws, Order of Reaction and the Rate Constant

(Higher Level)

Collision theory tells us that rate increases with concentration, but it does not tell us by how much. The quantitative relationship between rate and the concentrations of reactants is captured by the rate law (or rate equation), which can only be determined by experiment, never by simply reading off the stoichiometric coefficients of the balanced equation.

For a general reaction, the rate law takes the form $\text{rate} = k[\text{A}]^m[\text{B}]^n$, where $[\text{A}]$ and $[\text{B}]$ are reactant concentrations, $k$ is the rate constant, and the exponents $m$ and $n$ are the orders of reaction with respect to A and B. The overall order is the sum $m+n$. Orders are usually small integers (0, 1, or 2) but can in principle be fractional. A key point is that a species with a l