Reaction rates give valuable insight into the mechanism of a reaction. The rate depends on the concentration of the reactant or reactants in the slowest step of the mechanism. Rapid steps after the slowest step do not change the reaction rate. Kinetics studies give no information about the mechanism of steps occurring after the slowest step. The slowest step always determines the rate of the reaction. Steps before the slowest step may alter the amount of a reactant present during the slowest step. Consequently, kinetics studies may help to elucidate information about the reactive species formed during these early rapid steps.

The collision of more than 2 molecules with sufficient activation energy at one time is very improbable, consequently most slow steps are either one molecule changing or 2 molecules colliding. Complex rate laws usually indicate that a rapid equilibrium produces a reactive species that is a reactant in the slowest step.

If one molecule changes without collision in the slowest step, the reaction is said to be first order. For example,

If one molecule collides with a second molecule in the slowest step, the reaction is said to be second order. For example,

For simplicity of data manipulation, most kinetic measurements are done with one reactant as the limiting reagent and a large excess of all other reactants. The concentration of these other reactants remains within experimental error of a constant concentration during the measurements. In the reaction,A + B -> C, if B is in large excess as A reacts, the reaction is said to be pseudo first order with respect to A. If A is in large excess as B reacts, the reaction is said to be pseudo first order with respect to B. Sometimes a reactant that appears in the overall balanced reaction does not change the reaction rate at all because the reactant is not a part of the slowest step. The reactant may add after the slowest step. That dependence is said to be zero order.

Reaction Mechanisms