Reactions usually occur by a series of steps called a reaction mechanism.

The rate of a chemical reaction normally depends on the concentrations of the reactants.

The rate of reaction is dependent on the slowest step, which is called the rate determining step.

The order of a reaction with respect to any one reactant is the power to which the concentration of that reactant is raised in the rate equation.

→ Orders of reaction are used to relate the rate of a reaction to the reacting species.

The overall order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the rate equation.

The order of a reaction can only be determined from experimental data.

The rate equation is an equation that relates the rate of a reaction to the rate constant, $k$, and the reactant concentrations, e.g. a generation rate equation for $A + 2B → C + D$ is shown below.

$$ rate=k[A]^x[B]^y $$

If changing the concentration of a reactant $A$ has no effect on the rate of the reaction, then the reaction is zero order with respect to $A$, i.e. $rate=k$.

If doubling the concentration of a reactant $A$ doubles the rate of the reaction, then the reaction is first order with respect to $A$.

The following equation expresses the rate of such a reaction, where $k$ is the rate constant and $[A]$ is the concentration of reactant $A$ in $mol \space l^{-1}$.

$$ rate = k[A] $$

If doubling the concentration of a reactant $A$ increases the rate of the reaction fourfold, then the reaction is second order with respect to $A$.

The following equation expresses the rate of such a reaction.

$$ rate = k[A]^2 $$

The rate equation and the rate constant, including units, can be determined from initial rate data for a series of reactions in which the initial concentrations of reactants are varied. These can be zero, first, second or third order.

Experimentally determined rate equations can be used to determine possible reaction mechanisms.