A common form for the rate equation is a power law:
The constant  is called the rate constant. The exponents, which can be fractional, are called partial orders of reaction and their sum is the overall order of reaction.
In a dilute solution, an elementary reaction (one having a single step with a single transition state) is empirically found to obey the law of mass action. This predicts that the rate depends only on the concentrations of the reactants, raised to the powers of their stoichiometric coefficients.
The differential rate equation for an elementary reaction using product notation is:
Where:
* is the rate of change of reactant concentration with respect to time.
*k is the rate constant of the reaction.
* represents the concentrations of the reactants, raised to the powers of their stoichiometric coefficients and multiplied together.