Experimentally it has been found that the power is almost always an integer. The value of that power, or in more complex reactions the sum of the values of the powers, is called the order of the reaction. The proportionality constant is called the rate constant of the reaction, and is normally indicated by the symbol k . No orders higher than the third order in any one reactant are known. If there are two or more reactants, however, a rate law could involve more than one of them.
Reactions occasionally appear to have non-integral or fractional orders.
These unusual orders, as well as zero-order rate laws, can arise as a result of several successive steps of different stoichiometry interacting in more complex mechanisms, as we shall see in later sections. The order which will be experimentally observed for a reaction cannot be determined from the overall stoichiometry of the reaction. Nor is the value of the rate constant determined by the driving force of a chemical reaction, the free energy change. Both the order of a reaction and the value of its rate constant must be determined experimentally. Neither the order of a reaction nor the value of its rate constant can be deduced from the form or value of any of the stoichiometric or thermodynamic properties of the reaction.