“The order of a reaction refers to the powers to which the concentrations are raised in the kinetic rate law”. Considering the reaction rate below:
The reaction is α order with respect to reactant A, and β order with respect to reactant B”. Thus, the overall reaction order, n, is calculated as below:
The units of –rA are always in terms of concentration per unit time, while the units of the specific reaction rate, kA, will vary with the order of the reaction”
Reference: Fogler, H.S., Essentials of chemical reaction engineering. 2011: Upper Saddle River, N.J. : Prentice Hall, .