mass action rate law for second order irreversible reactions mass action like rate law for second order irreversible reactions, one reactant, one essential stimulator Reaction scheme where the products are created from a reactant and the change of a product quantity is proportional to the product of the reactant and the stimulator activities. The reaction scheme does not include any reverse process that creates the reactant from the products. The change of a product quantity is proportional to the quantity of the reactant and the stimulator.