mass action rate law for first order reversible reactions mass action rate law for first order forward, third order reverse, reversible reactions Reaction scheme where the products are created from the reactants and the change of a product quantity is proportional to the product of reactant activities. The reaction scheme does include a reverse process that creates the reactants from the products. The rate of the forward process is proportional to the quantity of one reactant. The rate of the reverse process is proportional to three product quantities.