text: Rudolph Marcus combined the assumptions of Hinshelwood with RRK theory to estimate this quantity.
full_question: Rudolph Marcus combined the assumptions of Hinshelwood with RRK theory to estimate this quantity. This quantity changes due to isotopic substitution in the kinetic isotope effect. This quantity is equal to the product of the concentrations of reactants raised to their order, all multiplied by a namesake constant of proportionality. The constant associated with this process is proportional to the exponential of negative activation energy divided by RT according to the Arrhenius equation. This quantity, which is equivalent to the change in reactant concentration over time, can be increased with a catalyst. For 10 points, give this value which refers to the speed of a reaction. 
first_sentence: Rudolph Marcus combined the assumptions of Hinshelwood with RRK theory to estimate this quantity.
raw_answer: reaction rate