An average reaction rate is calculated as the change in the concentration of reactants or products over a period of time in the course of the reaction. an instantaneous reaction rate is the rate at a particular moment in the reaction and is usually determined graphically. the reaction of compound a forming compound b was studied and the following data were collected: time (s) [a] (m) 0. 0.184 200. 0.129 500. 0.069 800. 0.031 1200. 0.019 1500. 0.016 what is the instantaneous rate of the reaction at t=800. s?