Changing the reactant concentration affects the rate of a complex reaction mechanism, but it does not affect the rate constant. The rate constant is a proportionality constant that is specific to a particular reaction and is independent of the concentration of reactants. It depends on factors such as temperature and the presence of catalysts.In a complex reaction mechanism, there are multiple elementary steps, each with its own rate constant. The overall reaction rate depends on the slowest step, also known as the rate-determining step. The rate law for the overall reaction can be determined by analyzing the rate-determining step and the concentrations of the reactants involved in that step.The relationship between reactant concentration and reaction rate can be described by the rate law, which is typically in the form:Rate = k[A]^m[B]^nwhere Rate is the reaction rate, k is the rate constant, [A] and [B] are the concentrations of reactants A and B, and m and n are the reaction orders with respect to A and B.By experimentally determining the reaction orders m and n and the rate constant k , you can predict how changes in reactant concentrations will affect the overall reaction rate. For example, if the reaction is first-order with respect to reactant A m=1 , doubling the concentration of A will double the reaction rate. If the reaction is second-order with respect to A m=2 , doubling the concentration of A will quadruple the reaction rate.In summary, while the rate constant is not affected by changes in reactant concentration, the relationship between reactant concentration and reaction rate can be used to predict changes in the overall reaction rate for a complex reaction mechanism. This is done by determining the rate law for the rate-determining step and using it to analyze how changes in reactant concentrations will impact the reaction rate.