In a complex reaction mechanism, the effect of concentration changes of reactants on the reaction rate can be quite significant. The reaction rate is influenced by the concentrations of the reactants involved in the rate-determining step RDS of the mechanism. The RDS is the slowest step in the reaction mechanism, and it determines the overall rate of the reaction.According to the rate law, the reaction rate is directly proportional to the concentration of the reactants raised to the power of their respective reaction orders. For example, for a reaction with the rate law: rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of reactants A and B, and m and n are their respective reaction orders.In a complex reaction mechanism, the reaction orders m and n may not be equal to the stoichiometric coefficients of the reactants in the balanced chemical equation. They are determined experimentally and can be fractional or even negative.When the concentration of a reactant increases, the reaction rate will generally increase as well, provided that the reaction order for that reactant is positive. If the reaction order is negative, an increase in the concentration of that reactant will lead to a decrease in the reaction rate. If the reaction order is zero, the reaction rate will be independent of the concentration of that reactant.In summary, the effect of concentration changes of reactants on the reaction rate in a complex reaction mechanism depends on the reaction orders of the reactants involved in the rate-determining step. An increase in the concentration of a reactant with a positive reaction order will increase the reaction rate, while an increase in the concentration of a reactant with a negative reaction order will decrease the reaction rate.