The effect of a change in reactant concentration on the rate of a reaction can be explained using the collision theory and the rate law. According to the collision theory, the rate of a reaction depends on the frequency of collisions between reacting particles. When the concentration of reactants increases, the number of particles per unit volume also increases, leading to more frequent collisions and a faster reaction rate.The reaction mechanism, which is the step-by-step sequence of elementary reactions by which the overall chemical change occurs, plays a crucial role in determining the effect of reactant concentration on the reaction rate. The rate law, which is an expression that shows the relationship between the rate of a reaction and the concentrations of reactants, can be derived from the reaction mechanism.For a simple reaction with a single-step mechanism, the rate law is often directly proportional to the concentration of the reactants. For example, for a reaction A + B C, the rate law can be written as:Rate = k[A]^m[B]^nwhere 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, respectively.Experimental evidence can be obtained by measuring the reaction rate at different reactant concentrations and determining the reaction orders m and n from the data. For instance, if the reaction rate doubles when the concentration of A doubles while keeping the concentration of B constant , the reaction is said to be first-order with respect to A m = 1 . If the reaction rate quadruples when the concentration of B doubles while keeping the concentration of A constant , the reaction is second-order with respect to B n = 2 .For reactions with multi-step mechanisms, the rate-determining step the slowest step in the mechanism plays a crucial role in determining the rate law and the effect of reactant concentration on the reaction rate. In such cases, the rate law is derived from the rate-determining step, and the reaction orders may not be equal to the stoichiometric coefficients of the reactants in the overall balanced equation.In conclusion, the effect of a change in reactant concentration on the rate of a reaction depends on the reaction mechanism and the rate law derived from it. Experimental evidence can be obtained by measuring the reaction rate at different reactant concentrations and determining the reaction orders, which provide insight into the reaction mechanism and the role of reactant concentration in the reaction rate.