The effect of changing the concentration of reactants on the reaction rate for the reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 solution can be explained using the collision theory. According to this theory, the reaction rate depends on the frequency of effective collisions between the reacting particles.The reaction between hydrochloric acid and sodium thiosulfate is as follows:2HCl aq + Na2S2O3 aq 2NaCl aq + H2O l + SO2 g + S s When the concentration of either hydrochloric acid or sodium thiosulfate is increased, the number of particles of that reactant in the solution also increases. This results in a higher probability of effective collisions between the reactant particles, leading to an increase in the reaction rate.In summary, increasing the concentration of either hydrochloric acid or sodium thiosulfate in the reaction will lead to an increase in the reaction rate. Conversely, decreasing the concentration of either reactant will result in a decrease in the reaction rate.