The effect of changing the concentration of reactants on the reaction rate in the reaction between sodium thiosulfate Na2S2O3 and hydrochloric acid HCl can be explained using the collision theory. According to this theory, the rate of a chemical reaction depends on the frequency of collisions between reactant molecules and the fraction of collisions that have sufficient energy to result in a reaction.In the reaction between sodium thiosulfate and hydrochloric acid, the following reaction occurs:Na2S2O3 aq + 2 HCl aq 2 NaCl aq + SO2 g + S s + H2O l When the concentration of either sodium thiosulfate or hydrochloric acid is increased, the number of particles of that reactant in the solution also increases. This leads to a higher probability of collisions between the reactant molecules, resulting in an increased reaction rate.In summary, increasing the concentration of either sodium thiosulfate or hydrochloric acid will increase the reaction rate in the reaction between these two substances. Conversely, decreasing the concentration of either reactant will result in a slower reaction rate.