Changing the concentration of the reactants in the chemical reaction between sodium thiosulfate Na2S2O3 and hydrochloric acid HCl can significantly affect the reaction rate and efficiency. The reaction between these two substances can be represented by the following balanced equation:Na2S2O3 aq + 2 HCl aq 2 NaCl aq + SO2 g + S s + H2O l The reaction rate is influenced by the collision theory, which states that the rate of a reaction depends on the frequency and energy of collisions between reactant molecules. When the concentration of reactants is increased, there are more molecules present in the reaction mixture, leading to a higher probability of collisions between reactant molecules. This results in an increased reaction rate.In the case of the reaction between sodium thiosulfate and hydrochloric acid, increasing the concentration of either reactant will lead to a faster reaction rate. This can be observed experimentally by the faster formation of the sulfur precipitate, which causes the reaction mixture to become cloudy.As for the efficiency of the reaction, it is typically measured by the percentage yield or the extent of the reaction. In this context, efficiency refers to how completely the reactants are converted into products. Changing the concentration of the reactants does not directly affect the efficiency of the reaction, as long as the stoichiometry of the reaction is maintained. However, it is important to note that a faster reaction rate may lead to side reactions or other factors that could potentially decrease the overall efficiency of the reaction.In summary, increasing the concentration of the reactants in the reaction between sodium thiosulfate and hydrochloric acid will increase the reaction rate, while the efficiency of the reaction may not be directly affected by the concentration changes, as long as the stoichiometry is maintained.