The reaction between sodium thiosulfate Na2S2O3 and hydrochloric acid HCl is a classic example of a reaction that is affected by the concentration of the reactants. The reaction can be represented by the following balanced equation:Na2S2O3 aq + 2 HCl aq 2 NaCl aq + H2O l + SO2 g + S s The effect of changing the concentration of reactants on the reaction rate can be explained using the collision theory. According to this theory, the rate of a reaction depends on the frequency of effective collisions between the reacting particles. When the concentration of reactants is increased, the number of particles per unit volume also increases, leading to a higher probability of effective collisions and thus a faster reaction rate.In the case of the reaction between sodium thiosulfate and hydrochloric acid, increasing the concentration of either reactant will result in a faster reaction rate. This can be observed experimentally by measuring the time it takes for a certain amount of sulfur S to be produced, which causes the solution to become cloudy.As for the sustainability of the reaction, it is important to note that this reaction produces sulfur dioxide SO2 , a gas that contributes to air pollution and acid rain. Therefore, increasing the concentration of reactants may lead to a faster reaction rate, but it will also result in the production of larger amounts of SO2, which is not environmentally sustainable.In summary, changing the concentration of reactants in the reaction between sodium thiosulfate and hydrochloric acid affects the reaction rate, with higher concentrations leading to faster reaction rates. However, increasing the concentration of reactants also leads to increased production of sulfur dioxide, which has negative environmental impacts.