The effect of concentration on the reaction rate of the reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 can be explained using the collision theory. According to this theory, the rate of a chemical reaction depends on the frequency of collisions between the reacting particles and the energy with which they collide.When the concentration of either hydrochloric acid or sodium thiosulfate is increased, there are more particles of the reactants present in the solution. This leads to a higher probability of collisions between the reacting particles, resulting in an increased reaction rate.In summary, increasing the concentration of hydrochloric acid or sodium thiosulfate in the reaction will lead to an increase in the reaction rate. This is because there are more particles available to collide and react, leading to a higher frequency of successful collisions and faster formation of products.