Changing the concentration of reactants in a chemical reaction affects the reaction rate according to the collision theory. The collision theory states that for a reaction to occur, reactant particles must collide with the correct orientation and with sufficient energy to overcome the activation energy barrier.In the reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 , the reaction can be represented by the following balanced equation:2HCl aq + Na2S2O3 aq 2NaCl aq + H2O l + SO2 g + S s When the concentration of either HCl or Na2S2O3 is increased, the number of particles of that reactant in the solution also increases. This leads to a higher probability of successful collisions between the reactant particles, resulting in an increased reaction rate.Conversely, if the concentration of either reactant is decreased, the number of particles in the solution decreases, leading to a lower probability of successful collisions and a decreased reaction rate.In summary, increasing the concentration of reactants in the chemical reaction between HCl and Na2S2O3 will increase the reaction rate, while decreasing the concentration will decrease the reaction rate. This relationship can be further quantified using the rate law and the reaction order with respect to each reactant.