The effect of varying the concentration of reactants on the rate of the reaction between sodium thiosulphate 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 the reacting particles and the energy with which they collide.When the concentration of either sodium thiosulphate 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 reacting particles, resulting in an increased reaction rate.In the reaction between sodium thiosulphate and hydrochloric acid, the products formed are sodium chloride NaCl , sulfur dioxide SO2 , sulfur S , and water H2O :Na2S2O3 aq + 2HCl aq 2NaCl aq + SO2 g + S s + H2O l The reaction efficiency can be affected by the rate of the reaction. A faster reaction rate may lead to a more efficient reaction, as the reactants are converted to products more quickly. However, it is essential to maintain a balance between the reaction rate and the control of the reaction, as an excessively fast reaction may lead to side reactions or difficulty in controlling the reaction conditions.In summary, increasing the concentration of either sodium thiosulphate or hydrochloric acid will increase the rate of the reaction between them. This can lead to a more efficient reaction, provided that the reaction conditions are controlled appropriately.