The effect of varying reactant concentrations on the reaction rate and efficiency in the chemical reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 can be explained using the collision theory. The collision theory states that for a reaction to occur, the reactant particles must collide with each other with sufficient energy and proper orientation. The reaction between HCl and Na2S2O3 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 collisions between the reactant particles, resulting in an increased reaction rate.In terms of efficiency, a higher concentration of reactants may lead to a faster reaction, but it does not necessarily mean that the reaction will be more efficient. Efficiency is typically measured by the percentage yield or the selectivity of the desired product. In this case, the reaction between HCl and Na2S2O3 is a simple reaction with only one set of products, so the efficiency is not significantly affected by the concentration of the reactants.To summarize, increasing the concentration of either hydrochloric acid or sodium thiosulfate in the reaction will increase the reaction rate due to a higher probability of successful collisions between reactant particles, as explained by the collision theory. However, the efficiency of the reaction remains relatively constant as it is not significantly affected by the concentration of the reactants.