The reaction between hydrochloric acid HCl and sodium thiosulphate Na2S2O3 is a classic example of a reaction that demonstrates the effect of varying the concentration of reactants on the reaction rate and efficiency. The reaction can be represented by the following equation:2HCl aq + Na2S2O3 aq 2NaCl aq + H2O l + SO2 g + S s When the concentration of one or both reactants HCl and Na2S2O3 is increased, the reaction rate will also increase. This is because there are more particles of the reactants in the solution, which leads to a higher probability of successful collisions between the particles, resulting in an increased rate of reaction.Conversely, when the concentration of one or both reactants is decreased, the reaction rate will decrease. This is due to the reduced number of particles in the solution, leading to a lower probability of successful collisions and a slower rate of reaction.The efficiency of the reaction, in this case, can be defined as the extent to which the reactants are converted into products. When the concentration of reactants is increased, the reaction will proceed faster, but the efficiency may not necessarily be affected. The efficiency of the reaction depends on factors such as the purity of the reactants, the presence of any catalysts, and the reaction conditions e.g., temperature and pressure .In summary, varying the concentration of reactants in the reaction between hydrochloric acid and sodium thiosulphate will have a direct impact on the reaction rate, with higher concentrations leading to faster reaction rates and lower concentrations leading to slower reaction rates. The efficiency of the reaction may not be directly affected by the concentration of reactants but is influenced by other factors such as reactant purity, catalysts, and reaction conditions.