$$Na_2S_2O_3(aq) + HCl(aq) → S(g) + SO_2(g) + 2NaCl$$
I. The reaction of 50.0mL of 0.1M sodium thiosulfate and 5.0mL 2M $HCl$ will go to completion more quickly than the reaction of 50.0mL of 0.05M sodium thiosulfate and 5.0mL 2M $HCl$.
II. Higher concentrations of reactants cause the frequency of effective collisions to increase.