AP Chemistry The Rate of an Iodine Clock Reaction Purpose In this experiment you will determine the rate law for an iodine clock reaction In an iodine clock reaction the completion of the reaction is signaled by the sudden appearance of the dark color characteristic of the reaction of I2 with starch The rate of an iodine clock reaction depends on the concentration of the reactants As a result the time required for the appearance of the dark color can be adjusted by manipulating the concentration of the reactants S2O82 2I 2SO42 I2 The form of the rate law is Rate k S2O82 q I r The object of the experiment is to determine the values for the rate constant k and the reaction orders q and r Several unfamiliar sulfur bearing ions appear in this experiment They are S2O82 Persulfate ion S2O32 Thiosulfate ion S4O62 Tetrathionate ion The rate at which persulfate disappears S2O82 is determined indirectly The black color won t appear until I2 can accumulate The rate of the reaction is given by Rate S2O82 t It will be necessary to know or measure S2O82 the initial change in concentration of S2O82 ions and t the time elapsed during the change as well as the initial concentration of S2O82 and I ions A way to obtain the S2O82 by coupling another reaction to the one we are studying The new reaction is the reduction of I2 by Na2S2O3 sodium thiosulfate I2 2S2O32 2I S4O62 This reaction consumes the I2 as fast as it s formed Only when S2O32 is gone will the I2 accumulate and the reaction complete Because of the stoichiometry of the above reactions every mole of thiosulfate consumes the iodine produced by half a mole of persulfate S2O82 1 2 S2O32 1 and S2O32 S2O32 initial S2O32 final Since S2O32 final is zero all the S2O32 is consumed S2O32 S2O32 initial And S2O82 1 2 S2O32 initial The new reaction is fast There can be no I2 present as long as there are S2O32 ions present However these ions are being consumed in the second reaction As soon as all the S2O32 ions have reacted I2 from the first reaction begins to accumulate in solution The sudden presence of I2 can be detected by the sudden appearance of the dark color from the reaction of the iodine with starch which is present in the reaction mixture The rate of the reaction is affected by the concentration of the reactants as would be expected from the rate law as well as the overall concentration of ions in the solution In order to maintain a constant ionic environment unreactive ionic substances will be added where necessary You will do three experiments in three separate containers each with a different concentration of one of the reactants in accordance with the initial rates method Tabulating this data will allow us to determine the exponents of the rate law values of q and r and the rate constant k Procedure 1 Obtain 3 serum bottles with stoppers a Mohr graduated pipette a pipette pump and a thermometer and a stopwatch Create a DATA TABLE for the three experiments 2 Record the temperature of the laboratory to the nearest 0 1 degree C 3 Mark each serum bottle with a number 1 2 or 3 4 Rinse the pipette with two 1 mL portions of the NaI solution Discard each of these portions 5 Using the table below pipette the appropriate amounts of each solution into each serum bottle Rinse the pipette with distilled water and the solution to be measured as in step 4 between dispensing different solutions DO NOT ADD THE LAST SOLUTION K2S2O8 UNTIL YOU ARE READY TO BEGIN TIMING MIXING TABLE Volumes of stock solutions in mL Expt 0 20M 0 20M 0 010M 2 starch 0 20M 0 20M NaI NaCl Na2S2O3 K2SO4 K2S2O8 1 2 0 2 0 2 0 1 0 2 0 2 0 2 2 0 2 0 2 0 1 0 0 4 0 3 4 0 0 2 0 1 0 2 0 2 0 2 6 Prepare the stopwatch to begin timing Fill the pipette with the amount of K2S2O8 required for experiment 1 Add the K2S2O8 solution immediately start the stopwatch and swirl the bottle to mix the solution thoroughly Place the bottle on a white piece of paper on the bench top 7 Note and record the exact time the dark color FIRST appears BE ALERT this happens very fast a minute or two and very suddenly 8 Repeat steps 6 and 7 with bottles 2 and 3 using the volumes of K2S2O8 specified in the mixing table Analysis 1 Calculate the initial concentration of persulfate ion and iodide ion in the three experiments Remember that dilution occurred when the reaction mixtures were prepared Example 2 mL NaI stock X 0 20M NaI stock 0 036M NaI 0 036M I11 mL mixture 2 Calculate the initial rates for the three experiments in bottles 1 2 and 3 Use Rate S2O82 t Give correct units 3 Obtain the reaction orders for the persulfate and iodide ions If you obtain non integer values for reaction orders round to the appropriate number of significant figures 4 What is the correct rate law 5 Calculate the rate constant k in the three experiments and obtain the mean Give correct units 3