K-1 KINETICS SEMI-MICRO SULFUR CLOCK REACTION A. INTRODUCTION: The purpose of this experiment is to determine the effect of concentration upon the rate of reaction. It is known that the thiosulfate ion, S2O32-(aq) decomposes in the presence of hydrogen ion in the following manner: S2O32-(aq) + 2 H+ -------> S(s) + H2SO3(aq) When sulfur is produced, it makes the solution opaque to a mark located below the reaction vessel (in this case a 24 well semi-micro plate). If one starts timing the reaction as soon as the acid has been added, and peers through the solution at a mark on the underside of the well and stops timing as soon as it is no longer possible to see the mark, this is a good measure of the rate of the reaction. BEFORE YOU BEGIN SAFETY AND WASTE DISPOSAL 1. Wear safety goggles at all times. 2. HCl is corrosive. Wash affected areas with water and notify your teacher. If HCl gets into your eyes flush with water for 20 minutes. 3. Dispose of all wastes as directed by the instructor. EQUIPMENT 1-24 Well semi-micro plate, 1-10 mL graduated cylinder, timers, cotton swabs, 3-Beral pipettes (ungraduated), 1- toothpick (stirring rod) MATERIALS Distilled water, 0.32 M Na2S2O3(aq), 0.2 M HCl(aq) B. Procedure 1. Using a Beral pipette, add 30 drops, 25 drops, 20, drops, 15 drops, and 10 drops of 0.320 M thiosulfate solution to each of 5 separate wells (see table). Now starting with the well with 25 drops of thiosulfate, add 5, then 10, then 15, then 20 drops of water to each well in sequence. This maintains the volume at 30 drops. 2. Put the plate on a piece of white paper and mark the paper with distinct "dots" such that, when the plate is placed over them, each of the wells will have a dot visible in the center. 3. BE PREPARED TO TIME THE FOLLOWING AS SOON AS THE FIRST DROP IS ADDED. 4. With the third Beral pipette, add 10 drops of HCl solution to the first well and stir with the toothpick. Time until the "dot" is not visible in the bottom of the well. Do the same with each of the other wells in succession. Record your results in the table provided. 5. If time permits, repeat the procedure as a check on your first run. 6. Clean out the plates using water and a cotton swab. THANKS. K-1 KINETICS Semi-Micro Sulfur Clock Reaction Name_____________________________________ Hour_____ Date__________ DATA Data Table 1 Well # Drops of Drops of Drops of Run #1 Run #2 Thiosulfate Water HCl Time, sec Time, sec 1 30 0 10 2 25 5 10 3 20 10 10 4 15 15 10 5 10 20 10 Data Table 2 Well # Concentration of Thiosulfate Time (sec) (Molarity) [Average if done twice.] Graph Construct a graph in which the time of reaction (X AXIS) is plotted against the concentration of the thiosulfate ion concentration (Y AXIS): A graphical analysis program for a computer should be used if possible. QUESTIONS 1. What relationship did you discover between the concentration of the thiosulfate ion and the time of this reaction to occur? 2. Compare your graph with the graph and discussion that is found in your text. What do you find? 3. What effect do you think it would have on the reaction if the number of drops of HCl were increased to 15? decreased to 5? WHY? (In fact, try it if you have time.) 4. What effect would increasing and decreasing the temperature of the mixture have on the reaction rate? WHY?