Experiment 7: The Clock Reaction In Experiment Five you observed several fascinating chemical reactions, most of which seemed to occur almost instantaneously. In today s experiment, you will carry out the socalled Iodine Clock reaction, which relies on a color change to indicate that the reaction has completed. Several different factors have can have an effect on the speed of this reaction; we will attempt to test each of these factors to determine which speed up and which slow down a given reaction. The chemical reaction is rather complex; it can be summarized (in words) as iodate ion + bisulfite ion iodine + sulfate ion + water + hydronium ion. Although you may not understand the details of this reaction, you will be able to observe how changing aspects of the reaction might cause the reaction to go more quickly or more slowly. We will be using a starch solution as an indicator in this reaction; its presence causes a color change at the end of the reaction which would not be observed otherwise. We will study indicators more in depth in later experiments. Procedure Note: You do not have to exclusively use 50 ml beakers throughout this experiment; you may substitute other sizes if necessary. A. The Effect of a Catalyst on Reaction Rate Test One With Catalyst 1. Pour 20 ml of 0.5% potassium iodate (KIO 3 ) solution into a 50-mL beaker, and add 8 drops of copper (II) sulfate (CuSO 4 ) solution (the catalyst) to the beaker. 2. Pour 20 ml of 0.2% sodium bisulfite (NaHSO 3 ) solution into a second 50-mL beaker, and add 5 drops of starch solution to the beaker. 3. Record the start time shown on your stopwatch. 4. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. Test Two Without Catalyst Repeat the steps of the previous part; however, on step one, do not add the copper (II) sulfate solution. The reaction should proceed more slowly; however, we have not observed this to consistently Page 7-1
B. Effect of Concentration on Reaction Rate Test 1 1. Pour 20 ml of 1.0% potassium iodate solution into a 50-mL beaker, and add 8 drops of copper (II) sulfate solution to the beaker. 2. Pour 20 ml of 0.2% sodium bisulfite solution into a second 50-mL beaker, and add 5 drops of starch solution to the beaker. 3. Record the start time shown on the stopwatch. 4. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. Test 2 1. Pour 20 ml of 0.5% potassium iodate solution into a 50-mL beaker, and add 8 drops of copper (II) sulfate solution to the beaker. 2. Pour 20 ml of 0.4% sodium bisulfite solution into a second 50-mL beaker, and add 5 drops of starch solution to the beaker. 3. Record the start time shown on your stopwatch. 4. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. Test 3 1. Pour 20 ml of 1.0% potassium iodate solution into a 50-mL beaker, and add 8 drops of copper (II) sulfate solution to the beaker. 2. Pour 20 ml of 0.4% M sodium bisulfite solution into to a second 50-mL beaker, and add 5 drops of starch solution to the beaker. 3. Record the start time shown on your stopwatch. 4. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. Page 7-2
C. Effect of Temperature on Reaction Rate Test 1: Below Room Temperature 1. Determine the ambient air temperature in the laboratory and record it in column 2, row 2, of the Part C data table. 2. Fill a 500-mL beaker about one-fourth full with crushed ice, and add about 300 ml of water. (It is fine to "eyeball" how much water you add. The beaker should be about threefourths full after you add the water.) 3. Pour 20 ml of 0.5% potassium iodate solution into a 50-mL beaker, and add 8 drops of copper (II) sulfate to the beaker. 4. Pour 20 ml of 0.2% sodium bisulfite solution into a second 50-mL beaker, and add 5 drops of starch solution to the beaker. 5. Place both beakers in the shallow tray, and carefully pour ice water into the tray until the water level in the tray is about even with the liquid level in the beakers. Leave the beakers in this ice bath until the solutions reach a temperature of about 10 C. Record the temperature of the solutions in column 2, row 3, of the Part C data table. 6. Record the start time shown on your stopwatch. 6. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. 7. Record the stop time and calculate the amount of time required for the color change to Test 2 Above Room Temperature 1. In a 500-mL beaker, heat about 300 ml of water to about 65 C. 2. Repeat steps 3 and 4 of Part C Test 1. 3. Place both beakers in the shallow tray, and carefully pour the hot water into the tray until the water level in the tray is about even with the liquid level in the beakers. Leave the beakers in this water bath until the solutions reach a temperature of about 40 C. Record the temperature of the solutions in column 2, row 4, of the Part C data table. 4. Record the start time shown on your stopwatch. 4. Start the stopwatch as you pour the contents of the first beaker into the second beaker, stirring the mixed solutions continuously. Stop the stopwatch when the reaction mixture turns deep blue. Page 7-3
Page 7-4
Report Sheet The Clock Reaction Name: Data Tables Part A: Effect of Catalyst on Reaction Rate Test Catalyst Added? Amount of Time Required (sec) 1 Yes 2 No Part B: Effect of Concentration on Reaction Rate Test % KIO 3 % NaHSO 3 Amount of Time Required (sec) Part A, Test 1 0.5% 0.2% 1 1.0 % 0.2 % 2 0.5 % 0.4 % 3 1.0% 0.4 % Part C: Effect of Temperature on Reaction Rate Test Temperature (ºC) Amount of Time Required (sec) Part A, Test 1 Room: Below Room Temp Above Room Temp Conclusions How is the rate of the reaction affected by the presence of copper (II) sulfate? Page 7-5
In many reactions, the use of a catalyst speeds the reaction up by a tremendous amount. Based on your data, do you think it was essential to use the catalyst in Parts B and C? Explain Briefly. How is the rate affected by doubling the concentration of KIO 3 NaHSO 3 What appears to happen to the reaction rate when the concentration of both of these chemicals is doubled? How is the reaction rate affected by lowering and raising the temperature? Try to explain these results based on differences at the molecular level. Page 7-6
Preparation Required for This Lab stopwatches ice Each in three regular bottles without droppers: 1.0% KIO 3 solution (about 10.28 grams KIO 3 per 1 L water) about 50 ml/student 0.5% KIO 3 solution (dilute 1.0% solution by a factor of 2) at least 100 ml/ student 0.4% NaHSO 3 solution (about 3.33 grams NaHSO 3 per 1 L water ) about 50 ml/student 0.2% NaHSO 3 solution (dilute 0.4% solution by a factor of 2) at least 100 ml/student In three dropper bottles, about 50-100 ml per bottle: 0.010 M CuSO 4 solution 1% starch solution Page 7-7