The aim of this experiment is to find the effect of varying the concentration of iodide ions on the rate of reaction between hydrogen peroxide and an acidified solution of potassium iodide: H2O2(aq) + 2H+(aq) + 2I⁻ → 2H2O(l) + I2(aq) The course of this reaction can be followed by carrying it out in the presence of small quantities of starch and sodium thiosulfate solutions. As the iodine molecules are produced they immediately react with the thiosulfate ions and are converted back to iodide ions: I2(aq) + 2S2O32⁻(aq) → 2I⁻(aq) + S4O62⁻(aq) During this period the reaction mixture remains colourless. But once the thiosulfate ions have been used up, a blue/black colour suddenly appears because the iodine molecules now get the chance to react with the starch. A series of experiments will be carried out in which only the concentration of the iodide ions will be varied. the concentration and volumes of the other chemicals involved will be kept constant as well as the temperature at which the experiments are performed. Since the amount of thiosulfate ions initially present will be the same in each experiment, the appearance of the blue/black colour will always represent the same extent of the reaction. So it t is the time taken for the blue/black colour to appear then we can take 1/t as a measure of the reaction rate*. Apparatus and Chemicals
selection of syringes 100 cm3 glass beakers white tile timer distilled water
1 mol dm⁻3 sulfuric acid 0.1mol dm⁻3 potassium iodide 0.1mol dm⁻3 hydrogen peroxide 0.005 mol dm⁻3 sodium thiosulfate 1% starch
Hazards: Both the sulfuric acid and hydrogen peroxide solutions irritate the eyes. Care: Wear eye protection. If any chemical splashes on your skin, wash it off immediately. When using
the syringes always keep them pointing downwards.
Procedure 1. Using syringes make up the following mixtures in five dry 100 cm3 beakers.
Mixture Volume of sulfuric acid/cm3 Volume of sodium thiosulfate/cm3 Volume of starch/cm3 Volume of potassium iodide/cm3 Volume of water/cm3
1 10 10 1 25 0
2 10 10 1 20 5
3 10 10 1 15 10
4 10 10 1 10 15
5 10 10 1 5 20
2. Place the beaker containing the mixture 1 on the white tile. 3. Measure 5 cm3 of hydrogen peroxide into a syringe. Add it to mixture 1 as quickly as possible and at the same time start the timer. 4. Carefully swirl the reaction mixture in the beaker from time to time. When the blue/black colour just appears stop the timer and record the time (in seconds). 5. Repeat steps 2 to 4 with each of the remaining solutions Note: Since the total volume of the reaction mixture was the same in each experiment we can assume that the volume of the potassium iodide solution is a measure of its concentration**. DATA COLLECTION Record your results in the most appropriate way. DATA PROCESSING AND PRESENTATION 1. For each set of results, calculate the value of 1/time. (This is a measure of the rate of reaction*). 2. Plot a graph of rate against volume of potassium iodide. (Remember that volume is a measure of concentration here**).
CONCLUSION AND EVALUATION 1. Give a valid conclusion to this experiment with an explanation. 2. Describe any sources of error in your experiment. 3. Describe ways of improving your experiment in order to reduce the errors you mentioned above. Data Collection: Data Table 1:The Time Taken for the Mixture to Turn Blue/Black Once Potassium Iodide is Added Volume of Potassium Iodide: (± 0.05 cm3) Mixture 1 (25cm3) Mixture 2 (20cm3) Mixture 3 (15cm3) Mixture 4 (10cm3) Mixture 5 (5cm3) 18.82 22.78 30.10 45.90 87.94 17.59 22.19 29.03 43.09 85.93 16.63 23.13 28.97 43.59 49.97 17.68 22.70 29.37 44.19 74.61 Trial 1 Time (± 0.01 sec) Trial 2 Time (± 0.01 sec) Trial 3 Time (± 0.01 sec) Average Time (± 0.01 sec) Rate of Reaction (s−1)
0.05656 0.04405 0.03405 0.02263 0.01340
Sample Calculation: Average Time: eg. Rate of eg.
Trial 1 + Trial 2 + Trial 3...