When a solution containing thiosulfate ions, S2O32-, is acidified the following reaction occurs. S2O32-(aq) + 2H+(aq) → S(s) + SO2(g) + H₂O(l) The solid sulfur that is formed makes the mixture become cloudy. The rate of reaction can then be measured by timing how long it takes for the mixture to become too cloudy to see through. You will investigate how changing the concentration of the thiosulfate ions affects the rate of reaction. Throughout these experiments care must be taken to avoid inhaling the SO₂ that is produced. It is very important that as soon as each experiment is complete the contents of the beaker are emptied into the quenching bath. FA 4 is 2.00 mol dm⁻³ hydrochloric acid, HCl. FA 5 is a solution of sodium thiosulfate, Na2S2O3. distilled water

1. (a)Method Experiment 1 • Use the 50 cm³ measuring cylinder to transfer 40.0 cm³ of FA 5 into the 100 cm³ beaker. • Use the 25 cm³ measuring cylinder to measure 20.0 cm³ of FA 4. • Add the 20.0 cm³ of FA 4 to FA 5 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printed text on the insert from above through the mixture in the beaker. • Note the time when the print on the insert becomes obscured. • Record this reaction time to the nearest second. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker so it is ready for use in Experiment 2. Experiment 2 • Use the 50 cm³ measuring cylinder to transfer 20.0 cm³ of FA 5 into the 100 cm³ beaker. • Use the 50 cm³ measuring cylinder to transfer 20.0cm³ of distilled water into the same beaker. • Use the 25 cm³ measuring cylinder to measure 20.0 cm³ of FA 4. • Add the 20.0 cm³ of FA 4 to FA 5 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printed text on the insert from above through the mixture in the beaker. • Note the time when the print on the insert becomes obscured. • Record this reaction time to the nearest second. • Empty the contents of the beaker into the quenching bath. • Rinse the beaker thoroughly. Record all your results in a table. You should include the volume of FA 5, the volume of distilled water, the reaction time and the rate of reaction for both experiments. The rate of reaction can be calculated using the following formula. rate of reaction = 1000 / reaction time[4]
2. (b)A student suggested that the rate of the reaction is directly proportional to the concentration of the thiosulfate ions. State whether your results support this suggestion. Explain your answer.[1]
3. (c)The student's suggestion in (b) could be made more reliable by carrying out further experiments. Prepare a table to show three further experiments you could carry out. Show clearly the volumes of FA 4, FA 5 and distilled water that you would use in each of these experiments. Do not suggest a volume of FA 5 that is greater than 40.0 cm³ or less than 20.0 cm³. DO NOT CARRY OUT THESE ADDITIONAL EXPERIMENTS.[2]