2 The reaction between thiosulfate ions and hydrogen ions produces a precipitate of sulfur. When the concentration of hydrogen ions, [H+(aq)], is kept constant, the rate is proportional to one of the following: • the concentration of thiosulfate ions, [S2O32-(aq)] • the square of the concentration of thiosulfate ions, [S2O32-(aq)]2. You will determine which relationship is correct by mixing solutions of sodium thiosulfate and sulfuric acid. You will measure the time taken for a fixed amount of sulfur to be precipitated. Throughout these experiments care must be taken to avoid inhaling any SO2 gas 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 and the beaker is rinsed thoroughly. FA 3 is 1.00 mol dm⁻³ sulfuric acid, H2SO4· FA 4 is 0.100 moldm⁻³ sodium thiosulfate, Na2S2O3.

1. (a)Method Experiment 1 • Use the 50 cm³ measuring cylinder to transfer 50.0 cm³ of FA 4 into the 100cm³ beaker. • Use the 25 cm³ measuring cylinder to measure 10.0 cm³ of FA 3. • Add FA 3 to FA 4 in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in Table 2.1. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready for use in Experiment 2. Experiment 2 • Use the 50 cm³ measuring cylinder to transfer 30.0cm³ of FA 4 into the 100cm³ beaker. • Use the same measuring cylinder to add 20.0cm³ of distilled water to the same beaker. • Use the 25 cm³ measuring cylinder to measure 10.0 cm³ of FA 3. • Add FA 3 to the mixture of FA 4 and distilled water in the beaker and start timing immediately. • Stir the mixture once and place the beaker on the printed insert. • View the printing on the insert from above, through the solution. • Stop timing when the print on the insert becomes obscured. • Record this reaction time to the nearest second in Table 2.1. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready for use in Experiment 3. Experiment 3 • Carry out one further experiment to investigate how the reaction time changes with a different volume of FA 4. Do not use a volume of FA 4 that is less than 20.0 cm³. Record your results in Table 2.1. Table 2.1 volume/cm³ experiment FA 3 FA 4 distilled water reaction time/s 1 10.0 50.0 0 2 10.0 30.0 20.0 3 [4][4]
2. (b)(i)Complete Table 2.2. Use the following expression to calculate the rate. rate = 1000 / reaction time Table 2.2 experiment concentration of FA 4/mol dm-3 rate/s-1 1 2 3 [2][2]
3. (b)(ii)Use data from two of your experiments to deduce which relationship for the rate is correct. Place a tick next to the statement in Table 2.3 which most accurately fits your results. Show your working. Table 2.3 The rate is proportional to [S2O32-(aq)]. The rate is proportional to [S2O32-(aq)]2. [2][2]
4. (c)The uncertainty in a 50cm³ measuring cylinder is ±0.5cm³. Calculate the maximum percentage error in the volume of FA 4 in Experiment 2. maximum percentage error in volume of FA 4 = ..........................% [1][1]
5. (d)Suggest improvements to your method and the processing of the results that would allow you to make a more reliable conclusion in (b)(ii). Do not suggest changes to the apparatus or to the concentrations of FA 3 and FA 4. Do not carry out any of your suggestions. [3][3]

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