The thiosulfate ion, S2O3²⁻, decomposes when an acid is added. S2O3²⁻(aq) + 2H⁺(aq) → S(s) + SO2(aq) + H2O(l) The rate of this reaction can be investigated by measuring how long it takes the solution to produce enough sulfur so that it cannot be seen through. You will investigate how the concentration of the thiosulfate ion affects the rate of the reaction. Note: A small amount of sulfur dioxide gas may be formed in the experiment. It is very important that you avoid inhaling any fumes. As soon as each experiment is complete, empty the reaction mixture into the quenching bath and rinse the beaker thoroughly. FA 1 is 0.100 moldm⁻³ sodium thiosulfate, Na2S2O3. FA 2 is 2.00 moldm⁻³ hydrochloric acid, HCl.

1. (a)**Method** Experiment 1 • Fill a burette with FA 1. • Run 40.00 cm³ of FA 1 into the 100 cm³ beaker. • Use the 25 cm³ measuring cylinder to measure 10.0 cm³ of FA 2. • Add the FA 2 to the FA 1 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 the space for results on page 4. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in Experiment 2. Experiment 2 • Refill the burette with FA 1. • Fill the other burette with distilled water. • Run 20.00 cm³ of FA 1 into the 100 cm³ beaker. • Run 20.00 cm³ of distilled water into the same beaker. • Use the 25 cm³ measuring cylinder to measure 10.0 cm³ of FA 2. • Add the FA 2 to the solution 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. • Empty the contents of the beaker into the quenching bath. • Rinse and dry the beaker and glass rod so they are ready to use in the next experiment. Experiments 3-5 • Carry out three further experiments to investigate how using different volumes of FA 1 affects the reaction time. Note that the combined volumes of FA 1 and distilled water must always be 40.00cm³. Do not use a volume of FA 1 that is less than 15.00 cm³. Record all your results in a table. You should include the volume of FA 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. The rate of reaction can be calculated using the following formula. rate = 1000 / reaction time **Results** [Empty table with rows I to VIII on page 4 of the question paper][8]
2. (b)On the grid, plot the rate (y-axis) against the volume of FA 1 (x-axis). Start each axis at the origin (0,0). Ring any anomalous points. Draw a line of best fit. [Empty grid with rows I to IV on page 5 of the question paper][4]
3. (c)In these experiments, the volume of FA 1 is a measure of the concentration of thiosulfate ions. A student suggested that the graph shows that the rate of reaction is directly proportional to the concentration of thiosulfate ions. Explain, using your graph, whether you agree with this student.[1]
4. (d)Use your graph to calculate the time you would expect to record if you had used 12.50 cm³ of FA 1 and followed the same method. Show clearly on the graph how you calculated this time. time = .............................................. s[2]
5. (e)Another student broke the beaker and decided to use a Petri dish instead. [Figure 1.1] What effect, if any, would this have on the times measured in the experiment in (a)? Explain your answer.[2]

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