Question 118 marks
Question
The thiosulfate ion, S2O32–, is unstable in the presence of acid. The following reaction occurs. S2O32–(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l) The rate of this reaction can be measured by timing how long it takes for the solid sulfur that is formed to make the mixture too cloudy to see through. You will investigate how the concentration of the thiosulfate ions affects the rate of this reaction. Throughout these experiments care must be taken to avoid inhaling any 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 and the beaker is rinsed thoroughly. FB 1 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. FB 2 is 2.00 mol dm–3 hydrochloric acid, HCl. distilled water (a) Method Experiment 1 ● Label one burette FB 1 and fill it with FB 1. ● Run 45.00 cm3 of FB 1 from the burette into the 100 cm3 beaker. ● Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FB 2. ● Add FB 2 to FB 1 and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the print on the insert from above the mixture. ● Stop timing when the print on the insert is no longer visible. ● 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 ● Fill the second burette with distilled water. ● Refill the burette labelled FB 1 with FB 1. ● Run 20.00 cm3 of FB 1 into the 100 cm3 beaker. ● Run 25.00 cm3 of distilled water into the same beaker. ● Use the 25 cm3 measuring cylinder to measure 10.0 cm3 of FB 2. ● Add FB 2 to the beaker and start timing immediately. ● Stir the mixture once and place the beaker on the printed insert. ● View the print on the insert from above the mixture. ● Stop timing when the print on the insert is no longer visible. ● 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 the next experiment. Experiments 3–5 ● Carry out three further experiments to investigate how the reaction time changes with different volumes of FB 1. The combined volume of FB 1 and distilled water must always be 45.00 cm3. Do not use a volume of FB 1 that is less than 20.00 cm3. Record all your results in a table. You should include the volume of FB 1, the volume of distilled water, the reaction time and the reaction rate for each of your five experiments. Calculate the rate of reaction using the following formula. 1000 rate = reaction time I II III IV V VI VII VIII [8] (b) On the grid opposite, plot the rate on the y-axis against the volume of FB 1 on the x-axis. Identify any anomalous points. Draw a line of best fit. I [4] II III IV (c) In these experiments, the volume of FB 1 is related to the concentration of the thiosulfate ions. Use your graph to suggest the relationship between the rate of reaction and the concentration of the thiosulfate ions. . …… . …… [1] (d) The quenching bath contains an aqueous mixture of sodium carbonate and universal indicator. (i) How does the quenching bath prevent the further production of SO2 from the reaction? . …… . …… [1] (ii) Suggest why the mixture contains universal indicator. . …… . …… [1] (e) (i) In each experiment the acid is in large excess. Show, by calculation, that the acid is in large excess in Experiment 1. [2] (ii) Suggest a reason why the acid used should be in large excess. . …… . …… [1] [Total: 18]
Mark scheme
1(a)(i) I Constructs a single 4 × 5 table for results for 5 experiments 1 AND at least one experiment attempted II Correct headings and units for data given. 1 • volume of FB1 and in cm3 or / cm3 or (cm3) • volume of water and in cm3 or / cm3 or (cm3) • time and in seconds or / s or (s) • rate and / s–1 or (s–1) III All times recorded to the nearest second 1 AND all volumes given to nearest 0.05 IV 3 additional volumes chosen with intervals not less than 5.00 cm3 1 AND all additional volumes of FB 1 ⩾ 25.00 cm3 AND In all 3 additional experiments water is added to make a total of 45.00 cm3 V All rates correctly calculated using 1000 / time 1 AND all recorded to the same number of dp or same number of sf. VI All five times increase with decreasing volume of FB 1 1 VII & VIII Compare ratio of time for 20.00 cm3 of FB 1 / time for 45.00 cm3 of FB 1 to 2 dp. 2 Award VII for ratio 2.10 – 2.60 Award VIII for ratio 2.20 − 2.50 1(b) I Rate on y axis and volume of FB 1 on the x axis 1 With correct labels and units. Do not penalise missing / incorrect unit if penalised in 1(a) II (ecf) II Linear scales chosen so that the graph occupies more than half the available length for both axes (7 × 5 big squares) 1 III All points recorded plotted correctly to within half a small square and in the correct square or on the line if it should be 1 on the line and not on the line if it shouldn’t. IV Draws a line of best fit. 1 This can be a straight line or smooth curve. 1(c) Rate is proportional to concentration of thiosulfate 1 1(d)(i) (Carbonate/ quenching bath) removes H+ / neutralises acidic solution (so reaction stops). 1 1(d)(ii) To know when 1 all the carbonate has been neutralised / no more carbonate (solution) remains / more carbonate needs to be added OR mixture becomes acidic / pH drops below 7 / pH is no longer ⩾ 7 1(e)(i) M1: moles of H+ = 2(.00) × 10–2 2 AND moles of S2O32– = 4.5(0) × 10–3 M2: 2(.00) x 10–2 > 2 × 4.5(0) × 10–3 / 9(.0) × 10–3 Other valid methods exist (e.g., comparison of volume of acid required to react and volume used). 1(e)(ii) So acid concentration remains (almost) constant throughout the experiment 1 OR only the conc of thiosulfate affects the rate.