RP3 - Rate of reaction with temperature

• The initial rate of the reaction between sodium thiosulfate and hydrochloric acid can be monitored by measuring the time taken for a fixed amount of sulfur to be produced.

  Describe an experiment to investigate the effect of temperature on the initial rate of this reaction. [6 marks] 2019 Paper 3

  • Include
  • • a brief outline of your method
  • Stage 1 Method
  • (1a) Idea of using disappearing cross or colorimetry
  • (1b) Puts acid or thiosulfate into container on/with cross or in colorimeter
  • (1c) Add second reactant and start timing
• Describe an experiment to investigate the effect of temperature on the initial rate of this reaction.

  • how you will measure the time taken for a fixed amount of sulfur to be formed

  • Stage 2 Measurements
  • (2a) Repeat at different temperatures (if number of temperatures stated, must be more than two)
  • (2b) Record time, t, for cross to disappear / defined reading on colorimeter
  • (2c) Idea of ensuring other variables (cross, volumes, concentrations) kept constant (apart from T)
• Stage 3 Use of Results

  • (3a) 1/t (or 1000/time, etc) is a measure of rate
  • (3b) plot of rate (or 1/t etc) (y-axis) against T (x-axis) can come from labelled axes on sketch (IGNORE T against rate)
  • (3c) sketch of plot as shown (ALLOW 3c if axes not labelled but NOT if incorrectly labelled)

  A) Rate

  B) T

  
• The student noticed that the temperature of each reaction mixture decreased during each experiment. Suggest how the student calculated the temperature values in Table 1. [1 mark]

  • Measure the temperature at the start and end of the reaction AND find the mean/average
  • Allow idea of doing reaction in a water bath
• The student uses a stopwatch to measure the time. The stopwatch shows each time to the nearest 0.01 s

  Suggest why the student records the times to the nearest second and not to the nearest 0.01 s [1 mark]

  • Idea that it is hard to judge to the nearest 0.01 second
  • Idea that it is hard to judge the exact moment (that it becomes too cloudy / the cross disappears)
  • OR idea of reaction time
• Suggest, by considering the products of this reaction, why small amounts of reactants are used in this experiment. [1 mark]

  • as it forms a toxic gas
  • or SO2 is toxic/poisonous or
  • to limit amount of SO2 formed
• The student could do the experiment at lower temperatures using an ice bath. Suggest why the student chose not to carry out experiments at temperatures in the range 1–10 °C [1 mark]

  • reaction would take too long / too slow / take a long time / very slow
  • Allow idea that it makes judging the moment when the cross disappears more difficult