AQA A Level Chemistry

Revision Notes

8.2.7 Autocatalysis Experiments

Autocatalysis Experiments

Autocatalysis Experiment – Titration with sodium thiosulfate
  • Autocatalysis occurs when one of the products catalyses the reaction
  • Potassium manganate(VII) will react with acidified ethanedioate ions as detailed by the following equation:
    • 2MnO4(aq) + 16H+(aq) + 5C2O42−(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l)
  • To determine the rate of reaction some of the reaction mixture can be removed, and any unreacted manganate(VII) ions can oxidise iodide ions to form free iodine, stopping the reaction shown above
  • The iodine formed can then be titrated with sodium thiosulfate, using starch as an indicator
  • This is the most common practical method of recording the rate of reaction for Mn²⁺ as the autocatalyst in the reaction between ethanedioic acid and acidified potassium manganate(VII)

Autocatalysis Experiment – Titration with sodium thiosulfate

  1. Measure 10.0 cm3 of 0.1 mol dm−3 potassium iodide solution into each of the six 250 cm3 conical flasks
  2. In a 250 cm3 beaker measure 50.0 cm3 of 0.2 mol dm−3 ethanedioic acid with 25.0 cm3 of 0.02 mol dm −3 potassium manganate(VII), 3.0 cm3 of 2.0 mol dm−3 sulfuric acid and 47.0 cm3 of distilled water
  3. Swirl the reaction mixture and start the stop watch
  4. After 30 seconds use the 10 cm3 measuring cylinder to remove 10.0 cm3 of reaction mixture and put into the first conical flask, this effectively stops the reaction in the mixture in the flask at this point
  5. Repeat every 30 seconds to remove 10.0 cm3 of the reaction mixture for 180 seconds
  6. Wash out the burette with 0.01 mol dm−3 sodium thiosulfate solution and then fill the burette with 0.01 mol dm−3 sodium thiosulfate solution
  7. Using 1 cm3 of starch solution as an indicator, titrate each of the reaction mixture samples with 0.01 mol dm−3 sodium thiosulfate solution and record the titre

Results Analysis

  • The volume of the titre of thiosulfate used in each titration will be proportional to the concentration of manganate(VII) ions in the reaction mixture at the time the reaction mixture was stopped (by adding it to the potassium iodide solution)
  • The rate of reaction is slow to start with, then speeds up, before slowing down and stopping

 

6.2.3 Rate graph- autocatalysis, downloadable AS & A Level Chemistry revision notes

Autocatalysis curve of time showing the varying rate speeds across the course of the reaction

Worked Example

Explain the shape of the autocatalysis curve.

Answer:

    • Initially the reaction is slow, however one of the products is Mn2+ which catalyses the reaction
    • Therefore, as the reaction continues more catalyst is released and the reaction reaches its maximum rate of reaction
    • Then as the concentration of potassium manganate(VII) continues to decrease, the rate also decreases

Autocatalysis Experiment – Colorimetry

  • Instead of monitoring this reaction by removing samples and titrating with sodium thiosulfate, colorimetry can be used
  • The absorption of light is dependent on the concentration of potassium manganate(VII), and some data loggers can be used in conjunction with a colorimeter producing a graph (absorption against time) directly without any calculation

Worked Example

State why it is possible to use a spectrometer to measure the concentration of the manganate(VII) ions in this reaction mixture, considering the properties of the reactants and products.

Answer:

    • Manganate(VII) ions are coloured purple
    • All other reactants and products are not coloured (or too faintly coloured to be detected)
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