Autocatalysis Experiments (AQA A Level Chemistry)

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

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

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