8.2.7 Autocatalysis Experiments

Aim:

To investigate autocatalysis in the reaction between ethanedioic acid (oxalic acid) and acidified potassium manganate(VII), using potassium iodide and titration with sodium thiosulfate to track the concentration of unreacted manganate(VII) ions over time.

Introduction:

Autocatalysis occurs when one of the products of a reaction acts as a catalyst for the reaction itself, increasing the reaction rate as the product concentration builds up. In this experiment, $Mn²⁺$ ions act as the autocatalyst in the redox reaction between acidified ethanedioate ions ($C₂O₄²⁻$) and potassium manganate(VII) ($MnO₄⁻$).

The reaction is:

$$2MnO₄⁻(aq) + 16H⁺(aq) + 5C₂O₄²⁻(aq) \rightarrow 2Mn²⁺(aq) + 10CO₂(g) + 8H₂O(l)$$

By removing samples of the reaction mixture at regular intervals, the reaction can be "stopped" by adding potassium iodide. The unreacted manganate(VII) ions oxidise the iodide ions to iodine, which is then titrated with sodium thiosulfate to determine the concentration of manganate(VII) at different times.

Materials and Equipment:

• 0.02 mol dm⁻³ potassium manganate(VII) ($KMnO₄$) solution
• 0.2 mol dm⁻³ ethanedioic acid ($H₂C₂O₄$) solution
• 2.0 mol dm⁻³ sulfuric acid ($H₂SO₄$)
• 0.1 mol dm⁻³ potassium iodide ($KI$) solution
• 0.01 mol dm⁻³ sodium thiosulfate ($Na₂S₂O₃$) solution
• Starch indicator solution
• Distilled water
• 250 cm³ conical flasks (6)
• 250 cm³ beaker
• 10 cm³ measuring cylinder
• 50 cm³ burette
• Stopwatch

Method:

Step 1: Prepare the Reaction Mixture

1. Measure the following into a 250 cm³ beaker:

• 50.0 cm³ of 0.2 mol dm⁻³ ethanedioic acid
• 25.0 cm³ of 0.02 mol dm⁻³ potassium manganate(VII)
• 3.0 cm³ of 2.0 mol dm⁻³ sulfuric acid
• 47.0 cm³ of distilled water

2. Swirl the mixture gently to ensure proper mixing. Start the stopwatch immediately after mixing.

Step 2: Prepare the Conical Flasks

1. Measure 10.0 cm³ of 0.1 mol dm⁻³ potassium iodide (KI) solution into six 250 cm³ conical flasks.

Step 3: Sample the Reaction Mixture

1. After 30 seconds, use a 10 cm³ measuring cylinder to remove 10.0 cm³ of the reaction mixture from the beaker.
2. Immediately transfer this to the first conical flask containing potassium iodide. The reaction is stopped when the manganese(VII) ions oxidise the iodide ions to iodine.
3. Repeat this sampling every 30 seconds for a total of 180 seconds (6 samples).

Step 4: Titrate the Samples

1. Wash the burette with 0.01 mol dm⁻³ sodium thiosulfate solution, then fill it with the same solution.
2. Add 1 cm³ of starch solution to each conical flask as an indicator. The iodine will form a blue-black complex with starch.
3. Titrate each sample with the sodium thiosulfate solution:

• As you add the thiosulfate solution, the iodine reacts with it, turning the solution from blue-black to colourless.
• Record the volume of sodium thiosulfate (titre) required for each sample to reach the endpoint.

Results Table:

Time (s)	Volume of Na₂S₂O₃ (cm³)	Concentration of MnO₄⁻ (mol dm⁻³)
30	...	...
60	...	...
90	...	...
120	...	...
150	...	...
180	...	...

Note: The volume of sodium thiosulfate used in each titration is proportional to the concentration of manganate(VII) ions remaining at that time.

Results Analysis:

• The volume of sodium thiosulfate used in each titration reflects the amount of unreacted $MnO₄⁻$at the time of sampling.
• The reaction shows typical autocatalytic behaviour:
  • The rate of reaction starts slow, increases (as $Mn²⁺$builds up and autocatalyses the reaction), and then slows down again as the $MnO₄⁻$is used up.
• Plotting a rate graph of concentration of $MnO₄⁻$versus time shows a curve typical of autocatalysis, where the rate increases, and then decreases as the reaction proceeds.

Key Points to Remember:

1. Autocatalysis: The reaction is autocatalysed by $Mn²⁺$ ions, which are one of the products of the reaction. This means the reaction speeds up as more Mn²⁺ is produced.
2. Titration with Sodium Thiosulfate: The iodine produced by the reaction of remaining $MnO₄⁻$with iodide ions is titrated with sodium thiosulfate. The amount of thiosulfate required is proportional to the concentration of $MnO₄⁻$ at that point.
3. Rate Profile: In an autocatalytic reaction, the rate typically starts slow, speeds up as the catalyst ($Mn²⁺$) is produced, and then slows again as the reactants are used up.