An oscillating reaction
Objective: Introduce the concept of oscillating reaction and reaction mechanism
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Laboratory materials
250 mL beaker
Magnet
Stirring plate
Spatula
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Reagents
Sulfuric acid (H2SO4), 100 mL
Malonic acid (CH2(COOH)2), 1.1 g
Potassium bromate (KBrO3), 1 g
Manganese(II) sulfate dihydrate (MnSO4·H2O), 0.22 g
Deionized water
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Safety
Don't forget the gloves, lab coat, and safety goggles!!!
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Questions
What is an oscillating reaction?
What do you observe when the manganese sulphate is added?
Procedure
- Add 100 mL of the sulfuric acid solution to a 250 mL precipitate jar and whirl it to create a vortex.
- Then, add malonic acid and potassium bromate. Once dissolved, incorporate manganese(II) sulfate and observe what happens over the course of 5 minutes.
- Note: To improve visualization, use a white background.
Theoretical explanation
When manganese(II) sulfate is added, the solution immediately turns red, which disappears after a minute. This process repeats over time with an increasing oscillation period.
The oscillation is caused by two autocatalytic steps. Bromate ions oxidize malonic acid to carbon dioxide. The reaction is catalyzed by manganese(II) ions. Upon combining the reactants and the catalyst, the reaction mixture oscillates between red (corresponding to bromine, which is an intermediate in the reaction) and colorless in a period of about 20 seconds.
This is one of the simplest oscillating reactions to demonstrate. It’s an example of a type of reaction called “Belousov-Zhabotinsky reactions” (BZ).
The reaction that takes place is:
3 CH2(COOH)2 + 4 BrO3– 4 Br– + 9 CO2 + 6 H2O
