B-Z Oscillating Reaction (Cerium Catalyzed)
- Page ID
- 127586
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Required Training |
Required PPE |
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UC Lab Safety Fundamentals |
Lab coat, safety glasses/goggles, nitrile gloves |
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Equipment – Large Scale |
Chemicals – Large Scale |
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2 L Graduated cylinder |
500mL bottles of solutions A, B, C, and D prepared by the dispensary (fresh within 6 months): |
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A: 0.125 M KBrO3 (20.9 g/L) in 3N H2SO4 B: 0.55 M Malonic acid (57.2 g/L) in 3N H2SO4 |
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C: 0.2 M Ceric ammonium nitrate (11.0 g/L) in 3N H2SO4 |
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D: 0.025 M Ferroin indicator (Sigma 318922 or 1.735 g 1,10-phenanthroline dehydrate with 695 mg FeSO4*7H2O in 1L DI) |
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Equipment – Small Scale |
Chemicals – Small Scale |
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Medium sized petri dish |
Solutions same as above |
Procedure:
- Large scale
- Pour 500 mL solution A and 500 mL solution B into the large graduated cylinder.
- Stir for a brief moment and ensure the solution is clear.
- Add 10 mL solution C and 30 mL solution D.
- Stir.
- After about a minute, the color oscillation will start with the green solution turning blue, then violet, then red, before quickly turning back to green.
- This oscillating reaction will continue for at least 20 minutes, often up to an hour with its frequency slowing over time.
- Small Scale
- Pipet 5 mL solution A, 5 mL solution B, 0.1 mL solution C, and 0.3 mL solution D into a petri dish.
- Swirl the dish to coat the entire bottom and leave sitting on the projector for about 5 minutes, undisturbed.
- Turn the projector on and note the slowly moving diffusion fronts as the colors oscillate. A few wave centers will show a clean oscillation pattern, but bulk color changes also make this scale less predictable.
Discussion:
This reaction was first discovered in 1958 by Belousov and was not readily accepted. Zhabotinsky studied this reaction further and discovered that citric or malonic acids could be replaced with any number of carboxylic acids of the form R-CO-CH2-COOH. The overall reaction is the cerium catalyzed oxidation of malonic acid by bromate ions in dilute sulfuric acid. The bromate ions are reduced to bromide ions, while the malonic acid is oxidized to carbon dioxide and water. The overall reaction can be simplified to:
3 CH2(CO2H)2 + 4 BrO3- → 4 Br- + 9CO2 + 6H2O
It is far more interesting how we get from products to reactants, however. Simplifying multiple reactions into processes makes this easier to understand. Process #1 involves ions, and the steps are two-electron transfers. Process #2 involves radicals and one-electron transfers. Which process is dominant at any particular time is dictated by the concentration of bromide ions. Process #1 is favored at high bromide concentrations, while process #2 is favored at low bromide concentrations. Oscillations occur because process #1 consumes bromide ions, thus leading to favorable conditions for process #2. Inversely, process #2 indirectly liberates bromide ions, leading to more favored conditions for process #1. Process #1 has the net reaction:
BrO3- + 5 Br- + 6 H+ → 3 Br2 + 3H2O
The amber red color developed is due to the elemental bromine produced. This color disappears when it reacts with malonic acid in the following reaction:
Br2 + CH2(CO2H)2 → BrCH(CO2H)2 +Br- + H+
Process #2 competes with the above reactions with the net reaction:
2 BrO3- + 12 H+ + 10 Ce3+ → Br2 + 6 H2O +10 Ce4+
Most importantly, the following reactions cause the HBrO species to increase autocatalytically:
BrO3- + HBrO2 +H+ → 2 BrO2 +H2O
BrO2 + Ce3+ + H+ → HBrO2 + Ce4+
As this is still an active field of research, much is known and yet to be discovered about this reaction other observables in chaos theory.
Hazards:
Be careful not to spill solutions while pouring. If spilled on person, wash with copious amounts of water after treating the affected area with dilute base wash, as the solutions in the BZ demo are acidic. Potassium Bromate is also a cancer suspect agent.
SOP:
Carcinogen – Potassium Bromate
Corrosive – Sulfuric Acid
Disposal (by Storeroom):
Cap the used demo solution with parafilm, as gas is evolved and could cause a tightly sealed bottle to rupture. Dispensary will properly submit waste solutions to EH&S.
Acknowledgements
Chemical Demonstrations Volume 2 by Bassam Shakhashiri pp. 257-261