Catalytic Decomposition of Hydrogen Peroxide by Potassium Iodide
Two solutions are mixed resulting in an eruption of foam resembling a huge stream of toothpaste. This is the classic “Elephant Toothpaste” reaction.
Procedure presents an example of a catalyzed reaction.
The rapid evolution of oxygen gas is produced by the following reaction:
- 2 H2O2 (aq) = 2 H2O (l) + O2 (g) + heat
The decomposition of hydrogen peroxide in the presence of iodide ion occurs in two steps:
- H2O2 (aq) + I- (aq) = H2O (l) + OI- (aq)
- H2O2 (aq) + OI- (aq) = H2O (l) + O2 (g) + I- (aq)
- 20 mL 30% hydrogen peroxide, available from chemical supply establishments
- Potassium iodide 2 M – Prepare stock solution by mixing 33.2 g KI in distilled water and dilute to 100 mL
- Dishwashing liquid – approximately 5 mL
- Food coloring
- Graduated cylinders – 500 mL and 10 mL
- Large serving tray and rubber gloves to assist in cleanup
- Measure 20 mL of 30% hydrogen peroxide into the graduated cylinder. Add 5 mL dishwashing liquid and 10 drops food coloring to the graduated cylinder and shake to mix contents, but not too vigorously to avoid foaming. Measure 5 mL of 2 M KI solution in 10 mL graduated cylinder.
Quickly pour the 2 M KI solution into the 500 mL graduated cylinder containing the hydrogen peroxide plus soap and stand back. The generated oxygen interacts with the soap creating a foam “snake” which quickly fills the cylinder and spills onto the large serving tray. Since this is an exothermic reaction, steam may be seen emerging from the cylinder. The possible presence of a brown color in the foam is evidence of iodine in the reaction vessel.
Care should be taken handling 30% hydrogen peroxide – it is severely corrosive to the skin, eyes and respiratory tract. Do not stand over the reaction – steam and oxygen are quickly produced. Potassium iodide is slightly toxic. Safety goggles and gloves should be worn during the demonstration.
The foam and solution left in the cylinder may be rinsed down the drain with copious amounts of water.