In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Modified Demonstration of the Catalytic Decomposition of Hydrogen Peroxide submitted by:
Carlos Alexander Trujillo Departamento de Química, Universidad Nacional de Colombia, Bogotá, Colombia;
[email protected] checked by:
Edward Senkbeil Department of Chemistry, Salisbury University, Salisbury, MD 21801-6860 Paul Krause Department of Chemistry, University of Central Arkansas, Conway, AR 72032
The catalyzed decomposition of hydrogen peroxide is a popular demonstration commonly called the “Elephants’ Toothpaste”1 (1). In the typical demonstration hydrogen peroxide is decomposed to produce molecular oxygen in the presence of a surfactant with iodide as a catalyst. The rapid decomposition produces a thick yellow foam. It is usually performed with a hydrogen peroxide solution at 30% or 50%. Concentrated hydrogen peroxide is a powerful oxidant that causes severe burns to the skin and should be handled with extreme care and as such this demonstration is only appropriate for experienced demonstrators. A safer demonstration has been developed that uses household hydrogen peroxide (3% H2O2) and yeast as a catalyst. Yeast contains catalase, an enzyme active in the hydrogen peroxide decomposition: catalase
2H2O2(aq)
2H2O(l) + O2(g)
In the presence of a surfactant, foam is produced by the rapid decomposition of the hydrogen peroxide, and it expands to overflow the container. Foam with the similar characteristics as in the traditional demonstration can be obtained by adding gelatin to the diluted hydrogen peroxide solution. The effect is similar with the advantage of a white foam that is easier to color.
sion to a 1-L plastic bottle or graduated cylinder. Disperse dried baker’s yeast (5 g) in a mixture of 10 mL of glycerin and 50 mL of carpet shampoo while stirring until no more granules can be seen. The dispersion process takes between 5 and 10 minutes. Food coloring can be added to this suspension to obtain foams with color. To perform the demonstration the mixture of yeast, detergent, and glycerin is added rapidly to the bottle with the hydrogen peroxide–gelatin dispersion. Shaking the bottle facilitates the mixture of the reagents. Foam fills the container and pours out with an effect similar to the traditional demonstration. It is only necessary to add about half of the catalyst suspension. The larger volume is prepared because its viscosity prevents rapid enough transfer to maximize the effect. About 4 L of foam is produced from 200 mL of household hydrogen peroxide when the components are mixed at 20 ⬚C. The foam begins to collapse about 5 minutes after the demonstration. The demonstration can be done without the glycerin and gelatin but the foam does not have the same consistency as in the traditional demonstration. Gelatin makes the foam thicker and the glycerin makes it more stable. The hydrogen peroxide reaction is exothermic, (∆H = ᎑98.0 kJ mol᎑1). However, with the concentrations involved in this demonstration is not possible to see the steam rising from the foam like in the traditional demonstration, but the bottle increases in temperature about 10 ⬚C.
Materials • Household hydrogen peroxide or a solution about 3% H2O2 prepared by dilution of commercial concentrated hydrogen peroxide • Dried yeast (baker’s yeast) • Dish-washing detergent or carpet shampoo • Gelatin without flavor and color • Glycerin • Food coloring (optional)
Procedure Heat 200 mL of 3.0% household hydrogen peroxide to 40 ⬚C in a 500-mL beaker and add gelatin (8 g) to the solution while stirring. It takes about 5 minutes to obtain a clear suspension. Transfer the hydrogen peroxide–gelatin suspenwww.JCE.DivCHED.org
•
Hazards Even at 3% hydrogen peroxide is a potent oxidizing agent (2). Wear goggles and gloves. The residual liquids can be flushed down the drain or disposed of in accordance with local ordinances. Notes 1. The name “Elephant’s Toothpaste” was coined by Ron Perkins while lecturing at the University of Wisconsin–Madison. (e-mail communication).
Literature Cited 1. Conklin, A. R.; Kessinger, A. J. Chem. Educ. 1996, 73, 838. 2. Young, J. A. J. Chem Educ. 2003, 80, 1132.
Vol. 82 No. 6 June 2005
•
Journal of Chemical Education
855
