In the Classroom
tested demonstrations
Demonstration of the Catalytic Decomposition of Hydrogen Peroxide Submitted by:
Alfred R. Conklin, Jr.1 and Angela Kessinger
Checked by:
Daniel T. Haworth
Marquette University, Milwaukee, WI 53233
Hydrogen peroxide (H2O2) is used in many types of demonstrations—for example, as an oxidation agent in the oxidation of lugigenin and luminol and in the preparation of oxygen gas (1). In the demonstration described, the decomposition of hydrogen peroxide is catalyzed by iodide (I–) releasing oxygen (eq 1) with the production of soap bubbles. This demonstration is known as “Elephant’s Toothpaste” as done by the Weird Science Group from Illinois. I
Wilmington College, Wilmington, OH 45177
Table 1. Hydrogen Peroxide Concentration Versus cm3 of Foam Produced under Various Conditions Temperature °C
Concentration of H2O2 (%)
Foam Produced (cm3)
20
20
751 ± 10
20
10
348 ± 52
20
5
178 ± 33
22
20
751
–
H2O2(aq) –→ H2O + O2(g) ↑
(1)
Variations of this demonstration were investigated. The effects of changing the hydrogen peroxide concentration, different cations and anions, and temperature on the reaction were examined.
22
5
178
42
20
970
Procedure
42
5
242
Chemicals and equipment for all tests are identical. Several 1000-mL graduated cylinders, a 10-mL graduated cylinder, liquid dish-washing soap, food coloring (optional), and a tray to catch the foam are needed. Reagents used were: 30–32% hydrogen peroxide (Aldrich 21,676-3), potassium iodide (J.T. Baker 1-3163), potassium chloride (Fisher 9330-500), sodium iodide (Fisher S-324), and potassium bromide (J.T. Baker 2998). CAUTION: Concentrated hydrogen peroxide can cause burns. Always wear goggles and neoprene, butyl rubber, or vinyl gloves when handling the reagents. Prepare hydrogen peroxide solutions containing 20, 10 and 5% H2O2 using the following relationship: mLrCr = mLdCd
(2)
where mLr = mL of reagent needed; Cr = concentration of reagent H2O2 in g/100 mL; mLd = mL of solution to be prepared; and Cd = desired concentration of H2O2 in g/100 mL. As obtained from Aldrich, hydrogen peroxide has an H2O2 content of 30 to 32%. Thus, the concentration (eq 2) after dilution is only approximate. Even sealed and refrigerated hydrogen peroxide decomposes with time. All solutions must be made up at the same time to obtain the results shown here. The amount of hydrogen peroxide solution added to the 1000-mL graduated cylinder can be increased or decreased as needed so that foam from all three dilutions is less than 1000 cm3. Saturated solutions of potassium iodide, potassium chloride, potassium bromide, and sodium iodide are prepared by adding an excess of the salt to distilled water. The supernate of these solutions is used in carrying out the reactions. In a 1000-mL graduated cylinder, add about 10 mL of 5% hydrogen peroxide solution, two or three squirts of liquid dishwashing detergent, and two drops of food coloring (for color). Swirl the contents of the cylinder and quickly add 5 mL of saturated potassium iodide solution before the
agitation has subsided. The amount of foam produced is recorded in cm3. This procedure is repeated with the 10% and 20% H2O2 solutions. The amount of foam produced is shown in Table 1. The same procedure was repeated with the variations described below. A regression analysis of the concentration of hydrogen peroxide vs. the average cubic centimeters of foam produced gives an r2 value of 99.8. In spite of the inaccuracy in the measurement of foam, there is a good relationship between amount of foam and concentration of hydrogen peroxide. Thus, the effect of concentration on the reaction is well demonstrated. CAUTION: This reaction is exothermic. Approximately the same volume of foam is produced when sodium iodide is substituted for potassium iodide; however, no reaction takes place when potassium chloride or potassium bromide is used. This demonstrates that it is indeed the I– anion which is the effective catalyst for this reaction. The effect of temperature on the reaction is given in the table. At higher initial temperatures more foam is produced. Because the reaction is exothermic, any initial temperature effect is obscured by the increase in temperature during the reaction. Literature Cited 1. Shakhashiri, B.Z. Chemical Demonstrations—A Handbook for Teachers of Chemistry. University of Wisconsin Press: Madison, 1983; Vol 1, p 180; Vol 2, p 144.
1
Author to whom correspondence should be addressed.
838
Journal of Chemical Education • Vol. 73 No. 9 September 1996
