"Soapy Kinetics" A Classroom Demonstration
The "Soapy Kinetics" demonstration offers an interesting addition to the traditional "Old Foamy" or "Elephant Toothpaste" demonstrations. Your students will be excited by the soapy eruption and learn kinetics concepts along the way. The traditional "Old Foamy" is still a great demo by itself. It’s a great motivational activity and provides insight to the illusive concept of catalysis.

Materials Needed: AlconoxTM detergent, 3 – 4 g (or 2 – 3 teaspoons of commercial dish washing detergent) Hydrogen peroxide, H2O2, 30%, 6%, 3%, 20 mL of each Sodium iodide solution, NaI, 2 M, 4 – 6 mL Graduated cylinders, 10-mL and 100-mL, 3 of each Large, plastic demonstration tray (the deepest one available) Graduated cylinder, 500 mL Lighter or matches and wood splints Spoon or scoop

Procedure:

Part 1: Effect of Concentration on the Reaction Rate

1. Place three 100-mL graduated cylinders in a large, plastic demonstration tray. Add 20 mL of 30% hydrogen peroxide to the first cylinder, 20 mL 6% hydrogen peroxide to the second cylinder, and 20 mL 3% hydrogen peroxide to the third cylinder.
2. Add 3 – 4 grams of solid Alconox^TM detergent to each cylinder and swirl to dissolve the detergent. If Alconox^TM is not available then 2 – 3 teaspoons of commercial dishwashing detergent will do nicely. (Note: Do not select a low-foaming detergent.)
3. Measure out 5 mL of 2 M sodium iodide solution in each of three 10-mL graduated cylinders. Ask your students to predict the order at which each of the peroxide solutions will react with the iodide.
4. Ask for three student volunteers. Make sure the students are wearing chemical splash goggles; warn them to step back as soon as they pour. Have the students simultaneously pour the sodium iodide solution into the three cylinders containing the different concentrations of hydrogen peroxide. Make observations. White foam erupts from the cylinder with the 30% peroxide the fastest; the 6% peroxide next with a modest rate, and only slowly rises up from the cylinder with 3% peroxide.

Part 2: Traditional "Old Foamy"
Observing a Reaction Intermediate and Products

1. Place a 500-mL graduated cylinder in a large, plastic demonstration tray. Measure out 20 mL of 30% hydrogen peroxide and add it to the cylinder.
2. Add one small scoop (3 – 4 g) of solid Alconox^TM and swirl the mixture to dissolve the detergent. If Alconox^TM is not available then use 2 – 3 teaspoons of commercial dishwashing detergent.
3. Measure out 5 mL of 2-M sodium iodide solution and, quickly but carefully, pour this into the cylinder. In a few seconds, copious amounts of white foam will be produced. Observe closely at the beginning of the reaction. A brown foam is produced at first but then turns white before it erupts out of the cylinder. This is due to the presence of the free iodine produced by the extreme oxidizing ability of the 30% hydrogen peroxide.
4. Notice the steam coming off the foam. This indicates that the decomposition reaction is quite exothermic. Light a wood splint and blow out the flame leaving only a glowing splint. Insert the glowing wood splint into the foam. The wood splint will reignite in the foam – this indicates the gas in the foam is (nearly) pure oxygen. Take the wood splint out of the foam and blow out the flame, reinsert it, and watch it reignite again. This can be repeated several times.

Discussion:
Hydrogen peroxide decomposes to produce oxygen gas and water according to the decomposition reaction shown below:

2H₂O_2(aq) -> 2H₂O_(l) + O_2(g) + Energy

The reaction is quite slow unless catalyzed by something such as iodide ions, manganese dioxide, or even yeast or the enzymes in human blood. A catalyst is a substance that, when added to a reaction mixture participates in the reaction and speeds it up, but is not itself consumed in the reaction. The iodide ion is used as a catalyst in this demonstration.

Safety Precautions:
Hydrogen peroxide solution, 30%, is a strong oxidizing agent; it is severely corrosive to the skin, eyes, and respiratory track; and a dangerous fire and explosion risk. Do not heat this substance! Sodium iodide solution is slightly toxic by ingestion. Although the Alconox^TM detergent is considered non-hazardous, do not ingest the material. Do not stand over the reaction; steam and oxygen are produced quickly. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron.

Disposal:
The foam and any solution left in the cylinder or on the plastic tray may be rinsed down the drain using plenty of tap water. Consult a good safety manual for additional instructions.

Acknowledgement:
The "Old Foamy" demonstration is in the traditional form and is described in many good manuals. The kinetics portion is added to provide insight into the relationship between concentration and the rate of the reaction.

More Teaching Tips from Quantum^® coming soon!