Magic Genie

Introduction

When potassium iodide is dropped into a flask containing 30% hydrogen peroxide, a “magical” genie appears in the form of water vapor and oxygen.

Concepts

• Exothermic reaction
• Catalysts
• Decomposition reactions

Materials

Safety Precautions

Hydrogen peroxide, 30%, will act as an oxidizing agent with practically any substance. It deserves the science teacher’s special handling and storage attention. This substance is severely corrosive to the skin, eyes and respiratory tract; a very strong oxidant; and a dangerous fire and explosion risk. Do not heat this substance. The reaction flask will get extremely hot; use only a Pyrex flask and hold with a towel around it to prevent burns. Do not point the mouth of the flask towards yourself or anyone else. Never tightly close a vessel containing hydrogen peroxide—it may explode. Wear chemical splash goggles, chemical-resistant gloves and chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Immediately clean up any liquid which may have spilled. Any liquid remaining in the flask may be flushed down the drain with excess water and disposed of by Flinn Suggested Disposal Method #26b. Remove any remains of the potassium iodide packet and dispose of in the regular trash according to Flinn Suggested Disposal Method #26a.

Procedure

1. Wrap 4 g of potassium iodide in a small piece of filter paper or tissue. Staple the filter paper so that no potassium iodide leaks out.
2. Add 50 mL of the 30% hydrogen peroxide solution to a 1000-mL Pyrex volumetric flask. Caution: Wear rubber gloves when handling 30% H₂O₂. Contact with skin may cause burns.
3. Set the flask on a counter and hold the flask with a thick cloth towel. Drop in one packet of the potassium iodide solid. Point the flask up and in a safe direction away from yourself and your students as the magic genie (water vapor) emerges from the flask. The flask will get extremely hot. The towel will hide the flask contents as well as protect your hand from the heat produced.

Student Worksheet PDF

12553_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 350 mL of 30% hydrogen peroxide, 33 g of potassium iodide and 7 pieces of filter paper.
• Other substances that catalyze the decomposition reaction of hydrogen peroxide include iodide ions, such as NaI, manganese metal, manganese dioxide, yeast and even blood. For example: Manganese(IV) oxide can be substituted for potassium iodide in the demonstration. Both chemicals catalyze the reaction and will cause the release of oxygen from hydrogen peroxide.
• It is very important that this demonstration be done in a borosilicate (i.e., Pyrex) flask. A flask that is not borosilicate glass can crack from the evolution of heat.
• A large flask (1000-mL) is necessary because a brownish liquid can spurt out at the end of the reaction. A large flask will help prevent this from happening. The brown liquid results from the presence of free iodine produced from the extreme oxidizing ability of the 30% hydrogen peroxide.
• A thick cloth towel will prevent your students from seeing what is happening in the flask, as well as protect you from the heat evolved in the reaction. Another option is to wrap the flask in aluminum foil and decorate it like a “genie bottle.” A more colorful options is to add food coloring to the flask.
• The potassium iodide packet can also be attached to a piece of thread and hung inside the flask. Attach the thread to the outside of the flask with tape or a stopper. Warning: Do not use a solid stopper or cap. If the reaction starts prematurely, the pressure buildup may explode the flask. Use a one- or two-holed stopper and place it loosely on the flask.

Answers to Questions

1. Describe what happened in this demonstration.
   

   A hydrogen peroxide solution was poured into a volumetric flask. Potassium iodide solid, enclosed in filter paper, was dropped into the flask, and a great deal of water vapor was produced.

2. Write the chemical equation for the decomposition of hydrogen peroxide. Include heat in the equation, after determining if the reaction was endothermic or exothermic. Hint: The towel was necessary to hold the flask because the flask got very hot.
   

   2H₂O₂(aq) → 2H₂O(g) + O₂(g) + Heat

3. What is a catalyst? Name the catalyst in this demonstration. Could it be included as a reactant in the chemical equation?
   

   A catalyst is a substance that increases the reaction rate but is not consumed in the course of the reaction. The catalyst in this demonstration was the iodide (I^–) in the potassium iodide. It would not be included as a reactant, because it was all recovered after the experiment, therefore it is a catalyst.

4. In general terms, how does a catalyst work?
   

   A catalyst changes the pathway of a reaction mechanism. A successful catalyst uses a reaction pathway with a lower activation energy helping the reaction to occur at a faster rate.

Discussion

The Magic Genie demonstrates the decomposition of hydrogen peroxide into oxygen gas and water vapor. The reaction is an exothermic reaction and will evolve a lot of heat. The reaction is:

The decomposition is catalyzed by iodide (I^–) which is not changed during the reaction.

A catalyst is a substance that increases the rate of a reaction without being consumed during the reaction. The rate of decomposition of hydrogen peroxide is quite slow without the addition of a catalyst. There are two ways to tell if a substance is acting as a catalyst. First, it must speed up the reaction. Second, it is not consumed in the reaction. The same amount of the catalyst can be recovered as was originally added. If it were consumed, it would be a reactant and not a catalyst. In this demonstration, iodide was used as a catalyst.

There is an energy barrier that all reactants must surmount for a reaction to take place. This energy can range from almost zero to many hundreds of kJ/mol. This energy barrier is called the activation energy, E_a.

Reactants need to possess this amount of energy both to overcome the repulsive electron cloud forces between approaching molecules and to break the existing bonds in the reacting molecules. In general, the higher the activation energy, the slower the reaction.

The activation energy is related to the rate constant by the Arrhenius equation:


Where A is the frequency constant and is related to the frequency of collisions, R is the universal gas constant, and T is the temperature in K.

Catalysts are substances that speed up a reaction, but are not consumed in the reaction. Catalysts work by lowering the overall activation energy of the reaction, thus increasing the rate of the reaction.

In this reaction, I^– acts as the catalyst and dramatically increases the rate of reaction.

References

Special thanks to Jim and Julie Ealy, The Peddie School, Hightstown, NJ, who provided us with the instructions for this activity.

Stone, Charles, H. J. Chem. Ed., 1944, 21, 300.