Dynamic Duo

Introduction

Two catalysts cooperate to accelerate the decomposition of hydrogen peroxide.

Concepts

• Co-catalysis
• Kinetics
• Decomposition reactions

Materials

Safety Precautions

Hydrogen peroxide solution is an oxidizer and an eye and skin irritant. Cupric chloride solution is toxic by ingestion. Ferric chloride solution is a skin and tissue irritant. Wear chemical-resistant gloves, chemical splash goggles and a 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 The resulting solutions can be flushed down the drain with excess water, according to Flinn Suggested Disposal Method #26b.

Procedure

1. Label the test tubes A, B, C. Label the 500-mL Erlenmeyer flasks A, B, C.
2. Add 100 mL of 6% hydrogen peroxide solution to each of the Erlenmeyer flasks.
3. Add 8 mL of 1 M cupric chloride solution to test tube A and carefully slide it into flask A without spilling the contents of either the flask or the tube.
4. Add 8 mL of 1 M ferric chloride solution to test tube B and carefully slide it into flask B without spilling the contents of either the flask or the tube.
5. Add 4 mL each of 1 M ferric chloride solution and 1 M cupric chloride solution to test tube C and carefully slide it into flask C without spilling the contents of either the flask or the tube.
6. Secure a balloon over the mouth of each of the three flasks (see Figure 1).
   {13053_Procedure_Figure_1}
7. Quickly, yet carefully, tilt each of the flasks, allowing the contents of the test tubes to mix with the hydrogen peroxide solution in the flasks.
8. Return the flasks to upright positions and observe the rates at which the balloons expand.

Student Worksheet PDF

13053_Student1.pdf

Teacher Tips

• In this kit, enough chemicals are provided to perform the demonstration seven times: 2100 mL of 6% hydrogen peroxide, 85 mL each of 1 M cupric chloride solution and 1 M ferric chloride solution, and 2 packages of 11" balloons.
• The decomposition reaction is exothermic and the flasks may become very hot. Be sure to let them cool before handling.
• Other substances that catalyze the decomposition reaction of hydrogen peroxide include iodide ions, manganese metal, manganese dioxide, yeast and even blood.

Answers to Questions

1. Draw a diagram of the set-up. Include all chemicals used. Which balloon was filled the fastest?
   {13053_Answers_Figure_5}
   The balloon on flask C filled the fastest.
   
   
2. Write the chemical equation for the decomposition of hydrogen peroxide?
   H₂O₂(aq) →H₂O(l) + O₂(g)
   
   
3. What is a catalyst?
   A catalyst is a substance that speeds up a reaction but is not used up in the reaction.
   
   
4. Based on the results of this demonstration, what conclusion can we make about the catalysts Cu²+ and Fe³⁺?
   Both Cu²⁺ and Fe³⁺ are catalysts that speed up the decomposition reaction of hydrogen peroxide. But the speed of the reaction can be increased even more when they are used together as co-catalysts.

Discussion

A catalyst is a substance that can increase the rate of a reaction without being consumed during the reaction. The decomposition of hydrogen peroxide, H₂O₂, which was demonstrated in this lab decomposes according to the following equation.

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. It speeds up the reaction. Second, it is not consumed in the reaction, there 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 cupric chloride and ferric chloride were used as catalysts. While the mechanism is not fully understood, it is known when the two catalysts are mixed, the Cu²⁺ and Fe³⁺ the ions act together as co-catalysts and dramatically increase the rate of reaction.

Cupric chloride and ferric chloride each act as a catalyst for the decomposition of H₂O₂:

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.

While the mechanism is not fully understood, it is known that when the two catalysts are mixed, the Cu²⁺ and Fe³⁺ ions act together as co-catalysts and dramatically increase the rate of reaction.

References

Bilash, B.; Gross, G. R.; Koob, J. K. A Demo A Day; Flinn Scientific: Batavia, IL, 1995; p 193.

Walton, J. H. J. Chem Ed., 1931, 8, p 303.