Elephant Toothpaste
A Carolina EssentialsTM Activity
Total Time: 50 min
Prep: 30 | Activity: 20
Physical Science | Chemistry
6-12
Middle | High School
- Total Time: 50 min [ Prep: 30 | Activity: 20 ]
- Subject: Physical Science | Chemistry
- Grade: Middle | High School
Introduction
This quick demonstration showing the decomposition of hydrogen peroxide, catalyzed by iodide ions, provides students with visual evidence of a chemical reaction. Students compare the reaction rate of the catalyzed and uncatalyzed reactions and use their data to sketch an energy diagram for the decomposition reaction. The energy diagram serves as a model of energy flow through the reaction, from reactants to products, with 2 different reaction pathways—catalyzed and uncatalyzed.
Phenomenon
A definitive test for separating synthetic and natural fabrics is the burn test. How the fabric responds to the heat of a flame, the color of smoke produced, the odor, and properties of the ash or residue remaining identifies the base unit of the fabric—cellulose, protein, or a manmade monomer. Make observations from the burn test demonstration your teacher performs or watch the 4-minute video “Textile Fibers Burning Test” with numerous examples.
What are your observations?
Essential Question
How does the addition of a catalyst affect reaction rate?
Demonstration Objectives
- Observe a catalyzed decomposition reaction.
- Compare the reaction rates of catalyzed and uncatalyzed reactions.
- Construct a potential energy diagram for the catalyzed and uncatalyzed reactions.
Next Generation Science Standards* (NGSS)
HS-PS1-4. Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
SCIENCE & ENGINEERING PRACTICES
Developing and Using Models
- Use a reaction time data and potential energy diagram to construct a model of catalyzed and uncatalyzed reactions.
DISCIPLINARY CORE IDEA
HS-PS1-4: Matter and Its Interactions
- Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
CROSSCUTTING CONCEPTS
Energy and Matter
- Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system.
Materials
- Dishwashing liquid
- Hydrogen peroxide, 30%, 30 mL
- Potassium Iodide, Laboratory Grade, 100 g
- 1 large garbage bag or large tray
- 2 plastic bottles or large graduated cylinder (250 to 500 mL)
- 100-mL graduated cylinder
- Digital balance
- Spatula
- Timer
- Gloves
- Safety goggles
Procedure
Upon completion of this demonstration, the bottles can be rinsed, dried, and put away. Keep PPE on in case the bottle contains any leftover hydrogen peroxide. Unless otherwise prohibited, the foam and catalyst on the trash bag can be carefully rolled up and discarded in the trash.
STUDENT PROCEDURES
- Put on gloves and safety goggles.
- Crush any lumps of potassium iodide (KI) into fine crystals with a spatula, and then weigh 1.5 g of it onto a weighing boat or filter paper.
- Lay a large garbage bag flat on a desk, table, or lab bench to protect the demonstration area. A large tray can also be used for this purpose.
- Prepare both bottles or cylinders at the same time. Label one “Uncatalyzed” and the other “Catalyzed”.
- Place the plastic bottles or large graduated cylinders in the middle of the demonstration area.
- Measure 15 mL of 30% hydrogen peroxide and pour it into the bottle or cylinder. Repeat for the second bottle or cylinder.
- Add 25 drops of dishwashing liquid to the hydrogen peroxide in both bottles or cylinders.
- Swirl the bottles or cylinders to mix the liquid soap and peroxide.
- Prepare students to begin timing the reactions as soon as the KI is added ot the bottle or cylinder labeled “Catalyzed”.
- Quickly add the KI crystals to the bottle or cylinder labeled “Catalyzed” and begin timing.
- Observe the oxygen gas as it forms bubbles in the soap, creating a plume of foam that is quickly expelled.
- Stop timing the catalyzed reaction when the foam stops flowing out of the container.
- Direct students to answer the demonstration analysis questions. Check for evidence of a reaction in the uncatalyzed container every 5 minutes until students finish. Stop timing if or when a noticeable volume of bubbles is produced in the uncatalyzed container.
TEACHER PREPARATION AND TIPS
- Make sure the bottles are identical and that students understand both bottles are getting the same amount of the same chemicals. The only difference is the addition of the catalyst.
- Dawn dishwashing liquid produces a high volume of bubbles.
- Announce for students to get ready to time the reactions. You may want to assign individual students, 2 students per group, or split the room in half so both reactions are timed.
- Begin timing as soon as the KI is added.
- Announce to students when to stop timing the catalyzed reaction.
- Check the uncatalyzed reaction periodically for any evidence of bubbles.
| Reaction | Time(s) | Observations |
|---|---|---|
| Uncatalyzed Reaction | Will vary with temperature | May see small bubbles |
| Catalyzed Reaction | Instantaneous–runs for about 2 minutes | Large amounts of foam spew out of the bottle |
Analysis & Discussion
What evidence indicates that a a reaction took place?
Bubbles were produced.
2. Why was dishwashing detergent added to the hydrogen peroxide?
The detergent traps the oxygen bubbles, making a foam that allows the bubbles to be more visible and last longer.
3. Why was the solid potassium iodide added to one of the bottles?
It serves as a catalyst.
4. Compare the times for the 2 reactions. Using the time data, write a statement explaining what catalysis is.
When a reaction time is relatively slow and needs to be faster, a catalyst can be added. A catalyst reaction has a shorter reaction time because the addition of a catalyst lowers activation energy. See the explanation of the reaction mechanism below.
Hydrogen peroxide (H2O2 ) is stable for at least a year if stored in an airtight opaque container at room temperature. Common in first aid kits, a 3% H2O2 solution can be applied to minor cuts and abrasions. When the solution contacts tissue and blood, it rapidly decomposes into water (H2O) and oxygen gas (O2).
2H2O2( l ) → 2H2O( l ) + O2( g )
The oxygen gas creates a foam that lifts and washes contaminants out of the wound. This rapid decomposition can only happen in the presence of a catalyst. In the human body that catalyst is catalase, a biological catalyst in blood and tissue. Catalase can lower the activation energy from 75 kJ/mol to about 8 kJ/mol. In this demonstration, solid potassium iodide (KI) dissolves in aqueous H2O2 , forming an aqueous iodide ion (I –) and catalyzing the reaction of aqueous H2O2. The steps of the reaction mechanism are as follows:
5. The decomposition of hydrogen peroxide is an exothermic reaction. If the uncatalyzed activation energy is about 75 kJ/mol and ΔH = -196.1 kJ/mol, sketch a potential energy diagram for the catalyzed and uncatalyzed reactions. Label the reactions, axes, reactants, products, and activation energy for both reactions.
SHOP THE KIT
SAFETY REQUIREMENTS
- Safety Goggles
- Safety Gloves
- Safety Apron
*Next Generation Science Standards® is a registered trademark of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of, and do not endorse, these products.
