Lesson 14:

Concepts conveyed | Materials | Procedure | Resource

Concepts conveyed:

The purpose of this classroom demonstration is to illustrate a typical decomposition reaction of hydrogen peroxide and how the kinetics of that reaction are influenced by presence of a catalyst.

Materials:

• Safety goggles
• 1 500 ml graduated cylinder
• 1/4 spoon (i.e. 1-2 g) of solid KI
• 50 ml of 30% hydrogen peroxide
• 1 pair of disposable chemically-resistant gloves
• dishwashing detergent
• blue food coloring
• 1 plastic tray (big enough to perform the experiment on it)
• 20 ml of 3% hydrogen peroxide
• 1 Petri dish
• 1 overhead projector
• a small amount of KI or MnI₂

Procedure:

Place the 500 ml graduated cylinder upright in the plastic tray and put on the safety goggles. Note: Use extreme caution. Use 30% hydrogen peroxide for best results and handle it with chemically-resistant gloves to protect your hands, avoiding all contact with it. Pour approximately 50 ml of 30% hydrogen peroxide into the cylinder. Add a small amount of dishwashing detergent and a drop of blue food coloring. Add about 1/4 of a spoon of solid KI. Observe the decomposition of the hydrogen peroxide to oxygen gas. The oxygen gas that is given off can be observed when soap bubbles are formed. The brown color of the foam is evidence of some iodine produced in the reaction.

Note: You can also show the decomposition of hydrogen peroxide to a large class by using an overhead projector. Place a small amount of the dilute 3% hydrogen peroxide in a colorless, transparent container, such as a Petri dish, on an overhead projector. Add a small amount of KI or MnO₂. The oxygen gas bubbles will be easily observed.

When the demonstration is completed, explain a general decomposition reaction and the specific reactions involved in the decomposition of hydrogen peroxide.

Interesting Fact: When a dilute 3% solution of hydrogen peroxide is placed on a wound, catalytic decomposition occurs. This reaction is catalyzed by catalase, an enzyme in the blood.

Decomposition Reaction

A decomposition reaction takes place when one substance breaks down to form two or more other substances. The general reaction is:

C —–> A + B

Chemical Reactions (for hydrogen peroxide)

• The net reaction of the rapidly catalyzed decomposition of hydrogen peroxide which produces oxygen gas and forms a foam with the liquid detergent is:

2H₂O₂ (aq) ==> 2H₂O (l) + O₂ (g)

• In the presence of iodide ions, this reaction occurs in two steps. The first is the rate-limiting reaction.

H₂O₂ (aq) + I ^– (aq) ==> H₂O (l) + OI ^– (aq)
H₂O₂ (aq) + OI ^– (aq) ==> H₂O (l) + O₂ (g) + I ^– (aq)

Notice that I^– is regenerated, and is therefore called the catalyst.

Note: The brown iodine (I₂) that can form may be due to a direct oxidation and reduction reaction with the H₂O₂:

H₂O₂ + I ^– —–> 1/2 I₂ + 1/2 H₂ + OH ^–

Resource:

• Chemical Demonstrations for Grade Schools; Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, 1996.