General Safety Guidelines
Major Concepts
Materials and Apparatus
Anticipated Student Results
Answers to Data and Calculations
Answers to Results and Discussion
Relevant National Science Education Standards

Major Concepts

This activity explores the concepts of catalysis and reaction rates, and offers a glimpse of the contrast between kinetic and thermodynamic considerations of chemical reactions.

The main focus of the experiment is the comparison of the half-lives of the decomposition of hydrogen peroxide facilitated by two different catalysts, namely KI and MnO₂. The students will make a simple comparison of the effectiveness of the two catalysts by measuring the half-lives of two decompositions, each using the same amount of hydrogen peroxide and equimolar amounts of the respective catalyst.

The students should learn the basic concept that a catalyst affects the rate of the reaction without themselves being consumed in the reaction. The students should understand that this is why the amount of the catalyst used doesn't relate to the stoichiometry of the reaction in any apparent manner.

In an extension of the experiment, the students are asked to repeat the procedure using a smaller amount to the faster catalyst (which they will have discovered to be KI). The students should observe that while the reaction takes longer with less catalyst, the same amount of gas is produced. This demonstrates a basic principle of catalysis: a catalyst alters the reaction only in terms of kinetics. The rate is altered. However, a true catalyst has no effect on the thermodynamics of a reaction. The equilibrium constant remains the same. The reaction stoichiometry remains the same. Given enough time, the same amount of product will be formed with or without the catalyst (in this case, 50-55 ml of oxygen gas).

Materials and Apparatus

Potassium iodide (KI) is a moisture sensitive irritant, and manganese(IV) oxide is an oxidizer and an irritant. Students should use caution and wear gloves and eye protection while handling these materials.

The apparatus in this activity may be somewhat tricky for some students to assemble, so we advise setting it up yourself before class. The key piece of equipment needed for the apparatus is the pneumatic trough. Specifically, this is a trough with a rubber hose attached to a hole in the bottom of the trough. Pneumatic troughs are available from scientific supply houses for less than $20.

The trickiest part of assembling the apparatus is setting up the inverted graduated cylinder. There are two ways to do this. The first method is the easiest, and it is shown on the table below. Each step is illustrated in the photo to its right.

Step 1: If your trough is deep enough, you may be able to fill the cylinder by submerging it in the water.	Click to enlarge.
Step 2: Once you have filled the cylinder with water, make sure there are no air bubbles in it. Next, simply invert the cylinder. Be careful not to raise the mouth above the surface of the water.	Click to enlarge.
Step 3: Stand the cylinder upside-down in the trough. Place the mouth of the cylinder over the hole where the rubber hose attaches to the trough.	Click to enlarge.
Step 4: Secure the cylinder with a clamp attached to a clamp stand.	Click to enlarge.

Anticipated Student Results

Stoichiometrically, the decomposition of ml of a 3 wt.-% solution of hydrogen peroxide should produce roughly 49 ml of oxygen gas, though this result will vary depending on the exact concentration of the solution used. Therefore, the half-life of the reaction in this procedure is the time needed for 23 ml of gas to collect in the graduated cylinder.

Additional Teacher Resources

Reaction Simulator — a set of computer models simulating the kinetics of various types of reactions, from the University of Southern California. In these models, variables such as reactant concentration may be manipulated in order to observe their effect on the reaction rate. The simulator features animations of reactant molecules and graphical data during simulated reactions.

Relevant National Science Education Standards

Unifying Concepts and Processes — The activity looks at one aspect of the interworkings of the components of a chemical reaction system, namely its kinetics, and the effect the catalyst has on the behavior of the whole system. The students draw conclusions based on observed evidence, and must explain and interpret what they observe in terms of the model that is the rate law of the chemical reaction being studied. The students also measure a rate of change, and must also discern what things change when the catalyst is altered (the rate of the reaction) from things that remain the same when the catalyst is altered (the equilibrium constant of the reaction and final amounts of products at equilibrium).

Physical Science — The activity centers on the study of one particular chemical reaction. An understanding of the molecular nature of matter is integral to the activity.

Earth and Space Science — The activity demonstrates the nature of catalysis. The students should then be able to apply their understanding of catalysis to the stratospheric geochemical cycles that destroy ozone.

Science in Personal and Social Perspectives — The understanding of catalysis gained in this activity demonstrates how small amounts of CFCs in the atmosphere can destroy large amounts of ozone.

Silberman, R., and Eubanks, L. ACS Small-Scale Laboratory Assessment Activities. Clemson, SC: ACS DivCHED Examinations Institute, 1996.