Control the Reaction Rate of a Foaming Reaction

Background Information for Teachers

This section contains a quick review for teachers of the science and concepts covered in this lesson.

Chemical reactions are essential for life. Some happen very fast, whereas others seem to take ages. The speed of a chemical reaction is determined by its reaction rate. For many industrial applications, it is essential to be able to control reaction rates to ensure that processes happen fast enough to be economically viable, yet not too quick, so as to prevent the risk of explosions. Studying chemical reaction rates allows students to investigate the factors that influence the speed of a reaction and explore reaction mechanisms in more detail.

How molecules or the reactants of a chemical reaction interact or react with each other is explained in the collision theory. This theory states that all reaction molecules are in constant motion, and for a chemical reaction to occur they have to collide in order to form a product. A collision only leads to successful product formation if the molecules collide with sufficient energy, as well as in the correct orientation. The collision frequency and the number of effective collisions determine how fast all the reactants are converted to the end product.

Any factor that affects the number of successful collisions will also change the speed of a reaction. This includes changing the number of reactant molecules (the reactant concentration) or the kinetic energy of the reactant molecules (the temperature), varying the nature of the reactants, or adding a catalyst or inhibitor to the reaction. Enzymes are biological catalysts that increase the rate of a reaction that otherwise might not happen or would take too long to be beneficial. Enzymes are proteins made by our cells that help transform chemicals in our body by reducing the activation energy of a chemical reaction, while interacting with its reactants (Figure 1).

Each enzyme has an active site, which is where the reaction takes place. These sites are like special pockets that are able to bind a molecule. The enzyme pocket has a unique shape so that only one specific substrate (target molecule) is able to bind to it (Figure 2). This means that unlike a non-biological catalyst, enzymes are usually highly specific for a particular chemical reaction. Once the molecule is bound to the enzyme, the chemical reaction takes place. Then, the reaction products are released from the pocket and the enzyme is ready to start all over again with another substrate molecule.

An enzyme's activity tells you how well an enzyme performs its function and how fast the reaction takes place. The study of how enzymes change the rate at which a chemical reaction occurs is called enzyme kinetics. Many factors determine the rate at which an enzymatic reaction occurs, such as temperature, pH, enzyme concentration, substrate concentration, or the presence of inhibitors or activators. Enzymatic activity, or the reaction rate of the enzymatic reaction is usually measured by doing an enzyme assay. An enzyme assay measures either the disappearance of the substrates or the appearance of products over time (Figure 3). It is also possible to measure other variables that are a proxy for either of those. The rate at which the substrates disappear, or the products appear, is a measure of the enzyme's activity.

In this lesson, students will explore the decomposition reaction of hydrogen peroxide, which is catalyzed by the catalase enzyme. Catalase is a very common enzyme that is present in almost all organisms that are exposed to oxygen. The purpose of catalase in living cells is to protect them from oxidative damage; for example, caused by hydrogen peroxide. The catalase enzyme helps get rid of hydrogen peroxide by decomposing it into harmless water and oxygen, as shown in Figure 4.

The assay mixture students will use for their catalase assay includes yeast, water, hydrogen peroxide, and a liquid detergent. The yeast catalase will decompose the hydrogen peroxide to produce water and oxygen. The detergent will trap oxygen bubbles and cause foam formation inside the test tube. The amount of foam is dependent on the activity of the catalase enzyme; the more active the enzyme, the more foam it produces. This allows students to quantify the reaction rate by measuring the foam height in each reaction. Students will be challenged to achieve a certain reaction rate by varying the substrate (H₂O₂) concentration in their assay mixture. With a lower substrate concentration, students will measure less foam formation, which is equivalent to a lower reaction rate. Higher hydrogen peroxide concentrations, however, will speed up the reaction, as more collisions between the substrate and the enzyme are possible (Figure 4).

Additional Background Links

• Chemical Reactions, from Ducksters
• Rates of reaction, from Teach It Science
• Introduction to kinetics, from Khan Academy
• Basics of enzyme kinetics graphs, from Khan Academy
• A Simple Assay for Measuring Catalase Activity: A Visual Approach, from Scientific Reports

Prep Work (15 minutes)

1. Prepare all materials for each student group.
2. Label one cup for each group with "hydrogen peroxide," one with "detergent", and one with "water." Fill each cup with the right amount of the respective liquid.
3. Prepare a yeast solution in a cup by dissolving 1 teaspoon of yeast in 50 mL of room temperature tap water.
4. Print one worksheet for each student.

Engage (20 minutes)

1. Ask students if they have ever heard about or seen the elephant toothpaste reaction. It they haven't, show them this video (stop at 0:24 seconds):
   

   https://www.youtube.com/watch?v=CnBaIFWn1gc
2. Show students a bottle of hydrogen peroxide and tell them that today they will explore the reaction—which is the basis of the elephant toothpaste reaction—the decomposition of hydrogen peroxide.
3. Write the reaction equation (see Figure 4) on the board and make sure students understand the reaction.

   What are the reactants in this reaction? What are the products?

   Let students name the reactants (hydrogen peroxide) and the products (water and oxygen). Point out that one of the reaction products is a gas (oxygen).

4. Pour a little (about 2–5 mL) of the hydrogen peroxide into a test tube and ask:

   Why don't we see the hydrogen peroxide decompose in the test tube?

   Listen to the students' ideas. Use their responses to point out that many chemical reactions don't occur spontaneously. This is because for a chemical reaction to happen, the reactant molecules have to collide with each other in the right orientation and with enough energy to react and form a product. The minimum energy a chemical reaction needs to get going is called the activation energy.

5. Mention that some chemical reactions are happening very slowly. The speed of a chemical reaction is also called its reaction rate. The reaction rate tells us how fast the reactants of a reaction are converted into its products. Point out that in the test tube there is actually some hydrogen peroxide decomposing, but the reaction is so slow that the gas production is not visible.
6. Add some (about 1-2mL) of your prepared yeast solution to the hydrogen peroxide in the test tube.

   What happened when I added the yeast to the hydrogen peroxide?

   Have students share their observations. They most likely saw some bubbling, which proves that oxygen gas has been produced.

   Why does the reaction suddenly happen when the yeast is added, although yeast is not part of the reaction itself?

   Have students speculate what the function of the yeast is. If you showed the elephant toothpaste video, they might have read that the yeast acts as a catalyst for the reaction.

7. Together with your class, define what a catalyst is and what it does.

   Does anybody know what a catalyst is and what it does?

   Gauge students' knowledge about catalysts. Some students might know that catalysts speed up chemical reactions. Explain that they do this by lowering the activation energy of a chemical reaction.

8. Tell students that the catalyst within the yeast is actually an enzyme called catalase. Enzymes are proteins that function as biological catalysts. Briefly explain how enzymes work. You can use the Schematic Drawing of an Enzyme Reaction to show how enzymes bind their substrates to their active site where the reaction takes place, and then release the products afterwards.
9. Come back to the elephant toothpaste reaction. State that now you know the role of the hydrogen peroxide and the yeast in the reaction. Then question them about the role of the detergent.

   Why is the detergent added to the elephant toothpaste reaction? What does it do?

   Listen to students' responses. Guide them to conclude that the detergent traps the produced oxygen gas, which results in foam formation. Without the detergent the oxygen bubbles are immediately released into the air and not much foam is created.

10. Tell students that today they will explore the decomposition of hydrogen peroxide in more detail. Specifically, they will investigate how they can control or change the rate of the reaction. They will use the height of the generated foam as a proxy for how much oxygen has been formed. Thus, the foam formation is a good measure of the reaction rate.

Explore (45 minutes)

1. Divide students into groups of three. Each group should get their own table with their own equipment. Provide each student with a printed student worksheet.
2. Set the stage for their investigation. Tell students that in their experiments their task is to regulate the rate of the reaction by varying the concentration of its reactant (or the catalase's substrate), which is the hydrogen peroxide. The goal is to add just enough hydrogen peroxide so that they create the following foam heights in their test tubes:
   • 3 cm
   • 5 cm
   • 7 cm

   The foam height, which includes the reaction liquid and the foam, will be measured with a ruler, as shown in Figure 5, 2 minutes after the start of the reaction.

   Image Credit: Svenja Lohner, Science Buddies / Science Buddies
   Figure 5. The foam height is measured with a ruler from the bottom of the tube to the top of the foam.
   
   If you are short on time, you can also assign a different foam height to each student group instead of having one group investigate all three foam heights.
3. Let students know that they will find the detailed instructions on how to perform the reaction on their student worksheet. Each group has 15 minutes to find the hydrogen peroxide concentrations that will generate 3 cm, 5 cm and 7 cm of foam.
4. Clear up any questions that students might have. Students in each group can work in parallel, as they have 6 test tubes. Mention that they need to rinse their test tubes with water in between each reaction until no more foam remains in the tubes. The reaction solutions can be discarded in the sink.
5. Tell students that they need to take notes of what hydrogen peroxide concentrations result in which foam height. They should use the table on their worksheet to record their results. They can express their H₂O₂ concentrations as percentages of the original H₂O₂ concentration (undiluted = 100%).
6. Then give them 15 minutes to carry out their investigations independently. Circulate from group to group to make sure students follow the instructions correctly and don't create an intentional mess.

Reflect (20 minutes)

1. Gather all students for a group discussion. Ask them about their observations and results.

   Were you able to find the right hydrogen peroxide concentrations?

   What are the three hydrogen peroxide concentrations that result in a foam height of 3, 5, and 7 cm?

   Have students share their observations and results. Summarize the results in a table (as shown here) on the board.

If all students followed the instructions, the results of each group should be pretty similar. Discuss potential discrepancies between data from different groups. These could, for example, be caused by incorrect foam height measurements, inaccuracy in making H₂O₂ dilutions, wrong timing for foam measurements etc.

2. Have students make a graph from their results on graph paper or let them use graphing software. The graph should show the H₂O₂ concentration [%] on the x-axis and the measured foam height [cm] on the y-axis, as shown in Figure 6.
3. Use the graph to engage students in a discussion about chemical reaction rates.

   When looking at your graph, what do you notice about the correlation between the H₂O₂ concentration and the rate of the reaction?

   The graph should show a linear relationship between the two parameters (Figure 6). The reaction rate (or foam height) linearly increases with the hydrogen peroxide concentration.

   Image Credit: Svenja Lohner, Science Buddies / Science Buddies

   The graph shows a linear relationship between the hydrogen peroxide concentration and the foam height. The more hydrogen peroxide is added to the reaction, the more foam develops.

   
   Figure 6. Example graph that shows the correlation between the H₂O₂ concentration and the foam height (reaction rate).

   Can anybody explain why the reaction rate linearly increases with increasing concentrations of hydrogen peroxide?

   Have students share their thoughts. Remind them of the collision theory that you mentioned earlier. The higher the concentration of hydrogen peroxide, the more frequently hydrogen peroxide molecules will randomly collide with the catalase enzyme, which then coverts it into oxygen and water. Point out that for enzymatic reactions the observed linear relationship only holds true for a certain concentration range. If you continue adding hydrogen peroxide, the curve will eventually plateau as the enzyme becomes saturated with substrate.

   Besides varying the reactant concentration, can you think of other ways to change the rate of a chemical reaction? How else could you increase the chances of collisions between the reactant molecules?

   Help students identify the temperature as another variable that can be used to change the rate of a reaction. With increasing temperatures, the kinetic energy of the reactant molecules gets higher, which means they are moving much more rapidly. The increased movement of the molecules results in more collisions between them, which leads to more product formation.

   Why do you think people are interested in regulating or controlling the rate of a chemical reaction?

   Listen to students' ideas. For many industrial applications, it is essential to be able to control reaction rates to ensure that processes happen fast enough to be economically viable, yet not too quick, so as to prevent the risk of explosions. This is also true for enzymatic reactions that are heavily employed in the food industry and that constantly happen inside our bodies. Scientists develop drugs to modulate the activity of enzymes within our bodies to treat diseases. Drugs that increase an enzyme's activity are called enzyme activators and drugs that decrease an enzyme's activity are called enzyme inhibitors.

4. Wrap up the lesson by having students summarize what they have learned about chemical reaction rates, enzymes, and how to control the rates of reactions.

Lesson Plan Variations

• Determine how temperature affects the rate of the hydrogen peroxide decomposition reaction. Have your students heat up or cool down your initial reaction solution before you add the yeast. How does the foam height change?
• Change the amount of yeast (catalase) added to the hydrogen peroxide. How does a change of the enzyme concentration change the reaction rate?
• Allow students to add food coloring to their reaction solutions to create colored foam.