Control the Reaction Rate of a Foaming Reaction

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).

The reaction curve without an enzyme shows a much higher peak which represents the activation energy of the reaction compared to the curve with enzyme.

Figure 1. Energy diagram for a chemical reaction with and without the presence of an enzyme as a catalyst.

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.

The enzyme binds its substrate at the active site to form an enzyme/substrate complex. Once the reaction is completed, the reaction products are released from the active site of the enzyme.

Figure 2. Schematic drawing of an enzyme reacting with its substrate.

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.

As the reaction takes place, the amount of substrate decreases whereas the amount of product increases.

Figure 3. Example graph showing the appearing products or disappearing substrates during an enzyme assay. As the reaction takes place, the amount of substrate decreases, and the amount of product increases.

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.

Figure 4. Decomposition of hydrogen peroxide catalyzed by catalase.

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).

At low substrate concentrations, few collisions happen between the enzyme and its substrate, whereas more collisions occur when the substrate concentration is high.

Figure 4. The reaction rate of an enzymatic reaction is dependent on how much substrate is available for collisions with the enzyme.