The Presence of a Catalyst

Hydrogen peroxide solutions foam when poured onto an open wound because substances in the exposed tissues act as catalysts, increasing the rate of hydrogen peroxide’s decomposition. However, in the absence of these catalysts (for example, in the bottle in the medicine cabinet) complete decomposition can take months.

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy without itself being consumed by the reaction. Activation energy is the minimum amount of energy required for a chemical reaction to proceed in the forward direction. A catalyst increases the reaction rate by providing an alternative pathway or mechanism for the reaction to follow (see the figure below). Catalysis will be discussed in greater detail later in this tutorial as it relates to mechanisms of reactions.

A graph is shown with the label, “Reaction coordinate,” on the x-axis. The x-axis is depicted as an arrow. The y-axis is also an arrow and is labeled, “Energy.” There is a horizontal line that runs the width of the graph and appears just above the x-axis. A segment of this line is blue and is labeled, “Reactants.” From the right end of this line segment, a solid black, concave down curve is shown which reaches the level just below the end of the y-axis. The curve ends at another short, blue line labeled, “Products.” The “Products” line appears at a higher level than the “Reactants” line. An arrow extends from the horizontal line to the apex of the curve. The arrow is labeled, “Uncatalyzed reaction activation energy.” A second, black concave down curve is shown. This curve also meets the reactants and products blue line segments, but only extends to about two-thirds the height of the initial curve. From the horizontal line is another arrow pointing to the apex of the second curve. This arrow is labeled, “Catalyzed reaction activation energy.”

The presence of a catalyst increases the rate of a reaction by lowering its activation energy.

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Chemical reactions occur when molecules collide with each other and undergo a chemical transformation. Before physically performing a reaction in a laboratory, scientists can use molecular modeling simulations to predict how the parameters discussed earlier will influence the rate of a reaction. Use the PhET Reactions and Rates interactive to explore how temperature, concentration, and the nature of the reactants affect reaction rates.

Key Concepts and Summary

The rate of a chemical reaction is affected by several parameters. Reactions involving two phases proceed more rapidly when there is greater surface area contact. If temperature or reactant concentration is increased, the rate of a given reaction generally increases as well. A catalyst can increase the rate of a reaction by providing an alternative pathway that causes the activation energy of the reaction to decrease.

Glossary

catalyst

substance that increases the rate of a reaction without itself being consumed by the reaction

This lesson is part of:

Chemical Kinetics

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