OpenStax Chemistry 2e

Among the factors affecting chemical reaction rates was the presence of a , a substance that can increase the reaction rate without being consumed in the reaction. The concepts introduced on reaction mechanisms provide the basis for understanding how catalysts are able to accomplish this very important function.

The picture below shows reaction diagrams for a chemical process in the absence and presence of a catalyst. Inspection of the diagrams reveals several traits of these reactions. Consistent with the fact that the two diagrams represent the same overall reaction, both curves begin and end at the same energies (in this case, because products are more energetic than reactants, the reaction is ). The reaction mechanisms, however, are clearly different. The uncatalyzed reaction proceeds via a one-step mechanism (one transition state observed), whereas the catalyzed reaction follows a two step mechanism (two transition states observed) with a notably lesser . This difference illustrates the means by which a catalyst functions to accelerate reactions, namely, by providing an alternative reaction mechanism with a lower activation energy. Although the catalyzed reaction mechanism for a reaction needn’t necessarily involve a different number of steps than the uncatalyzed mechanism, it must provide a reaction path whose rate determining step is faster (lower Ea).

Reaction diagrams for an endothermic process in the absence (red curve) and presence (blue curve) of a catalyst. The catalyzed pathway involves a two-step mechanism (note the presence of two transition states) and an intermediate species (represented by the valley between the two transitions states). Source: OpenStax Chemistry 2e


Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019). Chemistry 2e. Houston, Texas: OpenStax. Accessed for free at


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