15. The Rates of Chemical Reactions   Previous PageNext Page
       A Simple Catalytic Mechanism

The energy released when two H atoms and a D2 molecule are bound to the catalytic surface is approximately equal to the energy required to dissociate the H2 molecule; thus panels one and three represent nearly the same energy states.

The contribution of the catalyst arises because the energy of the activated complex in panel four is not nearly as high as that of the intermediate complex in the gas phase:

 

The metal atoms help to hold the complex in place and stabilize it. The activation barrier therefore is lower, and the reaction takes place faster. Reaction energy profiles for uncatalyzed and catalyzed reactions are shown opposite.

This particular kind of assistance by a catalyst is known as a "rack" mechanism, because it literally pulls molecules apart and weakens bonds, thereby making them more susceptible to attack. The molecules that bind to a catalytic surface and are acted upon are called "substrate" molecules.

A catalytic surface, whether it be a clean surface of a finely divided metal or metal oxide, or the active site of an enzyme molecule, must be structured in such a way that it can bind substrate molecules from the reaction that it catalyzes, enable them to react, and release the products afterward.

In the words of Emil Fischer, a turn-of-the-century enzyme chemist, a catalyst and its substrate molecules must fit one another like a lock and key.

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