(thing) by SourKey Sun Apr 21 2002 at 2:34:03
A catalyst is a substance which is used in chemical reactions to accelerate the rate of said reaction. The catalyst accomplishes this by changing the Activation Energy (EA) of the reaction. Contrary to popular belief, though, the catalyst does not do so without reacting itself. If you think about it logically, how could a substance which is supposedly completely inert and unreactive possibly affect a reaction at all? I said substance, so let us disregard volume, pressure, and temperature. That being said, logic dictates that only a reactive substance (or reactant) could affect a reaction. Since a catalyst affects the reaction, it is, in essence, a reactant. The catalyst does not change the EA of a reaction, it in fact changes one reaction into several reactions (by reacting itself with the original reactants), each of which has a lower EA than the original reaction. The largest of the EAs of the lower reactions is taken to represent the EA of the overall reaction. In the intermediate reactions, the catalyst substance is reproduced completely.

Take for example the following reaction:

I- + OCl- --> IO- + Cl-

Left to its own devices, this reaction will take an exceedingly long time to occur due to its great EA. When the catalyst of water (H2O) is added, the reaction proceeds as follows:

1. OCl- + H2O --> ClOH + OH-
2. I- + ClOH --> IOH + Cl-
3. IOH + OH- --> IO- + H2O

Now cross out all the intermediate substances (being those substances which are produced in one step and consumed in one of the succeeding steps) and you're left with the overall reaction:

I- + OCl- + H2O --> IO- + Cl- + H2O

The catalyst is unchanged, and the overall reaction is the same as that of the original, but with a lower EA due to the less energy demanding intermediate reactions.

Since the catalyst is, in fact, consumed and regenerated fully (or nearly enough) during the course of the reaction, it is plain to see why the common misconception of catalysts is that they remain unreacted and unconsumed, but this simply isn't so.

"A catalyst is not a substance which 'takes no part in a reaction' (as you may have heard previously); rather, the catalyst is an active participant which is regenerated in a later step of the reaction mechanism."
-- James A. Hebden, Ph. D.
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