Catalysis by Doris Kolb
Illinois Central College East Peoria, Illinois 61635
If you mix together some powdered zinc and crystals of iodine and then wait for reaction to begin
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Zn + G
ZnIz
. . .nothing happens. The reaction should be spontaneous, since the change in free energy is negative (AG = -50 kcal). bur it is so ex&edingly slows( roomtemperature that even if you let the mixlure stand for several hours, it still shows no evidence of reaction. On the other hand, if you add a drop of water to the mixture, the reaction takes place immediately. A large puff of purple vapor (sublimed iodine) testifies to the considerable heat generated by the reaction. I t is still the same reaction indicated by the equation given ahove (the water, in fact, can be recovered unchanged), hut the presence of the water makes it happen much faster. In other words, water is a catalyst for the zinc-iodine reaction. Its accelerating effect on this reaction is just one of thousands of examples of a once mysterious (and still not completely understood) phenomenon known as catalysis. What Is A Catalyst?
The more a student learns about catalysis the more trouble he is likely to have in trying to define it adequately. If he looks up the word "catalyst" in six different textbooks, he might conceivablv come n o with six different definitions. There is no single "&ectn definition, but certain definitions are better than others. Here are some that most chemists would ~robablv consider acceptable.
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A catalvst is a substance that increases the rate of a reaction but doe< nut appear as a tractant in the rtoiehit,rnetrie equation. A mtalyat IS n ~ulnrnnrethar speeds u p n chemical reaction hut does r m r crdinarily krvrnr pan oftheprudurt. A catalyst is a substance that accelerates a chemical reaction but can usually be recovered unchanged when the reaction is over. A catalyst is a substance that enables a chemical reaction to go faster by providingan alternative reaction pathway. A eatalvst is a substance that aooears .. in the rate eauation for a chemical reaction but not in the overall stoiehiometrie eouation. . A catalyst ica dutance that iuuers theenerm ofactivatiw~ fur a c.hen
