Sunday chemistry blogging: catalysts

Ok, it's a day late, sorry, and it will be a bit short.  But during the camp when we were having a bit of trouble getting a fire started (meaning a guy kept throwing huge pieces of wood on before it got started), someone produced a plastic bag.  "Any of you taken chemistry?  This is called a catalyst!" he said, and cast the bag into the flames, where it burned about like you'd expect (very quickly).  I imagine this reflects the popular understanding of catalyst as something that makes a reaction go faster, like pouring gasoline on a fire.

Of course, I couldn't help gleefully pointing out that I actually have a degree in chemistry, and that the guy's definition of catalyst was completely wrong.  (Why study science if you can't be an asshole about it at least once in awhile?)  Here's the real definition: a catalyst is something that changes the rate of a reaction while not being consumed by the reaction.  So the common movie trope when someone sprinkles a bit of "catalyst" into a big vat of chemicals and it starts to boil ferociously is close to the mark, but it's worth thinking about why this is.

The critical part of the catalyst definition above is the "not consumed" part.  Most catalysts do something like this:

R1 + R2 + C → R1-C-R2 → P + C

Here we've got a reaction where two reactants, R1 and R2, are reacting to form a single product P.  The catalyst, C, is helping things out by forming an intermediate stage on the way to the product—it's kind of like a chemical matchmaker that steps back after making introductions.  (Like anything in science, the details of each individual catalytic reaction can get stupendously complex, and sometimes work in ways completely different from my picture above.  But this is a decent way of thinking about it in general.)

Most catalysts are used to speed up reactions (called positive catalysis), but some are used to slow them down (negative catalysis).  Now you too can correct people at parties!