Catalysts 7/22/22

Science with Richard Bleil

Heat is not a catalyst.  There, I said it.  It’s done.  There’s really nothing more to say.  Okay, I’ll explain.

By definition, heat is really not a catalyst.  Yes, it makes reactions occur faster, but a catalyst is an additive, a chemical that makes a reaction occur faster but is not consumed in the reaction.  For example, a chemist of old was invited to a factory known as the house of screaming pots.  “Here’s something I’ll bet you can’t explain,” he said smugly to the chemist, “when the stirring paddles are lowered to the point of scraping the bottom of my iron reaction vessels, my yield goes up, and the louder they scream, the greater my yield.”  The chemist looked at him and said, “why don’t you throw a handful of iron nails into the pots before starting and raise the paddles.”  The owner tried it, and his yield jumped enormously, as did his profits because not only was he making more product, but he didn’t have to keep replacing worn out iron pots. 

The iron was acting as a catalyst.  When the paddles were scraping the pot, they were scraping off iron shavings, so the chemist just replaced those shavings with nails.  In the reaction, the iron was not reacting, however (except for the possibility of an oxidation side reaction wherein the iron was rusting).  This means that as much iron as the owner put into the pots was exactly the amount of iron he was taking back out.

There is always an energy barrier for any chemical reaction to occur, called activation energy.  This is why reactants can be added together, like sugar in your bowl and oxygen.  There is a lot of energy stored in sugar, and when it burns (reacts with the oxygen), all of that energy will be released, but the reason that it does not happen is because there is not enough energy to start the reaction, or, if you prefer, to overcome this activation energy.  Once started, the sugar will burn on its own because the energy released by each sugar molecule will provide the heat necessary for the surrounding sugar molecules to overcome the activation energy and so forth.  Heat can initiate this reaction, but it doesn’t catalyze it.

Activation energies can be very high, like sugar and oxygen, or very low requiring no outside heat at all to get started.  The way that heat speeds up reactions is to uniformly provide more energy to the reactants, so they are closer to overcoming that activation energy.  Right now, there is fear of heat induced forest fires because of excessively higher than normal temperatures.  This is because of a couple of reasons, first the drying of dead wood and fuel in forests, but also because the wood is closer to overcoming that activation energy.

A catalyst, a true catalyst, acts by providing a different reaction pathway that has lower activation energies.  This pathway is often longer, but faster.  A catalyst can either make the reaction faster or have no effect because if it was a slower pathway, the original reaction pathway would still be the favored. 

Growing up in the mean streets of upper middle-class suburbia, my friend and I would ride around the neighborhood on our bicycles.  There was one street in my neighborhood, however, with a relatively long and steep hill.  Yes, we would sometimes try to tackle this hill, but it took a long time, and, in our youth, we would often get off of our bikes and push them to get up it.  We almost always avoided it, however, because the next street over was a less steep climb.  It was longer because we would go over the block which was also an incline, up the parallel incline hill, and back over a third incline to get to the point that the steep hill would have taken us, but because those hills were less steep, the pathway was faster despite the fact that it was longer.

On your car is a platinum catalyst designed to convert carbon monoxide into carbon dioxide.  This occurs naturally, but the reaction is very slow requiring carbon monoxide to collide with oxygen or ozone in just the correct orientation and with the correct amount of energy to react.  What’s happening in the catalyst is the platinum is trapping the carbon monoxide by the oxygen and holding it stable, forming a kind of platinum-carbon monoxide “complex”, and adjacent to other complexes like so many bristles on a brush.  Remember your parents telling you that if you get separated to pick one spot and just stand there so it’s easier for them to find you?  The same thing happens here.  The carbon monoxide is going nowhere, so it’s easier for an oxygen molecule to find them and attach to the carbon side of the molecule.  The oxygen will then also attach to the carbon of an adjacent molecule.  Once the oxygen bond breaks, two new carbon dioxide molecules have formed, and they are released by the platinum.  It’s a longer reaction pathway, but much faster.

And now you know more about catalysts than you ever wanted.

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