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Topic: Why can't a catalyst effect equilibrium?  (Read 10625 times)

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Offline shoofy2

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Why can't a catalyst effect equilibrium?
« on: August 11, 2009, 10:22:38 PM »
Suppose you have a reaction with a high activation energy, and no particles have high enough energy to surmount the barrier. A catalyst is introduced that drops the activation energy. Now some particles can surmount the energy barrier.

It would seem now that the catalyst has allow the reaction to go forward, NOT just increased the rate, since the reaction never could have happened without the catalyst.

In other words, if the exponential factor were zero before introduction of catalyst, and something greater than zero afterwords, how could it be said that the catalyst merely raised the rate of reaction?

Everything I've read says a catalyst only effects rate. I can't figure out where I am going wrong.

Thank you in advance.

Offline renge ishyo

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Re: Why can't a catalyst effect equilibrium?
« Reply #1 on: August 11, 2009, 11:10:47 PM »
Think about it this way, when the catalyst lowered the activation energy for the forward reaction what did it do to the activation energy for the *reverse* reaction? It lowered that too by the *same* amount. So it made it easier for the reaction to go forward, but it also made it easier for it to go backwards. The result is that the reaction goes faster in both directions. Getting to equilibrium faster is the only net result.

Another way to think about it is this, equilibrium in a given system is defined by the concentrations of the products at equilibrium. Systems that have these concentrations to start with are already at equilibrium and those that have different concentrations move towards the equilibrium concentrations and then the reaction stops when they get there. So how does the concentration of a catalyst change when a reaction goes from reaction to product? Well, a catalyst gets used up early on in the reaction and then gets regenerated at a later step. In other words, the concentration of the catalyst *doesn't change* in the reaction, it is the same concentration at equilibrium as it is when the reaction starts. So it can't be a factor affecting the equilibrium itself.

Hope this helped.

Offline shoofy2

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Re: Why can't a catalyst effect equilibrium?
« Reply #2 on: August 12, 2009, 12:18:24 AM »
The idea that the reverse reaction is effected in the same way is very helpful, but I still run into a conceptual dilemma.

Perhaps it is just times when the reaction would not proceed at all without a catlyst (a problem with an exponential factor of zero). At times when the activation energy cannot be surmounted by any molecules, and a catalyst overcomes this activation energy, the catalyst has now allowed the reaction to move forward, where previously it would be unable to.

There are products being created here. Where previously there were none. Granted, the activation energy is lowered in the reverse direction as well, but still it seems as if the equilibrium constant would now be higher, because surely it could not be zero.

But, by the logic of the above, the reverse reaction's activation energy would be lowered, and thus when equilibrium was reached, the concentrations would be 100% reactants and 0% products, since thats what it was before the catalyst was introduced. But the concept of dynamic equilibrium seems to break down here. How can you have dynamic equilibrium with 0 products?

Thanks for the help so far!


Offline Yggdrasil

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Re: Why can't a catalyst effect equilibrium?
« Reply #3 on: August 12, 2009, 02:19:45 AM »
At the beginning of the reaction, when there are no products and everything is reactants, the system is not at equilibrium.  Sometimes these non-equilibrium states can be extremely stable because the activation energy for the reaction is too high (e.g. The diamond --> graphite equilibrium strongly favors graphite at standard temperature and pressure, but because the activation energy for converting diamond into graphite is extremely high, the reaction does not occur in practice.  Hence, the saying that "diamonds are forever").

Basically, you can think of catalysts as things that make a reaction move toward equilibrium more quickly.

Offline renge ishyo

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Re: Why can't a catalyst effect equilibrium?
« Reply #4 on: August 13, 2009, 03:08:28 PM »
Yggdrasil is right, if the system is not at equilibrium it will spontaneously move towards equilibrium once you supply it with a reactant molecule that is energetic enough for the reaction to take place. This is true whether you add a catalyst to speed things up or not. If you don't supply it with a reactant molecule that it energetic enough, it is true that the reaction will not proceed or that it's rate is very slow...say a million years or even "never", but so long as the reaction is away from equilibrium it is still *ready* to proceed when you *do* add a molecule that has enough energy. A catalyst lets you still do the reaction faster by lowering the "energy requirement" for the molecule so that rather than taking a millions years you can do it in less time.

As an illustration of how a catalyst works, one model of a catalyst has it bind to a reactant in such a way that it "stretches" the bond out in one of the reactants to make the bond strained. Now a less energetic molecule can ram into this strained molecule and cause it to break whereas without the catalyst there the less energetic molecule would just bounce off the unstrained reactant, and you'd need to *wait* for a faster moving molecule with enough "umph" to come along to break the bond...which could take awhile. The catalyst doesn't create the product...it just makes it easier for the original reactants to get to the product.

It's like if I told your little sister to cut a rubber band so that it forms a single long piece instead of a loop. Your sister might be 3 years old and too weak to cut the rubber band. But if you "stretch out" the rubber band by holding it for her, maybe now she can cut through the rubber band. In this example, you would be acting like the catalyst. You could have just waited for your little sister to grow up and then maybe one day she would have enough strength to cut the band herself without you stretching it, but that would obviously take longer, right? Either way, you aren't creating anything in either example...you still have only one rubber band whether you have cut it yet or not.

Hope this helped.

Offline shoofy2

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Re: Why can't a catalyst effect equilibrium?
« Reply #5 on: August 13, 2009, 05:33:05 PM »
Great imagery, thanks to both of you for that -- it really helped me nail the concepts of kinetics versus equilibrium.

One of the things that was holding me back from fully grasping the concept was my understanding (or lack thereof) of why a reaction moves toward equilibrium (mainly, the concept of entropy, which is ill explained in most undergraduate textbooks). Putting together the concepts of equilibrium, kinetics, and thermodynamics has definetely strengthened my chemistry basis, but is something that none of my professors have stressed.   

Thanks again for the help.

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