Catalyst affect on pH

[msw27]

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Q: Suppose a reaction is acid-catalyzed by a solution of pH 3.0. What can be said about the pH of the resulting solution.

A: It can't be predicted without information on the acidity of the reactants and products. The catalyst is not the only factor affecting pH.

Under what circumstances would an inorganic catalyst affect pH and why?

 

[RSAgator]

Q: Suppose a reaction is acid-catalyzed by a solution of pH 3.0. What can be said about the pH of the resulting solution.

A: It can't be predicted without information on the acidity of the reactants and products. The catalyst is not the only factor affecting pH.

Under what circumstances would an inorganic catalyst affect pH and why?

I think you're misunderstanding the solution a bit. You have a reaction that is catalyzed by an acid at a certain pH. The catalyst has already affected the pH in making it pH 3.0. The answer is saying that you can't predict the pH of the resulting solution because you don't know anything about the other things that are in the solution. If your product is more basic than your reactants then the pH of the solution is going to go up, and if the opposite is true it is going to go down. However without knowing how the product and reactants affect pH there's no way of knowing the resulting pH.

To answer your question in THIS case an inorganic catalyst affects pH, it makes the solution pH 3. The point is that once the reactants are placed in the solution and the reaction takes place you don't have enough information to determine in which direction the pH changes.

 

[msw27]

Hmm. I guess I didn't expect them to be speaking of the reaction prospectively. I appreciate your help. If you wouldn't mind, would you elaborate more on how inorganic catalysts affect pH directly? I understand this concept for biological systems, but I don't understand how various hypothetical catalysts would do the same.

 

[RSAgator]

Hmm. I guess I didn't expect them to be speaking of the reaction prospectively. I appreciate your help. If you wouldn't mind, would you elaborate more on how inorganic catalysts affect pH directly? I understand this concept for biological systems, but I don't understand how various hypothetical catalysts would do the same.

Well I'm not entirely sure what your definition of organic catalysts, inorganic catalysts, and catalysts in biological systems are. The essence of a catalyst is that it allows to reaction to proceed faster by making the reaction conditions more favorable. When something is acid catalyzed, it generally means that the pH is sufficiently low for there to be enough protons available to help the reaction proceed. This could involve protonation of a hydroxyl group turning it into a water which acts as a better leaving group for example. The hydrogen is later regenerated by deprotonation by something else, hence the catalysis (the catalyst isn't used up).

Common catalysts are also metal ions which are able to freely donate and accept electrons which in turn make the reactants that are acted upon more reactive. These are very common in biological system and I would suspect in inorganic ones as well.

To answer your question of how the catalyst affects pH directly, in this case my understanding is that the hydrogen ions ARE the catalyst. They affect pH directly because their concentration IS pH. I think when people think of catalysts the basic biological definition that comes to mind is something like "induced fit" or "lock and key" but in reality a catalyst can be almost any compound that makes a reaction proceed faster than it would otherwise. In this case it's the protons contributing to the reactivity (it's also why the pH in the stomach has to be low), in some cases it's electron donation, and in still other cases it's simply binding to a molecule so that its reaction site is more accessible (the typical biological definition). Hope that clarifies something =)

 

[msw27]

Thanks so much, that definitely helps.

Since the MCAT is pretty much about trying to figure out what the question is asking...

...Minus ten points on my part. lol

 

[Vihsadas]

If I could add, I'd like to state that this is a good MCAT question. You might be drawn to: 'No change. Since the acid catalyst is regenerated at the end of a reaction the pH should stay the same'. The question, however, didn't say anything about the reactants and products of the reaction which can change. What if one of the products was basic in nature? Then you'd need to know the concentration of the reactants and the products to know how pH changes even though the concentration of the acid catalyst did not change.

 

[Kaustikos]

If I could add, I'd like to state that this is a good MCAT question. You might be drawn to: 'No change. Since the acid catalyst is regenerated at the end of a reaction the pH should stay the same'. The question, however, didn't say anything about the reactants and products of the reaction which can change. What if one of the products was basic in nature? Then you'd need to know the concentration of the reactants and the products to know how pH changes even though the concentration of the acid catalyst did not change.

But if the reactant was basic and used the catalyst (acid), wouldn't that by definition eliminate it as a catalyst since it IS being used?

Let me clarify; if the catalyst was the [H] in the solution, like in the acid-catalyzed elimination reaction of alkenes, and the proton is put back into solution - this is still by definition a catalyst.

BUT if the reactant was basic and put into the same situation, then it wouldn't be an acid-catalyzed reaction, but basically a titration/acid-base reaction, right?

 

[RSAgator]

in essence that's exactly the point of the question Kaustikos. You can't tell what's going to happen without knowing the product and reactants.

 

[Kaustikos]

in essence that's exactly the point of the question Kaustikos. You can't tell what's going to happen without knowing the product and reactants.

Yeah. No, I understand that. I'm just making sure what I am saying is correct.😀