[fatcat]

Hi, I just wanted some confirmation on this, is the reaction order of a catalyzed reaction the same as a regular one because well, the catalyst does not actually react with anything and thus is not present in the rate law expression. If that is so, does the rate constant, k, change because of the change in rate due to the catalyst?

Thanks in advance

[rucik5]

Hi,
I will answer your question with more questions
First, I wouldn't say that catalyst doesn't react with anything. It must remain unchanged at the end of the reaction [is regenerated] but it doesn't appear in the rate law since it does its job 'oustside' of the rate determining step. [Anyone is more then welcome to correct me here!].
Now, what role does the catalyst actually play in the reaction? What's its main purpose and how does it alter the reaction pathway? Can it still be considered 'the same' reaction? Would it have the same rate constant? Think about the energy profile too.

[fatcat]

Well as far as I know without diving into any reading, a catalyst decreases the amount of kinetic energy needed to successfully react.. but I guess I will have to look into that right now.

Edit: Alright, I now know that alright, the catalyst may be indeed reacting with the reactants, however not consumed and given back out as a product. I am still lost as to if it changes the rate constant, k. I would believe it would because if it didn't, it would imply that the reaction rate at which the reaction goes normally will use the same constant and thus be the same as the catalyzed reaction, which cannot be true.

I'm getting confident in my thinking that the reaction order remains unchanged except the rate constant, k, is increased upon addition of a catalyst, correct?

[DevaDevil]

the catalyst can indeed change the order of reaction if the pathway changes. For example:
A + B --> C:  2nd order

catalyzed:
A + catalyst --> A* (intermediate)
A* + B --> C*  (C* = product C attached to catalyst)
C* --> C + catalyst, rds

then the reaction is no longer 2nd order as C* --> C is rate determining (you fill as much catalyst as you can, and regeneration is the slowest part).

...but it doesn't appear in the rate law since it does its job 'oustside' of the rate determining step....

it may appear in the rate law, if the catalyzed step involves an intermediate with the catalyst (like a ligand-bond), but usually the catalyzed step is no longer rds, so most of the time the catalyst is not in the rate equation.
if in the aforementioned example A + Catalyst --> A* would be rate determining, then the catalyst is in the rate law.


yes, rate constants (kinetic parameters) will change. Remember: catalysts change kinetics, but NOT thermodynamics

[fatcat]

How would I determine if it changed the rate order of the reaction? I'd guess by comparing it to a known reaction rate order of the uncatalyzed reaction? I keep trying to find tables of these, but I know there must be thousands of reaction orders for every single reaction but I thought maybe there would be like a sort of database, maybe not?

[DevaDevil]

unless it is a well-known catalyzed reaction, it will be hard to find a table of reaction orders. Catalysis is a busy field of research with lots of scientific papers published all the time, expanding the knowledge. In literature you could try to see what has been done and what is known.

and no, there aren't thousands of orders for every reaction luckily, most reactions aren't that complex :p

[fatcat]

I meant thousands of reactions* Not thousands of orders for each reaction  Thanks for your answer