Why Is Catalyst 'C' Less Effective Than Catalyst 'B'?

[dRic2]

Hi, PF

I have to comment this graphic about a reaction with and without catalysis.

It's clear that the 'b' and 'c' line represent the catalysed processes, but I don't get one thing: my professor said that 'c' is a worse catalyst than 'b'. Why?

Thank you

 

[HAYAO]

Think about the intermediate state. Which is more kinetically favorable to proceed from that intermediate state to the final state? B or C?EDIT: Isn't this thread more suitable in the Homework forum?

 

[dRic2]

Sorry for the late replay

So basically C is worse because, although it requires a smaller activation energy, his intermediate state needs more energy to evolve into the products than B intermediate state ?

EDIT: Isn't this thread more suitable in the Homework forum?

Think you're right, I was in hurry and didn't notice there is a section about chemistry homework

 

[HAYAO]

Sorry for the late replay

So basically C is worse because, although it requires a smaller activation energy, his intermediate state needs more energy to evolve into the products than B intermediate state ?

Pretty much summarizes it.

The energy of the intermediate state of C is about the same as the product. If this is a thermodynamics dominated reaction, then at thermal equilibrium, we'll get (very) roughly something like 4:48:48 of initial, intermediate, and final state. If this is a kinetics dominated reaction, then the reaction from the intermediate to final state is significantly slower for C than B. In fact, it is more probable for the intermediate state of C to return back to the initial state than the final state.

 

[jim mcnamara]

Moved to Chemistry homework - no template, which is okay for this thread.

 

[dRic2]

The energy of the intermediate state of C is about the same as the product. If this is a thermodynamics dominated reaction, then at thermal equilibrium, we'll get (very) roughly something like 4:48:48 of initial, intermediate, and final state. If this is a kinetics dominated reaction, then the reaction from the intermediate to final state is significantly slower for C than B. In fact, it is more probable for the intermediate state of C to return back to the initial state than the final state.

Very clear! Nice explanation. I didn't think about this in particular:

In fact, it is more probable for the intermediate state of C to return back to the initial state than the final state.

Thank you, I like this kind of comments.

 

[HAYAO]

Thank you, I like this kind of comments.

Excuse me, I made a mistake. I thought that that initial energy potential barrier is lower than the second one. So from this diagram, kinetically, it is just as probable to produce initial state from the intermediate state as producing the final state.

 

[dRic2]

Ahahaha Same here! I didn't notice either (I didn't draw this)... strange optic effect

 

[epenguin]

I wouldn’t say that c is a worse catalyst than b. I think you could say that both b and c induce a change of mechanism relative to what is happening in a. They catalyse a mechanism that goes through a relatively stable, macroscopically accumulating intermediate which corresponds to the troughs.The intermediate would drain away with nearly the same rate constant in both cases but there would be more of it in case c because it is created the faster, and overall rate of accumulation of product consequently would be higher in case c .

 

[dRic2]

I think that c intermediate is thermodynamically equivalent (more or less) to the products, so I don't see a reason for the products to be favored. Therefor I have to give more energy to the system resulting in the possibility to reverse the reaction.

this is what I got thinking about @HAYO's reply and it sounds reasonable

 

[epenguin]

Yes, in c the intermediate is actually favoured over the final product. Not in b, so in that sense your Prof is right, b gives you more final product.

 

[dRic2]

Yes, problem solved. Thanks