[art1]

Hi everyone, I'm having some trouble with reaction mechanisms...

How does adding a catalyst speed up the rate determining step of a reaction, and how can I tell what catalyst will work? Is there any other way to speed up the reate determining step?

ex:

NO2+F2 ------> NO2F+F

how could I speed this up?

One thought i has was to have the NO2 be reacted spereately in a different equation but I'm not sure if that would work.

Any help would be appreciated

kudos

art

[Equi]

Catalysts don't "speed up", they provide a "different route". Most commonly I think is transition-state stabalisation.

[tennis freak]

actually the catalyst suplies some of the energy needed to get to the activation energy allowing the reaction to proceed. most commonly i have used an increase in temperature to provide some of the energy.

[mike]

A catalyst lowers the activation energy of the rate determining step, as Equi said it provides and alternate pathway for the reaction.

Reaction rate can also be increased by heating, or increasing the concentration of one or more of the reactants from the rate determining step. Note though that increasing the concentration of a reactant not in the rate determining step will not work.

I have not heard of a catalyst providing energy for a reaction though

[mrdeadman]

actually the catalyst suplies some of the energy needed to get to the activation energy allowing the reaction to proceed. most commonly i have used an increase in temperature to provide some of the energy.

wth are you talking about, a catalyst doesn't give any energy to anything. when you add catalysts, they make different reaction mechanisms, they participate, but in the end you end up with the unchanged catalyst again, even though they may change throughout. it will be found the same on the products side as the reactants side.

[tennis freak]

well i was doing one of the practice tests and i thought that was what was meant by the correct answer. maybe i assumed wrong. perhaps what it meant was what you said but i don't understand how that is an alternate route.

[syko sykes]

i think what he means by providing an alternate route is that the catalyst will react in one of the intermediate steps but will then go back to its original form when it appears in the final products. this can increase the rate of the reaction because each intermediate step is faster than the one step neccessary when a catalyst is not used. I'll see if i can find an example...

[mrdeadman]

i think what he means by providing an alternate route is that the catalyst will react in one of the intermediate steps but will then go back to its original form when it appears in the final products. this can increase the rate of the reaction because each intermediate step is faster than the one step neccessary when a catalyst is not used. I'll see if i can find an example...

yeah, how was that not clear though in my post?

[syko sykes]

i'm not sure what wasn't clear about that but here's an example:

consider the reaction 2H₂O₂(aq) --> 2H₂O(l) + O₂(g)

this is a single step decomposition reaction. the reaction rate can be increased by adding the catalyst I^- so that it becomes a two step reaction as follows:

H₂O₂ + I^- --> H₂O + IO^-
H₂O₂ + IO^- --> H₂O + O₂ + I^-

The rate determining step in this two step reaction goes quicker than in the unaided decomposition of H₂O₂. I^- is the catalyst and IO^- is an intermediate. As stated before, the catalyst remains unchanged in the reaction.

[tennis freak]

oh ok now i get it. thanks a lot guys

[mike]

A catalyst can even simply provide a good site for a reaction to occur. For example a reaction between two anions may proceed faster if the anions can be brought closer together (ie overcoming some of their repulsion). An example of this is using copper cations to increase the rate of the the iodide persulfate clock reaction.