Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: zephyrblows on February 09, 2007, 03:09:28 AM
-
Well, I know this may be somewhat meaningless...
Anyway, considering the same reaction, can the prefactor A in Arrhenius Equation be changed by catalysis ?
Thanks
-
the prefactor A is a constant that is based on the empirical relationship between temperature and reaction rate. catalysis merely changes the activation energy Ea, and hence the answer to your question is no.
-
Are you sure? The pre-factor A in the Arrhenius equation is a measure of the frequency of collisions while the exponential term determines how likely such collisions are to be successful. Therefore the question could be re-worded to "Can the number of collisions that would lead to a reaction be increased by a catalyst?"
Which would be yes.
-
A catalyst by traditional definition is something that modifies the activation energy; not the rate of collisions.
Sure, if you want to word it like that, then yes. However, if you are allowed to modify a definition into what you mean, then everything can be true.
What you said is true, but by the strict common definition of the word catalyst, what xiankai said is true.
-
...considering the same reaction, can the prefactor A in Arrhenius Equation be changed by catalysis ?
Enzymes are great catalysts because they can increase the A factor and decrease the energy of activation at the same time, both of which increase the reaction rate without increasing the temperature.
They can affect A by grabbing the reactants out of solution by binding to the active site of the enzyme, thus increasing the probability the reactants will approach each other. They are also good at orienting the reactants just right for the reaction, improving the steric factor considerably.
-
A catalyst by traditional definition is something that modifies the activation energy; not the rate of collisions.
What I'm wondering is, if a catalyst are to change Ea, shouldn't it change the reaction mechanism , and thus collision frequency, at the same time? ???
-
A may also be considered to be the orientation factor i.e. the proportion of collisions involving particles with sufficient energy to react that actually do.
Catalysts may be either homogeneous or heterogenous and certainly in the case of heterogenous catalysis the template effect must have an effect on the orientation factor A.
In the case of homogeneous catalysis, if the mechanism proceeds via an alternative route (with a different slowest step) then it stands to reason that the orientation factor for the slowest step must also change.