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Topic: Frustration with Chemical Kinetics  (Read 1652 times)

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Offline Twigg

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Frustration with Chemical Kinetics
« on: April 19, 2011, 01:49:03 PM »
I'm dying to know what advancements have been made in chemical kinetics. I've looked online for more sophisticated equations for rate constant: but I have not found anything other than Arrhenius and Eyring. I know the break down for the pre-exponential factor, but I think it's a flawed theory--a somewhat valuable heuristic. A=pz is just a two condition probability. Even if we assume that the values accurately reflect the kinetic situation, it's still going to be insufficient for a realist (like me! lol). Empirical probabilities have very little value to theory in my mind, if they cannot be characterized. I know there are theoretical explanations of p and z, orientation factor and collision frequency respectively, but these can only be determined from the empirical, undefined value of A. Let me show you what I mean:

So, let's say I'm trying to make a kinetic/collision model for an ideal reaction in the lab. I determine the reaction orders and I determine Ea and A at standard thermodynamic conditions. So, theoretically, I should be able to find collision frequency and orientation factor. One problem: A = pz has infinite solutions of p and z. I'd need one more equation involving p and z in terms of known constants to solve for the values. What's the point of having theoretical values if they have infinite solutions at constant conditions? Isn't that arbitrary by definition?

If I make further hypothetical provisions, I might be able to get a bit further. Let's say there's a definition for z from kinetic molecular theory (there probably is one that I don't know about, I'm just leaving room for the unknown here). So I can solve for p, woohoo. There's nothing mathematically wrong with this, but I have an objection to the use of theoretical components.

To solve for p from a kinetic molecular definition and the collision model equation (A = p z) is to fix a conjectured theoretical value (p) as the solution to a certain system of equations. That system of equations consists of (#1) the conjecture-based collision model equation (A = p z) and (#2) the kinetic mechanisms describing the likelihood of molecular collision. The problem here is pretty obvious: the solution depends on a conjecture that cannot possibly be compared to its theoretical explanation. In other words, we can determine A from kinetic data and determine z from kinetics, but there's no way of finding out what p means (for a realist). What I mean to say is this: if we assume A = p z, and we define A and z, then p depends on A and z! How can we call it the "orientation factor" when it's determined blindly as some probability of some condition included in A (or some product of probabilities perhaps)?

Also, I have one simple question. Where can I find practical kinetic data for common lab reactions? My CRC 82nd edition has nothing on kinetics. I looked at a 91st edition from my local library, and it only had kinetic data for stratospheric modeling (gas phases). I need data for aqueous reactions. It doesn't have to be super accurate, just accurate enough to be practically useful. Any help?

Also, if anyone knows of any interesting advancements or researches in chemical kinetics, I would be so grateful if you could post something about it.

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