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Topic: Enthalpy in the Boltzmann distribution ?  (Read 8048 times)

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Offline GeLe5000

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Re: Enthalpy in the Boltzmann distribution ?
« Reply #15 on: March 21, 2016, 04:37:33 PM »
I think that I understand what you say.

I tried to underlie the following problem (my own problem).

When a teacher tells the students that an endothermic reaction is favored by an increase in Temperature, there are two explanations :

1)   LeChatelier's principle
2)   The equation of  the response of chemical equilibrium to temperature

But there's no mechanistic explanation like the collision theory in Kinetics.

Thermodynamics is well known to be interested only in the difference between initial and final states, not the path between them.
There's a bit (or much) of thermodynamics in Kinetics but no Kinetics in Thermodynamics.

Offline Irlanur

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Re: Enthalpy in the Boltzmann distribution ?
« Reply #16 on: March 22, 2016, 08:56:05 AM »
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1)   LeChatelier's principle
2)   The equation of  the response of chemical equilibrium to temperature

I don't think these are two different explanations, 1) follows from 2).

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But there's no mechanistic explanation like the collision theory in Kinetics.

That's an odd statement. Of course there is no "mechanistic" explanation, because a mechanism is always concerned with some reaction pathway. But there is still a microscopic eplanation: Statistical Thermodynamics and Quantum Mechanics. But you cannot use these theories easily to give a DETAILED AND GENERAL explanation of the LeChâtelier principle. However, for a given reaction, you can give explanations for the equilibrium constants and so on.


Offline GeLe5000

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Re: Enthalpy in the Boltzmann distribution ?
« Reply #17 on: March 22, 2016, 10:55:58 AM »
Yes, LeChatelier's principle is only a "rule of thumb".

Concerning a "DETAILED AND GENERAL explanation ", I've always had the (most probably) false impression that Kinetics and Thermodynamics could be merged together in one theory.

I've always been intrigued by the fact that the equilibrium constant is the ratio of rate constants. Isn't it a relation between Kinetics and Thermodynamics ?
What about the fact that reactions are dépendent of the Temperature, like there is a Temperature dependence of equilibrium constants.
And today I read : "Kinetic and thermodynamic control of reactions" (terms used in organic chemistry).
There's also the transition state theory where ΔEo/RT is "thermodynamic".

I don't know if an autodidact can really get clear ideas about that.

Offline Irlanur

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Re: Enthalpy in the Boltzmann distribution ?
« Reply #18 on: March 23, 2016, 12:15:43 PM »
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I don't know if an autodidact can really get clear ideas about that.

I don't know either, but I wouldn't know why not. Even if you have Professors, you still have to understand it yourself...

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I've always been intrigued by the fact that the equilibrium constant is the ratio of rate constants. Isn't it a relation between Kinetics and Thermodynamics ?

of course. 

Offline Corribus

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Re: Enthalpy in the Boltzmann distribution ?
« Reply #19 on: March 23, 2016, 02:11:10 PM »
Basically it boils down to the fact that rates of reactions (kinetics) depend on both thermodynamic factors (energetics of transition state energies, mostly) and non-thermodynamic factors (molecular geometric factors, e.g.). Especially, because reactions can proceed through any number of pathways, many of them with multiple stages and steps, kinetics in practice is largely phenomenological. While it technically may be possible to predict kinetics from first principles considerations, the escalating complexity of all but the most basic chemical systems renders the study of kinetics predominantly experimental in nature (with much of the modelling work taking the form of semiempirical approaches). On the other hand, reaction thermodynamics depend mostly on only the initial and final states of the system, making thermodynamics as a discipline more directly susceptible to theoretical treatments. Of course certain aspects of kinetics are amenable to first-principles approaches (often applied ad hoc, after semiempirical relationships have been formulated) but it's difficult to construct a comprehensive theoretical framework that leads to easy prediction of how all complex reaction systems will evolve in time.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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