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Topic: relationship btw temperature and Arrhenius activation energy  (Read 3074 times)

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Offline youngtreeest

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relationship btw temperature and Arrhenius activation energy
« on: February 01, 2014, 11:45:43 AM »
There is no relationship btw temperature and Arrenhius activation energy for a reaction, right?

Thank you very much!

Offline curiouscat

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Re: relationship btw temperature and Arrhenius activation energy
« Reply #1 on: February 01, 2014, 02:30:37 PM »
One's an independent variable. Other's a constant (to some extent).

No, there's no non-trivial functional relation between a constant and a variable.

Offline youngtreeest

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Re: relationship btw temperature and Arrhenius activation energy
« Reply #2 on: February 03, 2014, 04:40:57 AM »
Thank you very much for the confirmation!

Offline Irlanur

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Re: relationship btw temperature and Arrhenius activation energy
« Reply #3 on: February 20, 2014, 04:38:13 PM »
As a reaction can occurr over several transition states, for large molecules some zillions, and these states don't have all the same energy, there certainly is a dependence between Temperature and the activation energy. After all, this dependence is small and assuming the activation energy is a constant is often reasonable.

Offline Corribus

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Re: relationship btw temperature and Arrhenius activation energy
« Reply #4 on: February 20, 2014, 05:42:54 PM »
In the traditional Arrhenius formulation, the activation energy is usually assumed to be temperature independent. In transition state theory, however, which use a more rigorous thermodynamical formulation, the activation energy (energy difference between the transition state and the reactants) is shown explicitly to be temperature dependent. If you have a p-chem book handy, you might look up the Eyring Equation. In general the Arrhenius activation energy is approximately equal to the activation enthalpy + RT.  (Because reaction coordinates are expressed on a Gibbs energy scale, it should be obvious that there is a temperature dependence because the Gibbs energy is temperature dependent: G = H - TS.) The effect of temperature is usually weak over small temperature ranges, which is why the Arrhenius equation works well in an overwhelmingly large number of cases.

You might find this paper a relevant example of the temperature dependence of the activation energy, if you have access to JACS.

http://pubs.acs.org/doi/abs/10.1021/j150609a003
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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