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Can an isolated system(of several phases) in equilibrium by itself move out of equilibrium? if a system contains for example an inflammable gas dispersed with another combustible gas such that the molecular speed are insufficient to cause the reaction. but as the molecular speeds keep fluctuating, at some point, it may cross the energy barrier and may start the reaction which will eventually spread evenly but till then the temperature there would be high and thus the system in inequilibrium. isn't this also an example that although the free energy is minimum and entropy maximum, some useful work would've been done if the temperature gradient had been utilised?

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    $\begingroup$ I'm slightly confused as to why you've specified multiple gases here -- an inflammable gas is, by definition, also combustible, and I'm not sure how that changes your question. Could you clarify? $\endgroup$ – Aesin Mar 31 '13 at 16:44
  • $\begingroup$ it is just to make sure that at the point in space where the reaction begins there is quite a large difference of temperature created and hence the condition of inequilibrium is unanimously accepted. you can ignore that considering only the thought experiment $\endgroup$ – Satwik Pasani Apr 1 '13 at 1:32
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The short answer to the question "Can an isolated system(of several phases) in equilibrium by itself move out of equilibrium?" is NO. Once the system is in equilibrium it will remain there.

If it is possible, as is asked in the second part of the question, to have some reaction be triggered at a higher temperature reached by fluctuation, then as has been commented, the answer is that the system was never in equilibrium.

We should never forget that there is no "absolute" starting temperature for a reaction. If the reaction is physically possible, it will take place, but the rate of that reaction can be horribly slow.

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    $\begingroup$ Technically it can move out of an equilibrium situation spontaneously, since a spontaneous increase in free energy is only statistically disfavoured. If the potential energy landscape has a shallow minimum, reasonable fluctuations might happen. Though now I have two questions. Firstly, do we know of some equilibrium that fluctuates appreciably, and more interestingly, do we know one that does so around room temperature? Or does the mere free energy contribution of mixing overwhelm perceptible fluctuations? Secondly, are there any reaction equilibria with more than one free energy minimum? $\endgroup$ – Nicolau Saker Neto May 12 '13 at 21:39
  • $\begingroup$ @NicolauSakerNeto The presence of two minima in free energy is present in regular solutions wherein the presence of two minima necessitates the splitting of the mixture into two phases of the compositions corresponding to the minima. But I couldn't find examples of such conditions in proper chemical 'reactions' $\endgroup$ – Satwik Pasani Jul 29 '13 at 16:50

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