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The Gibbs energy increases from the reactants to the transition state, but according to the Second Law of Thermodynamics, entropy of the universe can only increase. Why is the transition state possible then?

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  • $\begingroup$ Oh no, not that dilemma again! Btw, welcome to chemistry.SE! $\endgroup$ – M.A.R. Mar 23 '15 at 10:29
  • $\begingroup$ Thanks. If you know the answer, though, could please explain? $\endgroup$ – Charles Mar 23 '15 at 10:45
  • $\begingroup$ Wait...You mean there can't be an entropy decrease? $\endgroup$ – M.A.R. Mar 23 '15 at 11:09
  • $\begingroup$ That's what I thought - doesn't the second law say that? $\endgroup$ – Charles Mar 23 '15 at 11:11
  • $\begingroup$ The thermodynamics laws that your studied are valid only for some conditions. It is needed that the system be macroscopyic. Also this law only gives probabilities. It is posible to extend the theory to other areas. Doing so, in molecular cases, it is not too much unprobable this change for one molecule. All the molecules changing (that is what your can calculate with your thermodynamics) is highly unprobable. $\endgroup$ – user1420303 Mar 23 '15 at 12:42
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...according to the Second Law of Thermodynamics, entropy of the universe can only increase.

Well, there was a nice similar question in bio.SE about this. A system is not universe. The overall entropy of the universe increases, but that isn't true for only a particular thermodynamic system.

Also, no one said the transition state is stable!

The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. Wikipedia

(High potential energy = lesser stability)

This chemwiki.ucdavis page is also good to help reshape your understanding of second law of thermodynamics.

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