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We were just wondering in our chemistry class: is there any reaction which is neither exothermic nor endothermic?

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    $\begingroup$ I suppose you mean single-step / elementary reactions? If so, it seems unlikely; since if there's no energy gain or loss in the reactant species at the end of a reaction, why bother reacting in the first place? In other words: If there's no energy change, then there are no atoms rearranged or charges created...meaning there'd be no reaction. (Not a chemist, but this is the way I see it) :-) $\endgroup$ – paracetamol Jan 10 '18 at 9:51
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    $\begingroup$ @paracetamol You think right, but for the wrong reason. $\endgroup$ – Ivan Neretin Jan 10 '18 at 10:04
  • $\begingroup$ @Ivan Could you clarify? :-) $\endgroup$ – paracetamol Jan 10 '18 at 10:04
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    $\begingroup$ Zero energy gain does not mean the reaction will not happen (there is also entropy to consider). Also, zero energy change does not mean that there are no atoms rearranged. It could just have happened that the new arrangement has the same energy as the old one, yet still they differ. True, this is unlikely, but not impossible. $\endgroup$ – Ivan Neretin Jan 10 '18 at 10:07
  • $\begingroup$ @Ivan Aha! I stand corrected! :-) $\endgroup$ – paracetamol Jan 10 '18 at 10:10
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You could just as well have wondered is there any number which is neither negative nor positive. Yes, there is such number, and it is called zero. It is rarely encountered among the real-life physical values, though, unless enforced by some law (which is not the case here). After all, there are immensely many positive numbers, and equally many negative numbers, and only one zero. You can hardly expect to hit it by choosing numbers at random.

Then again, reaction enthalpies are subject to variation with temperature, so if we pick any reaction with low enthalpy, it is quite probable that there is a certain temperature at which the said enthalpy is exactly 0. That would be the answer to your question. It is for entropy to decide whether the reaction will occur spontaneously at this temperature. Chances are it will.

On a side note, there is a huge number of "void reactions" like $\ce{H2 + H -> H + H2}$ with guaranteed zero enthalpy at any temperature, but many would object against calling these things reactions in the first place.

So it goes.

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  • $\begingroup$ Now it becomes the matter of controlling the temperature to arbitrary precision ... $\endgroup$ – Weijun Zhou Jan 10 '18 at 12:33
  • $\begingroup$ Precision similar to that of enthalpy measurements will suffice. $\endgroup$ – Ivan Neretin Jan 10 '18 at 13:37
  • $\begingroup$ As it happens $\ce{H2 + H}$ is a very interesting reaction when studied in molecular beams as the transient $\ce{H3}$ species formed has proved very difficult to understand. There is much information to be obtained about the potential energy surfaces when the amount of energy in rotational and vibrational levels is observed under conditions of different collision energies . $\endgroup$ – porphyrin Jan 10 '18 at 14:08
  • $\begingroup$ @porphyrin Good to know you are not one of those many who would object. $\endgroup$ – Ivan Neretin Jan 10 '18 at 14:15

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