I was thinking lately about activation energy's dependency on time. I couldn't find any information on the internet. Only answer that came up to my mind is the fact that activation energy is constant, and doesn't usually change with time.

But what about auto-catalytic reactions? Maybe their activation energy changes with time after creation of small amount of product, at least theoretically. What do you think?

  • 1
    $\begingroup$ Can you provide an example of the process you are thinking about? Of course if the process changes in some way the activation energy can change. In particular, if you modify the transition path for a reaction. This is called catalysis. $\endgroup$
    – Buck Thorn
    May 30, 2019 at 8:38
  • $\begingroup$ Ican only think of iodination of acetone, which is some of the well known autocatalytic reactions. $\endgroup$
    May 30, 2019 at 9:33
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    $\begingroup$ Imagine that you repeatedly measure the height of a stone wall, and on one day you find it significantly lower than the day before. Do lengths and heights change with time on their own? Maybe they do. But maybe it was just a group of guys with hammers and other demolition equipment that did the trick. Same thing here. $\endgroup$ May 30, 2019 at 9:56
  • $\begingroup$ As Ivan Neretin writes, the activation energy is constant and is caused by the electronic energy in the bonds. Time does enter into a reaction, however, because only occasionally (say 1000 times/sec) do the reactants, by random thermal motion, come together in the right orientation and with the right energy to react. $\endgroup$
    – porphyrin
    May 30, 2019 at 10:09
  • $\begingroup$ If you add catalyst you can certainly expect the activation barrier for those molecules reacting through the catalytically-activated process to be reduced. The fraction of those molecules will change over time in accordance to the catalyst concentration. $\endgroup$
    – Buck Thorn
    May 30, 2019 at 10:50

1 Answer 1


The activation energy of a single reaction does not change with time but there might be more than one reaction happening

An activation energy is always constant for a given single reaction. But that doesn't mean the reaction speed will be constant (it might depend on concentration or temperature).

But what about autocatalytic reactions? There are two possible sources of confusion here. A very simple autocatalytic reaction might involve a changing concentration of an intermediate. In this case the reaction might go faster because that concentration is changing as the intermediate builds up (but the activation energy stays the same). In more complex cases, the catalytic route to the product involves a different reaction. What you have is not a changing activation energy for a specific reaction but the addition of a different reaction pathway to give the same product. This reaction will have a different activation energy but the activation energy of the original pathway is not changed.


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