Can anyone tell me exactly what happens during a chemical reaction in terms of ionisation energy? I know that during a reaction there is an exchange of electrons (except for those where electrons are shared), more specifically the valence electrons. But where exactly does the ionisation energy come from? Does it come from the charge of the other atom or...?

I'm asking this because I'm being asked to describe the reaction of sodium and water in terms of valance electrons and ionisation energy - and I just can't seem to understand where the ionisation energy comes from for there to be a reaction. I tried putting the same question as above to some AI chatbots and they seemed to suggest that the ionisation energy comes from the positive charge of the water attracting the single valance electron of the sodium. But water is neutral, so that can't really be the case, can it?

  • 1
    $\begingroup$ Why would you think that in the actual reaction there would be some single atoms of Na? Surprisingly, quite a lot of electrons are actually lost from a chunk of sodium reacting with water, but it's whole phase getting charged and I think you're over your head here. chemistry.stackexchange.com/questions/2606/… $\endgroup$
    – Mithoron
    Oct 14, 2023 at 17:16
  • $\begingroup$ Does this answer your question? Definition of activation energy $\endgroup$ Oct 15, 2023 at 19:48
  • $\begingroup$ Thank you Mithoron and DrMoishe Pippik :D!! The answer provided by @anon below proves to be very useful, but the threads which you have linked is also quite interesting. Maybe I will check them out in the future! $\endgroup$ Oct 16, 2023 at 8:47

1 Answer 1


In most reactions, the activation energy -- that is, the energy required to start the reaction -- comes from the motion of the molecules. In other words, it comes from heat.

(There are other possibilities. In photochemical reactions, some or all of the activation energy is provided by light. But the reaction of sodium and water is not photochemical. It happens in total darkness.)

There is an important difference between ionization energy and activation energy. As Poutnik explains in his answer, ionization energy is like escape velocity. It's the amount of energy needed to take an electron from sodium and fling it away to an infinite distance. This is a large amount of energy: 496 kJ/mol for sodium. That is way too much to be provided by the ambient heat at room temperature.

The reaction between sodium and water proceeds at room temperature anyway. This is because the electron does not actually need to be ripped away and sent to an infinite distance. It only needs to be pulled far enough away to create a transition state: a situation where the electron is balanced between the sodium it came from and a water molecule. In the transition state, the water molecule is quite close to the sodium, not at an infinite distance. Therefore only a small fraction of the ionization energy is required to reach the transition state.

  • $\begingroup$ Thank you for your answer! I got it now :D! $\endgroup$ Oct 16, 2023 at 8:45

Not the answer you're looking for? Browse other questions tagged or ask your own question.