3
$\begingroup$

In an answer to this question, it was asserted that:

Student always tend to forget that a solution must remain electrically neutral by all means. If you want a certain ion to leave water, you must provide another ion of the same charge that has left the solution.

As per discussion in the comments to that answer, is the second quoted sentence strictly true?

Can electrical neutrality not be achieved by electrons (rather than ions) migrating one way or another to provide balance? For instance, perhaps imagine an aqueous solution under electrolysis which is divided by an appropriate membrane. A quick search brings up AEM (Nature paper) which appears to depict unidirectional migration of hydroxyl ions.

$\endgroup$
8
  • 2
    $\begingroup$ The requirement for neutrality is not as strict as to a single ion excess. It just means forming local excess of one kind of ions rapidly changes local potential, which is acting against such excess. $\endgroup$
    – Poutnik
    Jun 9 at 6:14
  • $\begingroup$ @Poutnik yes indeed, but I don't think that's what the quoted text meant to exclude - I suspect the author meant "in order for ions to continue to pass in one direction you must provide ions going in the other" (or similar). Perhaps they will reply here. $\endgroup$
    – abligh
    Jun 9 at 6:21
  • 3
    $\begingroup$ Can electrical neutrality not be achieved by electrons (rather than ions) migrating one way Electrons react with water in fractions of milisecond, what can be seen as short flashes of blue color of hydrated electrons during reaction of alkali metals with water. It was observed using an ultrafast camera with thousands FPS. $\endgroup$
    – Poutnik
    Jun 9 at 7:14
  • 1
    $\begingroup$ @htmlcoderexe Search for articles (co)authored by Czech chemist Pavel Jungwirth. Some of them, like the one in Nature, may be behind a paywall. the main novelty is the explosion of alkali metals in contact with water is primarily physical coulombic explosion, not explosion of hydrogen. See e.g. Google: "Pavel+Jungwirth" alkali metal explosion electrons blue $\endgroup$
    – Poutnik
    Jun 9 at 12:37
  • 1
    $\begingroup$ @htmlcoderexe See also Jungwirth group publications, with possible search filtering on site and Nature: Coulomb explosion during the early stages of the reaction of alkali metals with water $\endgroup$
    – Poutnik
    Jun 9 at 12:47

1 Answer 1

2
$\begingroup$

Yes-but.

An inert cathode (e.g. the standard hydrogen electrode used as a cathode) does release electrons into water, but those electrons almost immediately react with the water, according to what we normally think of as a half-reaction,

$$\ce{2e- + 2H_2O -> H_2 + 2OH-}$$

so the effect is to produce hydroxide ions in solution (and hydrogen gas).

A series of experiments involving high-speed cameras and alkali metals have demonstrated that free electrons are very briefly in solution in water before reacting, e.g.

Coulomb explosion during the early stages of the reaction of alkali metals with water
Mason PE, Uhlig F, Vaněk V, Buttersack T, Bauerecker S, Jungwirth P.
Nature Chemistry 7(3):250-4 (2015)

A Non-Exploding Alkali Metal Drop on Water: From Blue Solvated Electrons to Bursting Molten Hydroxide
Mason PE, Buttersack T, Bauerecker S, Jungwirth P.
Angewandte Chemie 55(42):13019-13022 (2016)

(Free electrons can be a stable solute for much longer in other solvents.)

$\endgroup$
2
  • $\begingroup$ ... but in the case of an aqueous solution under electrolysis, don't the electrons go through the wire (and power supply)? Which is why we see electron flow in the wire and (I presume) no free electrons / blue flashes of light. $\endgroup$
    – abligh
    Jun 11 at 17:25
  • $\begingroup$ @abligh The electrons go through the wire and power supply, yes, but those are in between the anode and the cathode. Once electrons reach the cathode, they go into solution and then react with the water as I described. I think the reason we don't observe blue flashes with the standard hydrogen electrode is just that the electrons enter solution much more slowly than they do when being shed by alkali metals, so the concentration never gets high enough to be observable. $\endgroup$
    – zwol
    Jun 13 at 16:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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