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If I do water electrolysis I will get hydrogen at one electrode and oxygen at the other.

Is it because a molecule of water somewhere in the middle splits into H and O and then the H and O travel to the appropriate electrodes? Or is it something more complicated?

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  • $\begingroup$ All explained here. $\endgroup$
    – Farcher
    Commented May 27, 2019 at 5:44

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Even in "de-ionized" water, some fraction of the water molecules in the bulk will dissociate into $\rm H^+$ and $\rm OH^-$ ions.

In electrolysis, the $\rm H^+$ ions migrate towards the cathode, where they find each other, steal electrons from their surroundings (including the cathode) and form $\rm H_2$ gas. Likewise the $\rm OH^-$ ions migrate to the anode, further dissociate, and form $\rm O_2$ gas. But these migrations are mostly of ions that are already near the electrical terminals. The bulk migration of ions in the fluid, like the "drift velocity" of electrons in a metallic circuit, is surprisingly slow.

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  • $\begingroup$ What happens to the H atoms from OH-? $\endgroup$
    – Criticizing Israel not allowed
    Commented May 27, 2019 at 5:32
  • $\begingroup$ I presume that they turn into $\rm H^+$, which is stable in the bulk, and their migration away from the anode is part of the charge-exchange process. But I don't know the details of that process. $\endgroup$
    – rob ♦
    Commented May 27, 2019 at 5:51
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    $\begingroup$ The Wikipedia link given in the comments to the question indicates that there are multiple reaction pathways at the anode, several of which start with neutral molecules instead of hydroxide ions. $\endgroup$
    – Emilio Pisanty
    Commented May 27, 2019 at 6:26

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