Is it easier to oxidise $\ce{Fe^2+}$ to $\ce{Fe^3+}$ in acidic or alkaline solution?

$$ \begin{align} \ce{Fe &-> Fe^2+(aq) -> Fe^3+(aq)}\\ \ce{Fe &-> Fe(OH)2 -> Fe(OH)3} \end{align} $$

In my textbook it is written that as soon as $\ce{Fe^{2+}}$ changes to $\ce{Fe^{3+}}$, it acquires $\ce{OH-}$ to form $\ce{Fe(OH)3}$. This promotes oxidation of $\ce{Fe^{2+}}$ changes to $\ce{Fe^{3+}}$. I don't quite get this why precipitate formation speeds up the reaction?

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    $\begingroup$ You said it yourself: the reactions are different. $\endgroup$ Commented Jul 18, 2020 at 10:31
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    $\begingroup$ @user Look, your question has been edited using standardized notations and formatting. Please make sure you use it as an example in your edits. If you have questions regarding formatting, visit this page, this page and this one. $\endgroup$
    – andselisk
    Commented Jul 18, 2020 at 11:04
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    $\begingroup$ The experience shows that a crystal or a solution containing the ion $\ce{Fe^{2+}}$ may be useful for a couple of hours without being significantly oxidized. On the contrary, a precipitate of $\ce{Fe(OH)_2}$ starts being oxidized by oxygen from the air in less than one minute. $\endgroup$
    – Maurice
    Commented Jul 18, 2020 at 11:31
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    $\begingroup$ @user Please note, it is $\ce{Fe^2+}$ and not $\ce{Fe^{+2}}$. See: chemistry.stackexchange.com/questions/24769/… $\endgroup$ Commented Jul 18, 2020 at 11:48
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    $\begingroup$ Also see: chemistry.stackexchange.com/questions/90008/… $\endgroup$ Commented Jul 18, 2020 at 11:50

1 Answer 1


Her is a source: THE MECHANISM OF IRON CATALYSIS IN CERTAIN OXIDATIONS that confirms Maurice's reported observations:

It is well known that ferrous iron is comparatively stable in acid solution and that it is rapidly oxidized to the ferric state by the oxygen of the air in alkaline solution.

On the question of why, I quote another work: Fe2+ adsorption on iron oxide: the importance of the redox potential of the adsorption system with citing references:

Nowadays the dominant hypothesis is that adsorption of Fe(II) by ferric and non-ferric oxides has a different mechanism. Fe(II) adsorbed on ferric oxide (like Fe2O3) can be easily oxidized by the transfer of an electron from adsorbed species to the solid (Gorski and Scherer 2011; Hiemstra and van Riemsdijk 2007; Larese-Casanova et al. 2012).

Note my added link to the work of Hiemstra based on surface complexation modeling.

I hope this helps


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