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I am planning on doing some reactions that lead to Manganese metal and a neutral solution containing $\ce{Mn^{2+}}$ as by-products, and I wanted to try some things out with the Manganese as well.

Since Manganese has common charges of II and III, will a relatively fine, grainy form of the metal burn (and at what temperature) in air to form a mixed "$\ce{Mn(II, II)}$" oxide, $\ce{Mn3O4}$, the way Iron does as it rusts, or is $\ce{MnO}$ more predominant?

I have the same question about whether or not $\ce{Mn3O4}$ is formed in "thermite-like" reactions such as reacting $\ce{CuO}$ with $\ce{Mn}$.

As for water, I know that Manganese metal is one of the more active metals among the transition metals, so I am also concerned with whether Aluminum can be used to reduce $\ce{Mn^{2+}}$ to the metal in boiling water without the formation of any Manganese Oxide or Hydroxide side products due to the water (especially due to the fact that reducing metals like this precipitates them out as a fine powder).

$\ce{ 2Al + 3Mn^{2+} -> 2Al^{3+} + 3Mn}$

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  • $\begingroup$ Mn has too low redox potential to be deposited from water solutions. H2 is formed instead, from H+ or H2O. $\endgroup$
    – Poutnik
    Commented Jun 17, 2022 at 3:39
  • $\begingroup$ @Poutnik There's an inconsistency with this case. Some sources say manganese don't react with water under normal conditions but somewhere else, it says it does reacts and liberates hydrogen gas. $\endgroup$
    – Nilay Ghosh
    Commented Jun 17, 2022 at 5:02
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    $\begingroup$ @NilayGhosh It has TD and kinetic aspects. I have written it in context of Mn deposition by metallic Al, not Mn dissolution. $\endgroup$
    – Poutnik
    Commented Jun 17, 2022 at 5:06
  • $\begingroup$ @Poutnik Is this explained by the fact that $\ce{2H2O + 2e^- -> H2 + 2OH^- }$ has a potential of -0.83 which is higher than $\ce{Mn^{2+} + 2e^- -> Mn}$'s potential of -1.18? $\endgroup$
    – User1618
    Commented Jun 17, 2022 at 7:34
  • $\begingroup$ Yes, it is. But you have omitted to add standard to the potential, as the potential is pH dependent by 0.059 V/pH unit, if 1 H+ or 1 OH- per 1 e- are involved. Aditionally, potential -0.83 V belongs to pH 14, where are practically no Mn^2+(aq) but Mn(OH)2(s), so redox potential for Mn/Mn^2+ would be much lower than -1.18 V. $\endgroup$
    – Poutnik
    Commented Jun 17, 2022 at 8:39

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Unlike your claim, manganese is relatively inert despite it being a little more electropositive than its neighbors in the periodic table. It is not particularly reactive to air. The surface of manganese lumps oxidize to a minor extent but when it is finely divided, it becomes pyrophoric and burns in air. It tends to form the oxide $\ce{Mn3O4}$ and the nitride $\ce{Mn3N2}$. Temperature for such reactions are reaches more than 500 °C.

$$\ce{3Mn(s) + 2O2(g) ->[\Delta] Mn3O4(s)}$$

$$\ce{3Mn(s) + N2(g) ->[\Delta] Mn3N2(s)}$$

Manganese is also said to not react with water under normal conditions although some sources say it reacts with water to liberate hydrogen gas.

For your thermite reaction, I haven't found any $\ce{Mn-CuO}$ reaction but I did find $\ce{Al-Mn^2+-}$ thermite reaction. You can find the details here.

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