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In my answer, I stated that manganese is said to not react with water under normal conditions although some sources say it reacts with water to liberate hydrogen gas. Does it implies that it reacts with water at harsh conditions?

WebElements and ChemLibreTexts states that "Manganese does not react with water under normal conditions" whereas Lenntech says "it reacts with water (it rusts like iron) and dissolves in dilute acids." What does rusting here mean? Does manganese rust like iron?

In some sources and Concise Inorganic Chemistry by J.D.LEE states "At room temperature very slowly reacts with water"

$$\ce{Mn + 2H2O -> Mn(OH)2 + H2}$$

and some says that it reacts and oxidizes all the way to permanganate. The results are inconsistent and sometimes contradictory.

Question: Does manganese reacts with water? If so, in what condition (standard or harsh)?

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    $\begingroup$ Many metals react with steam, which may be considered a reaction with water at harsh conditions. Manganese is among them. $\endgroup$ Jun 18, 2022 at 11:41

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The Lenntech phrasing "reacts with water (it rusts like iron)" is incomplete. Wikipedia states that the reaction occurs with "water containing dissolved oxygen". This would actually be like iron, which also requires dissolved oxygen to react in water. With either metal the rusting reaction involves the metal being oxidized to ions while water and oxygen are reduced to hydroxide ions, leading to the formation of $\ce{Mn(OH)2}$ or $\ce{Fe(OH)2}$; but these hydroxides are prone to additional oxidation by oxygen or air to form oxides or oxide-hydroxides with the metal in higher oxidation states.

The above mechanism requires dissolved oxygen as one of the reactants to form the hydroxide and oxide ions; neither metal appreciably displaces hydrogen from otherwise pure water at room temperature. However manganese, again like iron, will react with steam to form oxide(s) plus hydrogen.

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  • $\begingroup$ Dissolved O2 oxidizes Fe+2 to Fe+3. It is not necessary for the reaction of Fe with water to give ferrous hydroxide. Water is reduced to H2 not to hydroxide ions. the latter have no change in oxidation state. $\endgroup$
    – jimchmst
    Jan 13, 2023 at 0:34
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    $\begingroup$ My interpretation is that water and oxygen are both reactants in the half equation $\ce{2 H2O + O2 +4e^- -> 4 OH^-}$. Rust formation mechanisms I have seen must include ferrous hydroxide be assembled iron(II) and hydroxide ions are formed to begin the process. See for instance en.wikipedia.org/wiki/Rust. $\endgroup$ Jan 13, 2023 at 1:00
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thank you for the excellent question. It was really thought provoking indeed. Here's the answer :

  1. Yes, it (Manganese) reacts with water.
  2. As Concise Inorganic Chemistry by J.D.LEE states it reacts with water in STANDARD conditions (if given enough time; which basically means the rate of the reaction is super slow)
  3. As it will react in standard conditions, it might also react in harsh conditions (ie. 100°C or 50°C). "Harsh" conditions weren't properly defined in the question. So, I am unsure about what you meant there.
  4. "Rusts like iron" means it oxidizes like iron. However, from the research done by Mingfei Zhou, Luning Zhang, Limin Shao, Wenning Wang, Kangnian Fan, Qizong Qin, it seems like Manganese Hydroxide is created upon reaction and not Manganese oxide as suggested by "rusts like iron".
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Reaction of manganese with water

The interaction of manganese with water can be described by the following reaction equation: $$\ce{Mn(s) + 2H2O(l) -> Mn(OH)2 (s) + H2 (g)}$$ Most likely, the reaction is slowed down due to the poor solubility of the resulting manganese (II) hydroxide which coats the surface of the reacting manganese and prevents the metal from reacting with water. It is reasonable to say that manganese will react with warm water faster as stated in [4].

References

Hope my answer will bring a bit of clarity to the problem. It is also supplemented with some reference links concerning the interaction of manganese with water:

  1. Ronald L. Rich. Inorganic Reactions in Water (1st ed.): "Water", p. 153.
  2. C.E. Housecroft, A.G. Sharpe. Inorganic Chemistry: "21.8 Group 7: manganese", p. 738.
  3. N.N. Greenwood, A. Earnshaw. Chemistry of the Elements (2nd ed.): p. 1044.
  4. P. Patnaik. Handbook of Inorganic Chemicals: p. 542.
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