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Like when iron reacts with water there forms iron oxide and hydrogen but how'll we come to know which of three equation will be right equation for reaction?

$$\ce{Fe + H2O → FeO + H2}$$

$$\ce{2Fe + 3H2O → Fe2O3 + 3H2}$$

$$\ce{3Fe + 4H2O → Fe3O4 + 4H2}$$

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    $\begingroup$ There are no general rules ! Of course the oxidation of iron passes always by the level Fe(II). But sometimes the oxidation goes immediately to the the level Fe(III). It depends on the nature of the oxidant. If $\ce{Fe}$ reacts with $\ce{HCl}$ it produced $\ce{FeCl2}$, as the $\ce{H2}$ produced would be able de reduce $\ce{FeCl3}$ to $\ce{FeCl2}$. If $\ce{Fe}$ reacts with $\ce{Cl2}$, it produces $\ce{FeCl3}$, as $\ce{Cl2}$ is able to oxidize $\ce{FeCl2}$ to $\ce{FeCl3}$ $\endgroup$ – Maurice Dec 17 '20 at 10:47
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    $\begingroup$ Note that there is an almost general rule, that oxides of metals with multiple (and close) oxidation states, like Fe, Mn, Co, Ni form frequently non-stoichiometric oxides, where atomic ratios are not a ratio of small integers. E.g. iron(II) oxide with theoretical composion FeO has the real composition Fe(0.84)O to Fe(0.95)O. $\endgroup$ – Poutnik Dec 17 '20 at 11:41
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    $\begingroup$ It depends, in many cases, on how much of the reactant is present to force the reaction to a higher oxidation state. See en.wikipedia.org/wiki/Law_of_mass_action $\endgroup$ – DrMoishe Pippik Dec 18 '20 at 3:05
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The stability of the various iron oxides depends on the oxygen fugacity (or oxygen partial pressure, which is a good approximation to fugacity in near-atmospheric pressures).

$\ce{Fe^3+}$ is the stable form of iron in atmospheric oxygen partial pressures, this in such conditions $\ce{Fe2O3}$ (hematite) will always be the stable product. The lower oxidation state oxides (such as magnetite $\ce{Fe3O4}$ and wüstite ~$\ce{FeO}$) can only stably form in deep earth environments, or in special controlled atmosphere reaction chambers.

That said, in normal conditions it is even more likely for iron hydroxides to form hydrated compounds, especially when water is a reactant, or is present. In these cases, you get a zoo of various Fe-oxides-hydroxides, some of which are amorphous (not crystalline), and others are metastable. These compounds are generally referred to as "rust".

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  • $\begingroup$ As Fe2O3 is reduced by H2 to Fe3O4, I bet rather Fe3O4 will be formed. Note that iron reacts with water mostly at conditions of hot fine iron particles passed by water vapour. See also en.wikipedia.org/wiki/Iron%28II%2CIII%29_oxide $\endgroup$ – Poutnik Dec 19 '20 at 13:47

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