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Characteristic Reactions of Iron (Fe³⁺)

  • Page ID
    97270
    • Most common oxidation states: +2, +3
    • M.P. 1535º
    • B.P. 2750º
    • Density 7.87 g/cm3
    • Characteristics: Iron is a gray, moderately active metal.

    Characteristic reactions of Fe²⁺ and Fe³⁺

    The \(\ce{[Fe(H2O)6]^{3+}}\) ion is colorless (or pale pink), but many solutions containing this ion are yellow or amber-colored because of hydrolysis. Iron in both oxidation states forms many complex ions.

    Aqueous Ammonia

    Aqueous ammonia reacts with Fe(II) ions to produce white gelatinous \(\ce{Fe(OH)2}\), which oxidizes to form red-brown \(\ce{Fe(OH)3}\):

    \[\ce{Fe^{2+}(aq) + 2NH3(aq) + 3H2O(l) <=> Fe(OH)2(s) + 2NH4^{+}(aq)}\]

    Fe3appt.gif

    Aqueous ammonia reacts with \(\ce{Fe(III)}\) ions to produce red-brown \(\ce{Fe(OH)3}\):

    \[\ce{Fe^{3+}(aq) + 3NH3(aq) + 3H2O(l) <=> Fe(OH)3(s) + 3NH4^{+}(aq)}\]

    Fe3bppt.gif

    Both precipitates are insoluble in excess aqueous ammonia. Iron(II) hydroxide quickly oxidizes to \(\ce{Fe(OH)3}\) in the presence of air or other oxidizing agents.

    Sodium Hydroxide

    Sodium hydroxide also produces \(\ce{Fe(OH)2}\) and \(\ce{Fe(OH)3}\) from the corresponding oxidation states of iron in aqueous solution.

    \[\ce{Fe^{2+}(aq) + 2OH^{-}(aq) <=> Fe(OH)2(s)}\]

    Fe4appt.gif

    \[\ce{Fe^{3+}(aq) + 3OH^{-}(aq) <=> Fe(OH)3(s)}\]

    Fe4bppt.gif

    Neither hydroxide precipitate dissolves in excess sodium hydroxide.

    Potassium Ferrocyanide

    Potassium ferrocyanide will react with \(\ce{Fe^{3+}}\) solution to produce a dark blue precipitate called Prussian blue:

    \[\ce{K^{+}(aq) + Fe^{3+}(aq) + [Fe(CN)6]^{4-}(aq) <=> KFe[Fe(CN)6](s)} \label{Prussian}\]

    Fe5a1ppt.gif

    With \(\ce{Fe^{2+}}\) solution, a white precipitate will be formed that will be converted to blue due to the oxidation by oxygen in air:

    \[\ce{2Fe^{2+}(aq) + [Fe(CN)6]^{4-}(aq) <=> Fe2[Fe(CN)6](s) }\]

    Fe5a2ppt.gif

    Many metal ions form ferrocyanide precipitates, so potassium ferrocyanide is not a good reagent for separating metal ions. It is used more commonly as a confirmatory test.

    Potassium Ferricyanide

    Potassium ferricyanide will give a brown coloration but no precipitate with \(\ce{Fe^{3+}}\). With \(\ce{Fe^{2+}}\), a dark blue precipitate is formed. Although this precipitate is known as Turnbull's blue, it is identical with Prussian blue (from Equation \ref{Prussian}).

    \[\ce{K+(aq) + Fe2+(aq) + [Fe(CN)6]^{3-}(aq) <=> KFe[Fe(CN)6](s)}\]

    Fe5b.gif

    Potassium Thiocyanate

    \(\ce{KSCN}\) will give a deep red coloration to solutions containing \(\ce{Fe^{3+}}\):

    \[\ce{Fe3+(aq) + NCS^{-}(aq) <=> [FeNCS]2+(aq)}\]

    Fe5cppt.gif

    No Reaction

    \(\ce{Cl^{-}}\), \(\ce{SO4^{2-}}\)