Iron(III) Aqua-Ion Hydrolysis and Colour
Dissolving an iron(III) salt in water rarely gives the simple, pale hexaaqua ion you might expect — instead, a hydrolysis equilibrium and ligand-exchange chemistry together produce a brown solution, and different acids added afterward pull that equilibrium in different directions with visibly different colours.
Fe³⁺ Aqua-Ion Hydrolysis Equilibrium and Colour Changes is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Why dissolving Fe(NO₃)₃.9H₂O in water gives a brown solution, not pale violet
The pure hexaaqua complex, [Fe(H₂O)₆]3+, is only very pale violet. But Fe³⁺ is small and highly charged, which makes the coordinated water molecules significantly more acidic than free water — so in ordinary water, the hexaaqua ion partially hydrolyses: [Fe(H₂O)₆]3+ + H₂O is in equilibrium with [Fe(OH)(H₂O)₅]2+ + H₃O⁺, and further hydrolysis can give [Fe(OH)2(H₂O)₄]+ as well. These partially hydrolysed, hydroxo-bridged species absorb light differently from the simple hexaaqua ion and are responsible for the BROWN colour actually observed when the pale violet crystals dissolve in plain water — the brown colour is a direct signature of this hydrolysis equilibrium being significantly populated, not of the pure hexaaqua ion itself.
Adding acid shifts the equilibrium back toward the pale violet hexaaqua ion
Adding concentrated nitric acid supplies a large excess of H₃O⁺ (or H⁺), which by Le Chatelier's principle pushes the hydrolysis equilibrium [Fe(H₂O)₆]3+ + H₂O <=> [Fe(OH)(H₂O)₅]2+ + H₃O⁺ back toward the LEFT (toward the simple hexaaqua ion), suppressing the brown hydrolysed species and restoring the pale violet colour of [Fe(H₂O)₆]3+ itself. Nitrate is a poor ligand for iron(III) here (it does not compete with water for coordination), so adding HNO₃ acts purely as a source of H⁺ to suppress hydrolysis, without introducing any new coloured complex.
Adding HCl instead gives a different colour, through ligand substitution rather than just pH
Adding hydrochloric acid instead introduces chloride ion, which — unlike nitrate — IS a good enough ligand to directly compete with water for coordination to Fe³⁺, displacing water ligands to form a new complex, [FeCl₄]-, which is YELLOW. This colour change is therefore not simply a hydrolysis-suppression effect (as with HNO₃); it is genuine ligand substitution, swapping some of the coordinated water/hydroxide for chloride, producing a chemically different complex ion with its own distinct colour. Comparing the HNO₃ and HCl results side by side is a clean way to see the difference between shifting an existing equilibrium (nitric acid, pale violet hexaaqua ion restored) and forming an entirely new complex through ligand substitution (hydrochloric acid, yellow tetrachloroferrate).
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- ✓The hexaaqua ion [Fe(H₂O)₆]3+ is pale violet on its own, but Fe³⁺ is acidic enough that it partially hydrolyses in plain water
- ✓Partially hydrolysed species like [Fe(OH)(H₂O)₅]2+ are responsible for the brown colour of an ordinary aqueous Fe³⁺ solution
- ✓Adding excess HNO₃ supplies H⁺, shifting the hydrolysis equilibrium LEFT and restoring the pale violet hexaaqua-ion colour — nitrate itself does not coordinate
- ✓Adding HCl instead introduces chloride, which DOES coordinate to Fe³⁺, displacing water to form yellow [FeCl₄]- — a ligand-substitution effect, not just a pH effect
Do not assume every colour change on adding acid to an Fe³⁺ solution works the same way. HNO₃ restores the pale violet hexaaqua-ion colour purely by suppressing hydrolysis (shifting an equilibrium); HCl gives a completely different yellow colour because chloride actually coordinates to the metal, forming a new complex — same starting solution, two different mechanisms.
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