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Iron(III) salts are generally yellow in aqueous solution. As a result of the highly charged ion, deprotonation takes place and solutions are acidic: |
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[Fe(H2O)6]3+(aq) |
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Addition of sodium hydroxide solution causes further deprotonation and gives a gelatinous, rusty brown precipitate of iron(III) hydroxide. This precipitate is not soluble in excess sodium hydroxide. |
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Fe3+(aq) + 3OH-(aq) ⇒ Fe(OH)3(s) |
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or [Fe(H2O)6]3+(aq) + 3OH-(aq) ⇒ Fe(H2O)3(OH)3(s) |
Addition of ammonia solution gives the same rusty brown precipitate of iron(III) hydroxide (ammonia solution is alkaline as it produces hydroxide ions in aqueous solution). No ammino complex of iron(III) is formed, so the precipitate is not soluble in excess ammonia.
Iron(II) and iron(III) are the common oxidation states of iron. Consequently, as iron(III) can be reduced to iron(II), iron(III) salts are oxidizing agents:
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Fe3+(aq) + e- ⇒ Fe2+(aq) |
A typical oxidation reaction occurs with colourless iodide ions. The yellow to green colour change of the iron is obscured by the production of brown iodine:
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2Fe3+(aq) + 2I-(aq) ⇒ 2Fe2+(aq) + I2(aq) |
Video - the oxidation of iodide by iron(III)
