Question #c2861

As you know, a Bronsted - Lowry acid is defined simply as a proton donor. A proton is also called a hydrogen ion, `"H"^(+)`.

This means that in order for a substance to be classified as a Bronsted - Lowry acid, it must be able to donate at least one hydrogen ion to a Bronsted - Lowry base, which is defined simply as a proton acceptor.

Out of all those four complex ions, the only one that can donate a proton (it can donate as many as three, actually) is the hexaaquairon(III) ion, `["Fe"("H"_2"O")_6]^(3+)`.

![)

Now, when this complex ion is formed, the central iron atom will bind with six water molecules. More specifically, it will bind with a lone pair of electrons from each of those six water molecules.

![http://www.chemthes.com/entity_datapage.php?id=3981]()

As you would imagine, the positive charge of the cation will attract these now bonding electrons from the oxygen atoms.

This will in turn disturb the electron density of the `"O"-"H"` bonds, which are already quite polar, since the bonding electrons that oxygen shares with hydrogen will spend even more time on oxygen.

Moreover, the `3+` charge will not remain concentrated on the iron cation, it will be distributed on the complex ion.

As a result, the partial positive hydrogen atoms will become even more partial positive, and thus easier to pick off by a base.

This means that in aqueous solution, one of the water molecules that are attached to the iron cation will donate one of those very partial positive hydrogen atoms to a free water molecule.

The structure of the complex ion will change, since it will now have five water molecules attached, plus an `"OH"` group.

![)

If you want, you can write the balanced chemical equation for the first ionization of the hexaaquairon(III) ion like this

`["Fe"("H"_2"O")_6]_text((aq])^(3+) + "H"_2"O"_text((l]) rightleftharpoons ["Fe"("H"_2"O")_5("OH")]_text((aq])^(2+) + "H"_3"O"_text((aq])^(+)`

None of the other complex ions have the capacity to donate a proton in aqueous solution, so your answer will be (3) `"Fe"("H"_2"O")_6]^(3+)`, the hexaaquairon(III) ion.