Question

When \(\mathrm{Fe}^{3+}\) ions dissolve in water, they immediately form \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{3+}\) aqueous ions in which six molecules of water bind to the iron via six Fe-O bonds. However, this is not all that happens. The solution also becomes acidic. Interestingly, \(\mathrm{Fe}^{2+}\) ions also form similar \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}\) ions, but the solution does not become acidic. Explain why one solution becomes acidic and the other does not. (Hint: The more positive the metal center, the more it attracts electrons to itself. Think about the effect this has on bonds within the ions.)

Short answer

The difference in acidity between solutions of \(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\) ions is due to the charge on the metal center and its effect on the polarization of Fe-O bonds. The higher positive charge on the \(\mathrm{Fe}^{3+}\) metal center makes its Fe-O bonds more polarized, leading to easier dissociation of O-H bonds in water ligands, generating H+ ions and forming an acidic solution. In contrast, the weaker polarization of Fe-O bonds in the \(\mathrm{Fe}^{2+}\) case does not lead to significant dissociation of O-H bonds, so the solution does not become acidic.

Step-by-step solution

Understanding the ions and complexes formed in each case

In both cases, we are dealing with iron ions, \(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\). When they dissolve in water, they form complexes with water molecules as ligands via Fe-O bonds. These complexes are \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{3+}\) and \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}\), respectively. The main difference between these two complexes is the charge on the metal center.

Evaluating the effect of charge on the metal center

The hint in the exercise suggests that the more positive the metal center, the more it attracts electrons to itself. This means that the Fe-O bonds in the \(\mathrm{Fe}^{3+}\) case would be more polarized than in the \(\mathrm{Fe}^{2+}\) case.

Analyzing the effect of bond polarization on acidity

When the Fe-O bonds are more polarized, the O-H bonds in the water ligands become more easily dissociable (as the oxygen atom is more attracted to the positively charged metal center and less to its own hydrogen atom). This dissociation generates protons (H+) in the solution, which makes the solution acidic.

Comparing the dissociation of water ligands in both cases

In the case of \(\mathrm{Fe}^{3+}\), the stronger polarization of Fe-O bonds leads to the easier dissociation of O-H bonds in the water ligands, generating more H+ ions and making the solution acidic. On the other hand, in the case of \(\mathrm{Fe}^{2+}\), the weaker polarization of Fe-O bonds does not lead to a significant dissociation of O-H bonds, and therefore, the solution does not become acidic.

Conclusion

The difference in acidity between the solutions of \(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\) ions lies in the charge on the metal center and its effect on the polarization of the Fe-O bonds. The higher positive charge on the \(\mathrm{Fe}^{3+}\) metal center makes its Fe-O bonds more polarized, which in turn results in the easier dissociation of O-H bonds in the water ligands and the formation of an acidic solution. In contrast, the \(\mathrm{Fe}^{2+}\) ions do not cause the same degree of bond polarization and dissociation, so the solution does not become acidic.

Key concepts

Iron Ion Complexes

When iron ions like \(\mathrm{Fe}^{3+}\) and \(\mathrm{Fe}^{2+}\) dissolve in water, they form complexes where water molecules act as ligands. These complexes can be represented as \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{3+}\) and \(\left[\mathrm{Fe}\left(\mathrm{OH}_{2}\right)_{6}\right]^{2+}\).
The main role of these iron ion complexes is to bind water molecules through iron-oxygen (Fe-O) bonds.

• \(\mathrm{Fe}^{3+}\) ions create a complex with a 3+ charge.
• \(\mathrm{Fe}^{2+}\) ions form a similar complex with a 2+ charge.

The charge difference on these complexes plays a crucial role in how they behave in solution, especially concerning acidity.

Charge Polarization

The concept of charge polarization is central to understanding why solutions of these complexes behave differently.
Charge polarization occurs when the positive metal center of the complex attracts electrons from its surrounding environment.
In our case, this refers to the electrons in the water molecules bonded to the iron.

• A higher positive charge means stronger attraction for electrons.
• \(\mathrm{Fe}^{3+}\) ions have a greater positive charge compared to \(\mathrm{Fe}^{2+}\).

This difference in charge affects how polarized the Fe-O bonds become, significantly influencing the solution's overall behavior.

Fe-O Bond

Fe-O bonds are the connections between iron ions and the water molecules in the complex.
The polarity of these bonds changes depending on the charge of the iron ion.

• In \(\mathrm{Fe}^{3+}\) complexes, Fe-O bonds are more polarized.
• In \(\mathrm{Fe}^{2+}\) complexes, these bonds are less polarized.

A more polarized Fe-O bond means that the oxygen in the water molecule is drawn more closely towards the iron ion, changing how it interacts with its hydrogen atoms. This sets the stage for potential O-H bond dissociation.

O-H Bond Dissociation

O-H bond dissociation is a key factor in understanding why the acidity of these solutions differs.
Due to the charge polarization, more polarized Fe-O bonds affect the O-H bonds in the water molecules.

• In \(\mathrm{Fe}^{3+}\) ions, the oxygen is more attracted to the positive iron center, weakening the O-H bond.
• This easier breaking of the O-H bond leads to the release of hydrogen ions (H\(^+\)), making the solution acidic.
• In \(\mathrm{Fe}^{2+}\) ions, less polarization means less breaking of O-H bonds, resulting in fewer hydrogen ions released and a neutral solution.

Thus, the strength and polarization of the Fe-O bond directly impact the potential for O-H bond dissociation and hence, the solution’s acidity.