Problem 31 - Chemistry: The Central Science

Question

A number of salts containing the tetrahedral polyatomic anion, \(\mathrm{BF}_{4}^{-}\), are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\) do not form ionic liquids. Explain this observation.

Step-by-Step Solution

Answer

In short, salts containing the tetrahedral polyatomic anion \(\mathrm{BF}_{4}^{-}\) form ionic liquids due to the weaker intermolecular forces and less polar nature of this ion compared to the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\). The lower polarity of \(\mathrm{BF}_{4}^{-}\) results in weaker interactions between the ions, allowing them to move more easily in the liquid phase. In contrast, the higher polarity of \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\) strengthens the interactions between ions, leading to higher melting points and a lack of ionic liquid formation.

Step1: Ionic Liquids

First, let's understand what ionic liquids are. Ionic liquids are salts that are typically a liquid at or below room temperature, having low melting points. This is due to their ionic nature, which allows them to remain in the liquid phase as a result of weak intermolecular forces between ions in the liquid.

Step 2: Analyzing Tetrahedral Structures

When looking at the two ions given, both have tetrahedral structures. The slightly smaller tetrahedral ion, \(\mathrm{BF}_{4}^{-}\), has boron at the central atom, surrounded by four fluorine atoms. The larger tetrahedral ion, \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\), has sulfur at the central atom, surrounded by four oxygen atoms.

Step 3: Comparing Electronegativity

The electronegativity difference between boron and fluorine in \(\mathrm{BF}_{4}^{-}\) is smaller compared to the difference between sulfur and oxygen in \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\). This is because boron has an electronegativity of 2.0, while fluorine is the most electronegative element with a value of 3.98. Meanwhile, sulfur has an electronegativity of 2.58, and oxygen has an electronegativity value of 3.44. Therefore, the \(\mathrm{BF}_{4}^{-}\) ion has a more evenly distributed electron density than \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\).

Step 4: Examining Ionic Bonding and Intermolecular Forces

Both anions are equally symmetrical in shape (tetrahedral), but \(\mathrm{BF}_{4}^{-}\) is much less polar than \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\). The lower polarity of \(\mathrm{BF}_{4}^{-}\) results in weaker intermolecular forces (IMFs) such as dipole-dipole interactions or hydrogen bonding, allowing the ions to slide past each other more easily. In contrast, \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\) exhibits stronger IMFs due to its higher polarity, which increases its melting point, making it harder to form ionic liquids.

Step 5: Conclusion

In conclusion, salts containing the tetrahedral polyatomic anion \(\mathrm{BF}_{4}^{-}\) form ionic liquids because of the weaker intermolecular forces and less polar nature of this ion compared to the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\). The lower polarity of \(\mathrm{BF}_{4}^{-}\) results in weaker interactions between the ions, allowing them to move more easily in the liquid phase, while the higher polarity of \(\mathrm{SO}_{4}{\underline{\phantom{xx}}}^{2-}\) strengthens the interactions between ions, leading to higher melting points and a lack of ionic liquid formation.