Question

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous \(\mathrm{BF}_{3}\) is bubbled through. Give reasons.

Short answer

AlF₃ dissolves due to formation of NaAlF₄ with added NaF; precipitates as BF₃ consumes free F⁻ ions, reducing AlF₃ solubility.

Step-by-step solution

Understanding Aluminium Trifluoride Solubility

Aluminium trifluoride (AlF₃) is generally insoluble in anhydrous hydrofluoric acid (HF). This insolubility can be attributed to the lack of a fluoride ion source in HF, as both AlF₃ and HF contain electronegative atoms that do not readily dissociate to interact and dissolve. In an anhydrous environment, the AlF₃ remains undissolved because there are no other agents present to assist in its solubility.

Role of Sodium Fluoride Addition

When sodium fluoride (NaF) is added to the mixture of AlF₃ and HF, the solubility dynamics change. NaF dissociates into Na⁺ and F⁻ ions in the solution, increasing the number of fluoride ions available. The fluoride ions then form a complex with AlF₃, specifically \[\text{\{NaAlF}_4\}\]\, which is soluble in the mixture. This allows the AlF₃ to dissolve due to the formation of the soluble complex.

Effect of Bubbling Gaseous \\(\mathrm{BF}_{3}\\)

When gaseous boron trifluoride (BF₃) is introduced into the solution, it reacts with the available F⁻ ions that were part of the NaAlF₄ complex. BF₃, being a strong Lewis acid, forms the complex \[\text{BF}_4^-\]\ with the free F⁻ ions. This reduces the concentration of free fluoride ions in the solution, disrupting the equilibrium and causing AlF₃ to precipitate out because it can no longer remain dissolved without sufficient free F⁻ to form the solubilizing complex.

Key concepts

Solubility

Aluminum trifluoride (AlF₃) has interesting solubility characteristics that differ based on the presence of other substances. In pure anhydrous hydrofluoric acid (HF), AlF₃ is insoluble. This is because HF lacks the free fluoride ions necessary to break up the solid AlF₃. Without the presence of other agents that can change the balance, AlF₃ simply does not dissolve.
However, when another compound like sodium fluoride (NaF) is added, the scenario changes. NaF releases fluoride ions (\( F^- \)) into the solution. These additional ions interact with AlF₃, effectively helping it to dissolve by forming new chemical species. This process exemplifies how solubility is not just about the solvent and solute, but also about the presence of other active ions and molecules that can alter the interactions between them.

Complex ions

The concept of complex ions involves creating stable chemical species that consist of a central metal atom or ion bonded to surrounding molecules or ions, called ligands. When NaF is added to aluminum trifluoride in solution, it produces fluoride ions that can form complex ions with aluminum.
Aluminum forms a complex ion with fluoride ions resulting in the formation of species like \( \text{NaAlF}_4 \), which are soluble. These complex ions are crucial in making certain otherwise insoluble compounds dissolve in solution. By binding to the central aluminum ion, the fluorides change its chemical environment, allowing the formation of this soluble complex, illustrating the dynamic nature of complex ion chemistry in altering solubility.

Lewis acids

In chemistry, Lewis acids are compounds that can accept an electron pair. Aluminum trifluoride can act as a Lewis acid because the aluminum atom in \( \text{AlF}_3 \) can accept electron pairs from fluoride ions.
Boron trifluoride (\( \text{BF}_3 \)), which is also a strong Lewis acid, plays an important role when it is bubbled through the solution containing NaF and \( \text{AlF}_3 \). It seeks out free fluoride ions to form the \( \text{BF}_4^- \) ion, pulling fluoride away from the aluminum complex. This shift leads the AlF₃ to precipitate out of the solution because it loses its fluoride ion complement that kept it in a dissolved state.

Fluoride ions

Fluoride ions (\( F^- \)) have a significant role in the chemistry of aluminum trifluoride. Their presence or absence in a solution can dramatically affect the solubility of \( \text{AlF}_3 \). Initially, in anhydrous HF, there are not enough free fluoride ions to dissolve \( \text{AlF}_3 \).
The addition of \( \text{NaF} \) contributes these necessary fluoride ions, which help form a complex ion that allows \( \text{AlF}_3 \) to dissolve. When \( \text{BF}_3 \) is introduced, it effectively "steals" these fluoride ions to create \( \text{BF}_4^- \) complexes. This leaves insufficient fluoride ions to maintain the solubility of aluminum trifluoride, enforcing its precipitation.

Chemical reactions

Chemical reactions involved in these processes highlight the balance and transformations that occur in solution chemistry. First, the dissolution of \( \text{NaF} \) and subsequent formation of \( \text{NaAlF}_4 \) illustrates a simple reaction where ions interact to produce a soluble complex.
The bubbling of \( \text{BF}_3 \) through the solution exemplifies a different type of reaction. Here, \( \text{BF}_3 \) acts as a strong Lewis acid, forming \( \text{BF}_4^- \) by removing fluoride ions from the solution. This reaction changes the chemistry of the solution significantly, causing the precipitation of \( \text{AlF}_3 \) that was previously dissolved. These chemical reactions show how the interaction between different species can alter states of dissolution and precipitation.