Question

Describe the solubility of CaF \(_{2}\) in each solution compared to its solubility in water. a. in a \(0.10 \mathrm{M} \mathrm{NaCl}\) solution b. in a \(0.10 \mathrm{M}\) NaF solution c. in a \(0.10 \mathrm{M}\) HCl solution

Short answer

The solubility of CaF2: a. is unaffected in 0.10 M NaCl solution, b. is decreased in 0.10 M NaF solution due to the common ion effect, and c. is increased in 0.10 M HCl solution due to the formation of HF.

Step-by-step solution

Understanding the Common Ion Effect

The solubility of a salt can be affected by the presence of a common ion, which is already present in the solution. Adding an ion that is a part of the salt can shift the dissolution equilibrium, resulting in reduced solubility due to Le Chatelier's Principle.

Assessing the Effect of NaCl Solution on Solubility

A 0.10 M NaCl solution provides chloride ions (Cl-), but since CaF2 does not contain chloride ions, the solubility of CaF2 will not be significantly affected by the NaCl solution. The Na+ ions also do not interact with CaF2, so the solubility remains similar to that in pure water.

Assessing the Effect of NaF Solution on Solubility

A 0.10 M NaF solution supplies fluoride ions (F-), which are common to CaF2. Due to the common ion effect, the addition of fluoride ions will suppress the solubility of CaF2 by shifting the dissolution equilibrium to the left, thus reducing its solubility compared to that in pure water.

Assessing the Effect of HCl Solution on Solubility

A 0.10 M HCl solution introduces chloride ions (Cl-) and hydrogen ions (H+). While chloride ions do not directly reduce the solubility of CaF2, the hydrogen ions can react with the fluoride ions to form HF, effectively removing fluoride ions from the solution and driving the dissolution equilibrium of CaF2 to the right. This can increase the solubility of CaF2 compared to that in water.

Key concepts

Solubility Equilibrium

Solubility equilibrium refers to the dynamic balance between a solid and its ions in a saturated solution. When a salt like calcium fluoride (CaF2) dissolves in water, it breaks down into calcium ions (Ca2+) and fluoride ions (F-). As some of these ions begin to recombine to form the solid, a point is reached where the rate of dissolution equals the rate of precipitation, establishing equilibrium. The expression of this equilibrium is called the solubility product constant (Ksp), which is specific to each salt at a given temperature.

When substances such as NaCl or NaF are introduced into the solution, this balance can be disturbed. Depending on whether these substances share ions with CaF2, they may either have no effect on the solubility or they can affect it significantly, which is described by the common ion effect.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that explains how systems at equilibrium react to external changes. According to this principle, if a stress is applied to a system at equilibrium, the system adjusts to minimize that stress and re-establish equilibrium. Stresses can include changes in concentration, pressure, or temperature.

In the context of solubility, adding more of an ion that is already present in the equilibrium will cause the system to adjust by favoring the formation of the solid, thereby reducing its solubility. Conversely, removing an ion from the solution, such as through the formation of a different compound, will shift the equilibrium towards dissolution to replace the removed ions.

Dissolution of Salts

The dissolution of salts is the process where ionic compounds separate into their constituent ions in the presence of a solvent, usually water. The interaction between the ions and the solvent molecules overcomes the forces holding the ions together in the solid lattice. Water, due to its polar nature, is particularly effective at solvating ions, allowing them to disperse throughout the solution.

However, the solubility of a salt can be notably influenced by the presence of other ions in the solution. When a salt shares a common ion with another compound in the solution, the increased concentration of that ion can change the extent of dissolution, due to the common ion effect.

Ionic Equilibrium

Ionic equilibrium pertains to the balance that occurs in an aqueous solution between the ions and the undissolved solid. This equilibrium is not static, but rather a continuous process of ions dissolving and precipitating. It's governed by the solubility product constant (Ksp), which quantifies the concentrations at which the solid will dissolve or precipitate under specific conditions.

Changes in the solution can shift this equilibrium. For example, adding an acid like HCl, which doesn't share a common ion with CaF2, wouldn't usually affect the solubility of CaF2. However, because HCl supplies extra H+ ions that can interact with F- ions in a side reaction to create HF, a new balance can be established that ultimately influences the solubility of CaF2.