Question

\(\mathrm{HF}\) is a weak acid, but its strength increases with concentration. Explain. (Hint: \(\mathrm{F}^{-}\) reacts with \(\mathrm{HF}\) to form \(\mathrm{HF}_{2}^{-}\). The equilibrium constant for this reaction is 5.2 at \(25^{\circ} \mathrm{C} .\) )

Short answer

HF strength increases with concentration as HF forms HF₂⁻, shifting equilibrium.

Step-by-step solution

Understanding Weak Acid

HF is a weak acid, meaning it only partially dissociates in water to form H⁺ and F⁻ ions. This is characterized by its relatively small ionization constant.

Considering Fluoride Ion Reaction

The fluoride ion (F⁻), produced from the partial dissociation of HF, can react with HF to form the bifluoride ion \(\mathrm{HF}_{2}^{-}\), as shown in the equilibrium reaction: \(\mathrm{F}^{-} + \mathrm{HF} \rightleftharpoons \mathrm{HF}_{2}^{-}\).

Analyzing the Equilibrium Constant

The reaction between F⁻ and HF to form \(\mathrm{HF}_{2}^{-}\) has an equilibrium constant of 5.2 at \(25^{\circ} \mathrm{C}\). A moderate equilibrium constant value indicates that a significant amount of \(\mathrm{HF}_{2}^{-}\) can form, especially as the concentration of F⁻ and HF increases.

Effect of Increased Concentration on Equilibrium

As the concentration of HF increases, more \(\mathrm{F}^{-}\) ions are available to react and form \(\mathrm{HF}_{2}^{-}\). This shifts the equilibrium towards more product formation, reducing the concentration of free F⁻ and thereby increasing the acidity of the solution.

Conclusion on Acid Strength

The formation of \(\mathrm{HF}_{2}^{-}\) effectively reduces the free F⁻ ion concentration, causing the original HF dissociation equilibrium to shift to the right, thereby enhancing acid strength with increased HF concentration.

Key concepts

Acid Strength

Acid strength refers to an acid's ability to dissociate in water and release hydrogen ions (H⁺). A strong acid dissociates completely, while a weak acid dissociates only partially. Hydrofluoric acid (HF) is a classic example of a weak acid. It releases some H⁺ ions, but not all molecules dissociate into ions.

This limited dissociation is due to the relatively weak ionization constant, reflecting its partial dissociation nature. As a result, not all fluoride ions (F⁻) are free in solution, and some remain bonded to hydrogen, forming HF.

The concept becomes interesting when considering how HF’s strength changes under varying conditions.

Equilibrium Constant

An equilibrium constant, denoted as K, measures the ratio of product concentrations to reactant concentrations at equilibrium. In the context of the bifluoride ion formation, \[ ext{F}^- + ext{HF} ightleftharpoons ext{HF}_2^- \] K is measured to be 5.2 at 25°C. This value indicates a balance between the reactants and the product, implying

• Significant formation of HF₂⁻ when equilibrium is reached
• The reaction favoring product formation slightly over remaining as reactants

The moderate K implies a stable interaction between F⁻ ions and HF molecules under equilibrium.

Bifluoride Ion

The bifluoride ion, denoted \( ext{HF}_2^- \), forms when a fluoride ion \( ext{F}^- \) reacts with a molecule of \( ext{HF} \). This reaction is reversible and reaches equilibrium, emphasizing the dynamic nature of chemical processes.

The formation of \( ext{HF}_2^- \) significantly affects the composition of the solution. By reducing the number of free fluoride ions \( ext{F}^- \), the behavior and properties of the solution change, thereby affecting the overall acidity of the solution. Over time, as \( ext{HF} \) and \( ext{F}^- \) continue interacting, the presence of \( ext{HF}_2^- \) increases, playing a crucial role in enhancing observable acid strength.

Dissociation

Dissociation involves the separation of molecules into smaller entities like ions. In a weak acid, such as HF, the process of dissociation is incomplete. HF dissociates to some extent in water, producing H⁺ and F⁻ ions.

This partial dissociation is due to competing reactions, including the formation of the bifluoride ion. Since \( ext{F}^- \) is consumed in forming \( ext{HF}_2^- \), the dissociation equilibrium shifts slightly, maintaining the solution's weakly acidic character despite changes in concentration.
Ultimately, the dynamic equilibrium involves both dissociation and recombination, which dictates the dissociation extent and characterizes HF as a weak acid.

Concentration Effect

Changes in concentration significantly affect the equilibrium state, especially in dynamic systems. For HF, increasing concentration enhances the formation of \( ext{HF}_2^- \), as more fluoride ions become available to react with HF molecules.

The concentration effect shifts the equilibrium toward more product formation, effectively reducing the number of free \( ext{F}^- \) ions. This reduction means fewer ions are available to shift the dissociation equilibrium back to the left, thereby increasing the perceived acidity.
By understanding the concentration effect, one can explain why concentrated HF solutions show increased acidity compared to dilute solutions.