Question

Which of the following salts will be substantially more soluble in acidic solution than in pure water: (a) ZnCO \(_{3}\) \(\mathbf{b} ) \mathrm{ZnS},(\mathbf{c}) \mathrm{BiI}_{3},(\mathbf{d}) \mathrm{AgCN},(\mathbf{e}) \mathrm{Ba}_{3}\left(\mathrm{PO}_{4}\right)_{2} ?\)

Short answer

The salts substantially more soluble in acidic solution than in pure water are: (a) \(ZnCO_3\), (b) \(ZnS\), (d) \(AgCN\), and (e) \(Ba_3(PO_4)_2\).

Step-by-step solution

Determine the dissolution reactions of the salts in water

When salts dissolve in water, they dissociate into their respective ions. Let's write down the dissociation reactions for each salt: (a) \(ZnCO_3 \rightarrow Zn^{2+} + CO_3^{2-}\) (b) \(ZnS \rightarrow Zn^{2+} + S^{2-}\) (c) \(BiI_3 \rightarrow Bi^{3+} + 3I^-\) (d) \(AgCN \rightarrow Ag^+ + CN^-\) (e) \(Ba_3(PO_4)_2 \rightarrow 3Ba^{2+} + 2PO_4^{3-}\)

Analyze the effect of acidic solution on each dissociation reaction

We need to consider the effect of H+ ions (from the acidic solution) on each dissociation reaction mentioned in Step 1. This will help us understand which salt will be more soluble in acidic solution than in pure water: (a) H+ ions will react with \(CO_3^{2-}\) to form \(HCO_3^-\) or \(H_2CO_3\), which will decrease the concentration of \(CO_3^{2-}\), shifting the equilibrium to the right and increasing the solubility of ZnCO3 in an acidic solution. (b) H+ ions will react with \(S^{2-}\) to form \(HS^-\) or \(H_2S\), which will decrease the concentration of \(S^{2-}\), shifting the equilibrium to the right and increasing the solubility of ZnS in an acidic solution. (c) H+ ions have no notable effect on the solubility of \(BiI_3\) in an acidic solution. (d) H+ ions will react with \(CN^-\) to form \(HCN\), which will decrease the concentration of \(CN^-\), shifting the equilibrium to the right and increasing the solubility of AgCN in an acidic solution. (e) H+ ions will react with \(PO_4^{3-}\) to form \(HPO_4^{2-}\), \(H_2PO_4^-\), or \(H_3PO_4\), which will decrease the concentration of \(PO_4^{3-}\), shifting the equilibrium to the right and increasing the solubility of \(Ba_3(PO_4)_2\) in an acidic solution.

Determine which salts are substantially more soluble in acidic solution

Based on the analysis in Step 2, we can conclude that the following salts are substantially more soluble in acidic solution than in pure water: (a) \(ZnCO_3\) (b) \(ZnS\) (d) \(AgCN\) (e) \(Ba_3(PO_4)_2\) The salt (c) \(BiI_3\) is not notably affected by the presence of the acidic solution. Thus, the answer should include salts (a), (b), (d), and (e).

Key concepts

Chemical Equilibrium

Chemical equilibrium in the context of solubility refers to the balance between a salt dissolving in water and the ions recombining to form a solid. When a salt is added to water, it reaches a point where its rate of dissolving equals the rate of precipitation. This state is dynamic, meaning that the dissolving and reforming processes happen continuously at the same rate.
The equilibrium point is influenced by the solubility product constant ( K_{sp} ). This constant is specific for each salt and dictates the maximum amount of the salt that can dissolve at a particular temperature.

• If the concentration of ions exceeds the K_{sp} , the salt begins to precipitate.
• If the concentration is below K_{sp} , the solution can dissolve more salt.

In acidic solutions, this equilibrium can shift because of interactions with H^+ ions, as seen in the solution analysis.

Dissociation Reactions

Dissociation reactions involve the separation of a compound into its individual ions when dissolved in solution. For salts, this process is essential because it determines how they interact in different environments, such as in acidic solutions.
When a salt, like those listed in the exercise, dissolves, it breaks into its constituent ions. For example, zinc carbonate ( ZnCO_3 ) dissociates into Zn^{2+} and CO_3^{2-} ions.

• Dissociation is expressed by a balanced chemical equation, showing the ions produced.
• Each dissociation is unique based on the bonds in the compound and the solution's characteristics.

Understanding these reactions helps predict how a salt will behave in acidic conditions, which is crucial for solving solubility problems as illustrated by the exercise.

Effect of pH on Solubility

The solubility of a salt can be significantly affected by the pH level of the solution it is in. This occurs because hydrogen ions ( H^+ ) from acids can react with the anions of the salt, altering the equilibrium and thus the solubility of the salt.
For example, consider ZnCO_3 . In an acidic solution, H^+ ions react with CO_3^{2-} to form HCO_3^- or H_2CO_3 . This consumption of CO_3^{2-} ions drives the equilibrium towards dissolving more ZnCO_3 to replace what was removed.

• Salts whose anions react strongly with H^+ tend to dissolve more in acidic solutions.
• The pH-dependent shift provides a way to control and increase solubility of certain salts in specific environments.

This principle also explains why salts like ZnS , AgCN , and Ba_3(PO_4)_2 show increased solubility in acidic environments as listed in the solution steps.

Acid-Base Reactions

Acid-base reactions play a critical role in determining the solubility of salts. These reactions involve the transfer of protons ( H^+ ions) between species, which can lead to changes in solute concentration and solubility dynamics.
In the case of salts, such as ZnS and AgCN , the H^+ ions from acids react with their anions ( S^{2-} and CN^- , respectively) to form HS^- and HCN . These reactions use up anions that were part of the dissociation equilibrium, pushing the equilibrium to dissolve more salt.

• The formation of new acid-base conjugates impacts the ions available in the solution.
• The interplay of acid-base reactions and solubility is important for both chemistry and industrial processes.

Understanding these interactions allows for predicting salt behavior under various acidic conditions, a skill needed for navigating complex chemical scenarios in real-world applications.