Problem 63 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

The salts that will be substantially more soluble in acidic solution than in pure water are \(\mathrm{ZnCO}_{3}\), \(\mathrm{AgCN}\), and \(\mathrm{Ba}_{3}(\mathrm{PO}_{4})_{2}\). This is due to the reactions of their anions with \(\mathrm{H^{+}}\) ions present in the acidic solution, shifting the solubility equilibria according to Le Châtelier's principle.

Step 1: Write the dissolution reaction for the salts

We begin by writing the dissolution reactions for each of the given salts: (a) \(\mathrm{ZnCO}_{3} \leftrightharpoons \mathrm{Zn^{2+}} + \mathrm{CO}_{3^{2-}}\) (b) \(\mathrm{ZnS} \leftrightharpoons \mathrm{Zn^{2+}} + \mathrm{S^{2-}}\) (c) \(\mathrm{BiI}_{3} \leftrightharpoons \mathrm{Bi^{3+}} + 3\mathrm{I^{-}}\) (d) \(\mathrm{AgCN} \leftrightharpoons \mathrm{Ag^{+}} + \mathrm{CN^{-}}\) (e) \(\mathrm{Ba}_{3}(\mathrm{PO}_{4})_{2} \leftrightharpoons 3\mathrm{Ba^{2+}} + 2\mathrm{PO}_{4^{3-}}\)

Step 2: Identify if a component reacts with acid

The next step is to see if any of the components of the dissolution reactions will react with acid (usually \(\mathrm{H^{+}}\)). The key to solving this problem is identifying which anions in the salts will react with the \(\mathrm{H^{+}}\) ions present in an acidic solution.

Step 3: Determine the effect of acid on solubility

By examining the solubility equilibria, we can see that the anions CO\(_{3}^{\mathrm{2–}}\), S\(^{\mathrm{2−}}\), I\(^{\mathrm{−}}\), CN\(^{\mathrm{−}}\), and PO\(_{4^{\mathrm{3−}}}\) can react with protons (H\(^{+}\)): (a) \(\mathrm{CO}_{3^{2-}} + \mathrm{H^{+}} \leftrightharpoons \mathrm{HCO}_{3^{-}}\) (b) \(\mathrm{S^{2-}}\) does not react with \(\mathrm{H^{+}}\) (c) \(\mathrm{I^{-}}\) does not react with \(\mathrm{H^{+}}\) (d) \(\mathrm{CN^{-}} + \mathrm{H^{+}} \leftrightharpoons \mathrm{HCN}\) (e) \(\mathrm{PO}_{4^{3-}} + \mathrm{H^{+}} \leftrightharpoons \mathrm{HPO}_{4^{2-}}\) Salts that have an anion which reacts with the \(\mathrm{H^{+}}\) ions present in an acidic solution will be more soluble in acidic solution than in pure water due to the shift in equilibria caused by the acid (Le Châtelier's principle).

Step 4: Identify the salts more soluble in acidic solution

Based on our analysis, we can conclude that: (a) \(\mathrm{ZnCO}_{3}\) will be substantially more soluble in an acidic solution than in pure water. (b) \(\mathrm{ZnS}\) will not be more soluble in an acidic solution. (c) \(\mathrm{BiI}_{3}\) will not be more soluble in an acidic solution. (d) \(\mathrm{AgCN}\) will be substantially more soluble in an acidic solution than in pure water. (e) \(\mathrm{Ba}_{3}(\mathrm{PO}_{4})_{2}\) will be substantially more soluble in an acidic solution than in pure water. Therefore, the salts that will be substantially more soluble in acidic solution than in pure water are \(\mathrm{ZnCO}_{3}\), \(\mathrm{AgCN}\), and \(\mathrm{Ba}_{3}(\mathrm{PO}_{4})_{2}\).

Key Concepts

Le Châtelier's Principle

Le Châtelier's Principle is a fundamental concept in chemistry that describes how a system at equilibrium responds to changes in concentration, temperature, or pressure. When a system in dynamic equilibrium is disturbed, it will adjust itself to counteract the imposed change, restoring a new equilibrium. This principle is especially helpful in understanding how solubility changes in response to different conditions.

For instance, when a salt dissolved in water reaches equilibrium, adding an acid introduces additional \(\mathrm{H^+}\) ions. If this new component reacts with one of the equilibrium components, it can shift the reaction in a direction that re-establishes balance.

• Consider the dissolution of \(\mathrm{ZnCO_3}\): \\(\mathrm{ZnCO_3 \leftrightharpoons Zn^{2+} + CO_{3}^{2-}}\).
• If \(\mathrm{CO_{3}^{2-}}\) reacts with added \(\mathrm{H^+}\) ions, forming \(\mathrm{HCO_3^-}\), the equilibrium shifts right, increasing solubility.

Understanding this principle is vital when predicting how the solubility of a compound may change in varying chemical environments.

Acid-Base Reactions

Acid-Base Reactions are integral to understanding many chemical processes, including the dissolution of salts in solutions. These reactions involve proton transfer between substances, typically resulting in the formation of water and salts.

Acidic solutions have high concentrations of \(\mathrm{H^+}\) ions, and these can interact with anions from dissociated salts.

• For example, in the presence of acid, \(\mathrm{CO_{3}^{2-}}\) in \(\mathrm{ZnCO_3}\) reacts with \(\mathrm{H^+}\) to form \(\mathrm{HCO_3^-}\), a new species.
• Similarly, \(\mathrm{CN^-}\) in \(\mathrm{AgCN}\) reacts with \(\mathrm{H^+}\) to form \(\mathrm{HCN}\), rendering the reaction more soluble in acidic solutions.

These reactions illustrate how altering the pH of a solution can significantly impact the solubility of certain compounds. The increased solubility allows ions to remain in solution, rather than precipitating out as solid salts.

Dissolution Reactions

Dissolution Reactions describe the process where solid substances dissolve in solvents, forming a homogeneous mixture. This process involves the breaking down of a substance's lattice structure and distributing its ions or molecules in the solvent.

In the context of salt dissolution in water, these reactions are largely influenced by the nature of the solvent and any additional components it contains.

• Salts dissociate into their respective ions, such as \(\mathrm{ZnCO_3}\) into \(\mathrm{Zn^{2+}}\) and \(\mathrm{CO_{3}^{2-}}\).
• Adding acid changes the dynamics as these ions may further interact with hydronium ions or other substances introduced, altering solubility.

Overall, understanding dissolution reactions helps in anticipating how different compounds behave in various chemical settings, a vital skill in both academic and practical chemistry applications.