Determine if each compound is more soluble in acidic solution than it is in pure water. Explain. a. \(\mathrm{BaCO}_{3}\) b. CuS c. \(\mathrm{AgCl}\) d. \(\mathrm{PbI}_{2}\)

The common ion effect is a principle in chemistry that explains how the solubility of a compound is affected by the presence of an ion already dissolved in the solution. When a soluble salt with a common ion is added to a solution, the addition increases the concentration of that particular ion. As a result, the solubility of the salt decreases due to the increase in ionic strength. This is because the system seeks to re-establish equilibrium, which might result in precipitation if the solution was initially saturated.

For example, if we dissolve barium carbonate \( \mathrm{BaCO}_{3} \) in water, it dissociates into barium \( \mathrm{Ba}^{2+} \) and carbonate \( \mathrm{CO}_{3}^{2-} \) ions. Adding an acid, which contains additional \( \mathrm{H}^{+} \) ions, reacts with \( \mathrm{CO}_{3}^{2-} \) to form carbon dioxide \( \mathrm{CO}_{2} \) gas and water, thereby reducing the concentration of the \( \mathrm{CO}_{3}^{2-} \) ion in the solution. This promotes the dissolution of more \( \mathrm{BaCO}_{3} \) to maintain equilibrium, illustrating the common ion effect.

Le Chatelier's principle, also known as the equilibrium shift, suggests that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. This principle is vital in understanding solubility in the context of acidic solutions.

When an acid reacts with an anion from a soluble salt, the equilibrium can shift. For instance, in the solubility of copper(II) sulfide \( \mathrm{CuS} \) in acid, the reaction of sulfide ions \( \mathrm{S}^{2-} \) with acid forms hydrogen sulfide gas \( \mathrm{H}_{2}\mathrm{S} \) that escapes the solution. The removal of \( \mathrm{S}^{2-} \) ions shifts the equilibrium to favor the dissolution of more \( \mathrm{CuS} \) to replenish the sulfide ions, thereby increasing its solubility in acid compared to pure water.

Solubility rules are a set of guidelines that predict whether a salt is soluble in water. These rules are essential for understanding the behavior of ionic compounds in water and can help predict the outcome of a reaction, like determining if a precipitate will form.

Soluble salts include those containing alkali metal ions and ammonium \( \mathrm{NH}_{4}^{+} \) ions, as well as nitrates \( \mathrm{NO}_{3}^{-} \) and acetates \( \mathrm{CH}_{3}\mathrm{COO}^{-} \) ions. Conversely, salts with sulfate \( \mathrm{SO}_{4}^{2-} \) ions are generally soluble, except for those with lead \( \mathrm{Pb}^{2+} \) ions, barium \( \mathrm{Ba}^{2+} \) ions, and a few others. Chlorides, bromides, and iodides are soluble except when paired with silver \( \mathrm{Ag}^{+} \) ions, lead \( \mathrm{Pb}^{2+} \) ions, or mercury \( \mathrm{Hg}_{2}^{2+} \) ions. These rules inform us, for example, that \( \mathrm{AgCl} \) and \( \mathrm{PbI}_{2} \) do not become significantly more soluble in acidic solutions because their anions do not react with the acid to form a weak acid or gas.

The reactivity of anions with acids is a significant factor in determining solubility in acidic solutions. Anions that can react with acids to form gases or weak acids will increase the solubility of their respective salts in acidic solutions compared to pure water.

For example, carbonate \( \mathrm{CO}_{3}^{2-} \) and sulfide \( \mathrm{S}^{2-} \) anions readily react with acids, leading to the formation of \( \mathrm{CO}_{2} \) and \( \mathrm{H}_{2}\mathrm{S} \) gases respectively. On the other hand, chloride \( \mathrm{Cl}^{-} \) and iodide \( \mathrm{I}^{-} \) ions found in \( \mathrm{AgCl} \) and \( \mathrm{PbI}_{2} \) do not form such reactions, thus their solubility is not substantially affected by the presence of acid. Recognizing these anion-acid interactions is instrumental for predicting solubility behavior in chemical systems.