Question

Which insoluble compound in each pair should be more soluble in nitric acid than in pure water? (a) \(\mathrm{PbCl}_{2}\) or \(\mathrm{PbS}\) (b) \(\mathrm{Ag}_{2} \mathrm{CO}_{3}\) or \(\mathrm{AgI}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) or \(\mathrm{AgCl}\)

Short answer

(a) \( \mathrm{PbS} \), (b) \( \mathrm{Ag}_{2} \mathrm{CO}_{3} \), (c) \( \mathrm{Al}( \mathrm{OH})_{3} \)

Step-by-step solution

Understanding Solubility in Nitric Acid vs Water

In this exercise, we need to determine which compound will be more soluble in nitric acid than in pure water. Nitric acid can increase solubility by reacting with anions from insoluble salts, forming soluble complex ions or changing the anion. We'll analyze each pair using this reasoning.

Pair (a) : Analyzing \( \mathrm{PbCl}_{2} \) vs. \( \mathrm{PbS} \)

Lead(II) chloride (\( \mathrm{PbCl}_{2} \)) is slightly soluble in water, while lead(II) sulfide (\( \mathrm{PbS} \)) is extremely insoluble. Nitric acid does not react with chloride ions to increase solubility significantly. However, it reacts with sulfide ions in \( \mathrm{PbS} \), converting \( \mathrm{S}^{2-} \) to \( \mathrm{H}_{2}\mathrm{S} \), thereby increasing solubility. Thus, \( \mathrm{PbS} \) is more soluble in nitric acid than water.

Pair (b): Analyzing \( \mathrm{Ag}_{2} \mathrm{CO}_{3} \) vs. \( \mathrm{AgI} \)

Silver(I) carbonate (\( \mathrm{Ag}_{2} \mathrm{CO}_{3} \)) can dissolve as nitric acid reacts with carbonate ions to form carbon dioxide and water. However, silver iodide (\( \mathrm{AgI} \)) remains insoluble because iodine ions do not react with nitric acid like carbonates. \( \mathrm{Ag}_{2} \mathrm{CO}_{3} \) is more soluble in nitric acid than in water.

Pair (c): Analyzing \( \mathrm{Al}( \mathrm{OH})_{3} \) vs. \( \mathrm{AgCl} \)

Aluminum hydroxide (\( \mathrm{Al}(\mathrm{OH})_{3} \)) dissolves in nitric acid as it reacts to form water and \( \mathrm{Al}^{3+} \) ions. Silver chloride (\( \mathrm{AgCl} \)), however, does not undergo any significant reaction with nitric acid. Therefore, \( \mathrm{Al}(\mathrm{OH})_{3} \) is more soluble in nitric acid than in water.

Key concepts

Insoluble Compounds

Insoluble compounds are substances that do not readily dissolve in water. They have very low solubility levels, which means that only tiny amounts can dissolve in a given amount of water. Solubility is influenced by various factors, including the nature of the compound and the solvent. For an insoluble compound, the attraction between its ions or molecules is stronger than the attraction between those ions or molecules and the solvent's molecules. However, insoluble compounds can sometimes become more soluble in certain conditions, such as in the presence of an acid.
For example, lead(II) sulfide (\( \mathrm{PbS} \)) is extremely insoluble in water, but when nitric acid is added, it can react with sulfide ions. Understanding these types of interactions helps in predicting whether an insoluble compound will dissolve better in pure water or in an acid.

Chemical Reactions

Chemical reactions occur when substances interact to form new substances. These reactions often result in the breakage and formation of chemical bonds. Reactions involving acids often change the solubility of insoluble compounds by altering the ions present.
When nitric acid interacts with insoluble compounds, it can transform certain anions. For instance, when nitric acid reacts with lead(II) sulfide, it converts sulfide ions (\( \mathrm{S}^{2-} \)) into hydrogen sulfide gas (\( \mathrm{H}_{2}\mathrm{S} \)), increasing solubility.
On the other hand, compounds like silver iodide (\( \mathrm{AgI} \)), which contain iodide ions, remain insoluble because iodide ions do not react with nitric acid in the same way. By understanding these reactions, students can predict the solubility behavior of compounds in different conditions.

Complex Ion Formation

Complex ion formation is a process where ions combine with other ions or molecules to form a complex. This often enhances solubility. When nitric acid is present, it can facilitate the formation of soluble complex ions from insoluble compounds.
Take, for instance, aluminum hydroxide \( ( \mathrm{Al(OH)}_{3} ) \), which typically forms a complex with the additional ions introduced by the acid. The reaction with nitric acid creates more soluble aluminum ions (\( \mathrm{Al}^{3+} \)), thus increasing the solubility of the originally insoluble substance.
Complex ion formation is a fundamental concept in chemistry that describes how certain reactions can make otherwise insoluble compounds soluble. Recognizing this can help in solving similar problems more effectively.

Anion Reactions

Anions, which are negatively charged ions, play a crucial role in solubility reactions with acids. Their interactions can either promote or inhibit dissolution, depending on the ion and the acid involved. Nitric acid often reacts with anions from insoluble salts, altering them into forms that have different solubility properties.
For example, when nitric acid reacts with carbonate ions in silver carbonate \( ( \mathrm{Ag}_{2} \mathrm{CO}_{3} ) \), it transforms these into carbon dioxide and water. This reaction significantly increases solubility compared to when the compound is in pure water. However, some anions such as iodide in silver iodide \( ( \mathrm{AgI} ) \) do not react with nitric acid, leaving the compound still insoluble. Understanding how anions react with acids is crucial in predicting solubility in various chemical environments.