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) PbS, (b) Ag2CO3, (c) Al(OH)3

Step-by-step solution

Understand the solubility concept

Many insoluble compounds can become soluble in acidified solutions because they can react with acids. Check if the compound can undergo a chemical reaction with nitric acid to increase its solubility.

Analyzing Part (a) - PbCl2 vs PbS

PbS (lead sulfide) can react with the acid, forming soluble Pb(NO3)2 and H2S gas. PbCl2 does not react with nitric acid notably. Thus, PbS is more soluble in nitric acid than in pure water.

Analyzing Part (b) - Ag2CO3 vs AgI

Ag2CO3 (silver carbonate) can react with nitric acid to form soluble AgNO3 and CO2 gas. AgI does not react with nitric acid. Therefore, Ag2CO3 is more soluble in nitric acid than in pure water.

Analyzing Part (c) - Al(OH)3 vs AgCl

Al(OH)3 (aluminum hydroxide) reacts with acids to form soluble Al(NO3)3 and water. AgCl does not react significantly with nitric acid. Therefore, Al(OH)3 is more soluble in nitric acid than in pure water.

Key concepts

Insoluble Compounds

Insoluble compounds are substances that do not dissolve easily in water. Imagine adding a spoonful of sugar to a cup of tea—sugar dissolves readily, but insoluble compounds do not behave like this. Solubility is influenced by the compound's chemical structure and the nature of its interactions with the solvent molecules. Insoluble compounds often form precipitates, which are solids that separate from solutions. For example, when a metal ion in solution binds tightly with anions forming a solid, it becomes insoluble. Some common insoluble compounds include certain salts, hydroxides, and sulfides. However, these compounds may become more soluble under different conditions, such as when exposed to acids.

Acid-base Reactions

Acid-base reactions occur when an acid and a base interact, producing a salt and water. This type of chemical reaction can alter the solubility of certain compounds. For instance, when acids, such as nitric acid, react with insoluble compounds, a new soluble product may form. The acid donates protons ( H^+ ions), which can react with the compounds to produce solubility-enhancing reactions. This is why certain insoluble compounds dissolve better in acidic solutions. Understanding these reactions is key in predicting solubility changes:

• The reaction of PbS with HNO_3 forms Pb(NO_3)_2 and H_2S , making PbS more soluble in acidic environments.
• Similarly, Ag_2CO_3 becomes soluble by reacting with HNO_3 to create AgNO_3 .

Chemical Reactivity

Chemical reactivity refers to how readily a substance undergoes a chemical change. In the case of solubility discussions, some insoluble compounds may be reactive with acids, which leads to increased solubility. For an insoluble compound to dissolve in an acid, its chemical reactivity must allow it to form new soluble compounds via a chemical reaction. For example, when comparing Al(OH)_3 and AgCl :

• Al(OH)_3 chemically reacts with acids to form Al(NO_3)_3 and water, showing high reactivity with acids.
• In contrast, AgCl does not show significant reactivity with nitric acid and remains insoluble.

Recognizing these reactivity patterns can help predict which compounds will dissolve better in acid.

Solubility in Acids

Solubility in acids is an essential concept for understanding how different compounds behave in acidic environments as opposed to pure water. Acidic solutions can change the solubility characteristics of otherwise insoluble substances. The increase in solubility is often due to favorable chemical reactions with the acid that convert the insoluble compound into a soluble form. Key points to consider include:

• Acids often generate ions or gases from insoluble compounds, enhancing their dissolution.
• This is seen in compounds like PbS and Ag_2CO_3 when reacting with HNO_3 .
• Such transformations make the substance dissolve readily in acidic mediums, bridging the gap between insoluble and soluble states.

By understanding the solubility differences in acids, students can predict outcomes of reactions in various chemical setups.