Question

Explain why zinc carbonate, \(\mathrm{ZnCO}_{3}\), is more soluble in hydrochloric acid than in water.

Short answer

Zinc carbonate reacts with hydrochloric acid, releasing carbon dioxide and forming soluble zinc chloride, increasing its solubility compared to water.

Step-by-step solution

Zinc Carbonate Solubility in Water

Zinc carbonate, \( \mathrm{ZnCO}_3 \), is only sparingly soluble in water. This is because it forms a solid in which the zinc ions \( (\mathrm{Zn}^{2+}) \) and carbonate ions \( (\mathrm{CO}_3^{2-}) \) are tightly held together within the crystal lattice, and very few ions are released into solution.

Reaction with Hydrochloric Acid

When \( \mathrm{ZnCO}_3 \) is added to hydrochloric acid \( (\mathrm{HCl}) \), it reacts chemically with the acid. The \( \mathrm{HCl} \) provides hydrogen ions \( (\mathrm{H}^+) \), which react with the carbonate ions \( (\mathrm{CO}_3^{2-}) \) to form carbonic acid \( (\mathrm{H}_2\mathrm{CO}_3) \). This decomposes quickly into carbon dioxide \( (\mathrm{CO}_2) \) gas and water \( (\mathrm{H}_2\mathrm{O}) \).

Enhanced Solubility Mechanism

The release of carbon dioxide gas drives the reaction forward, according to Le Chatelier's principle, further dissolving \( \mathrm{ZnCO}_3 \). Additionally, the \( \mathrm{Zn}^{2+} \) ions are also stabilized in solution by reacting with the chloride ions \( (\mathrm{Cl}^- ) \) from hydrochloric acid, forming soluble zinc chloride \( (\mathrm{ZnCl}_2) \).

Conclusion of Solubility Comparison

In water, \( \mathrm{ZnCO}_3 \) remains mostly undissolved, whereas in \( \mathrm{HCl} \), the reactions produce products that remove \( \mathrm{CO}_3^{2-} \) from solution and dissolve \( \mathrm{Zn}^{2+} \) as \( \mathrm{ZnCl}_2 \). These processes increase the apparent solubility of \( \mathrm{ZnCO}_3 \) in \( \mathrm{HCl} \) compared to in water.

Key concepts

Chemical Reaction with Hydrochloric Acid

When zinc carbonate, \( \mathrm{ZnCO}_3 \), is mixed with hydrochloric acid \( (\mathrm{HCl}) \), a chemical reaction occurs that significantly enhances its solubility. This process starts as the hydrogen ions \( (\mathrm{H}^+) \) from \( \mathrm{HCl} \) interact with the carbonate ions \( (\mathrm{CO}_3^{2-}) \) present in the zinc carbonate structure. These carbonate ions combine with hydrogen ions to form carbonic acid \( (\mathrm{H}_2\mathrm{CO}_3) \).

However, carbonic acid is not stable and quickly breaks down into carbon dioxide gas \( (\mathrm{CO}_2) \) and water \( (\mathrm{H}_2\mathrm{O}) \). This degradation process is pivotal as it helps to "remove" the carbonate ions from the solution, which in turn shifts the balance of the reaction and encourages more zinc carbonate to dissolve. Moreover, the leftover zinc ions \( (\mathrm{Zn}^{2+}) \) in the solution react with chloride ions \( (\mathrm{Cl}^-) \) from \( \mathrm{HCl} \) to form soluble zinc chloride \( (\mathrm{ZnCl}_2) \). This reaction pathway ensures that zinc carbonate dissolves much more readily in hydrochloric acid than it would in water alone.

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemistry that explains how a system at equilibrium responds to external changes to maintain balance. In the context of zinc carbonate's solubility in hydrochloric acid, Le Chatelier's Principle plays a crucial role.

The chemical reaction between zinc carbonate and \( \mathrm{HCl} \) involves converting \( \mathrm{CO}_3^{2-} \) ions into carbon dioxide gas and water. As the carbon dioxide gas is released from the solution, it causes a shift in the equilibrium, favoring the forward reaction according to Le Chatelier's Principle.

• The continuous removal of carbon dioxide from the reaction mixture decreases the concentration of \( \mathrm{CO}_3^{2-} \) ions.
• This decrease prompts the system to dissolve more zinc carbonate to restore equilibrium, thus increasing solubility.

This principle ensures ongoing zinc carbonate dissolution as long as conditions, such as excess \( \mathrm{HCl} \), facilitate the gas release.

Solubility Comparison

Solubility refers to how much of a substance can dissolve in a solvent at a given temperature. Zinc carbonate's solubility in water and hydrochloric acid exemplifies the effects of chemical reactions on solubility.

• In water, zinc carbonate is sparingly soluble due to its stable crystal lattice, which holds ions tightly together.
• Few ions dissociate into the water, keeping its solubility low.
• In contrast, in hydrochloric acid, chemical reactions enhance solubility dramatically.

By reacting with \( \mathrm{HCl} \), both the carbonate ions are "removed" as gas and water, while zinc ions form soluble complexes with chloride ions. These reactions create conditions where zinc carbonate dissolves much more freely than in plain water. This comparison illustrates how the presence of a specific acid can transform the behavior and solubility of a compound.