Problem 45 - General Chemistry: Principles and Modern Applications

Question

Which of the following solids is (are) more soluble in an acidic solution than in pure water: \(\mathrm{KCl}\), \(\mathrm{MgCO}_{3}\), \(\mathrm{FeS}, \mathrm{Ca}(\mathrm{OH})_{2,}\) or \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH} ?\) Explain.

Step-by-Step Solution

Answer

\( \mathrm{MgCO}_3 \), \( \mathrm{FeS} \), and \( \mathrm{Ca(OH)}_2 \) are more soluble in acidic solution than in pure water because these have the ability to react with the acid to increase its solubility.

Step 1: Identifying the nature of each molecule

Let's start by categorizing these molecules: \n \( \mathrm{KCl} \) is a neutral salt and won't be affected by the acidity of the solution. \n \( \mathrm{MgCO}_3 \) is a basic salt, anions of which will react with the H+ ions present in the acid. \n \( \mathrm{FeS} \) is an amphoteric compound, can show both acidic and basic properties. It reacts with acid and forming H2S. \n \( \mathrm{Ca(OH)}_2 \) is a strong base and it will react with extra H+ ions present in the acid. \n \( \mathrm{C}_{6}\mathrm{H}_{5}\mathrm{COOH} \) is an acid and it won't react with another acid to raise its solubility.

Step 2: Categorizing the compounds

Now based on step 1, we can categorize the molecules which are more soluble in acid than in water are \( \mathrm{MgCO}_3 \), \( \mathrm{FeS} \), and \( \mathrm{Ca(OH)}_2 \) because these are the only compounds among these that can react with acids to increase its solubility.

Key Concepts

Acid-base reactions

Acid-base reactions are fundamental in chemistry. They involve the transfer of protons (\(H^+\)) between reactants. An acid donates a proton, while a base accepts it. This interaction is essential in understanding solubility changes in different environments, such as acidic solutions.
For instance, some salts can dissolve more readily in acidic solutions. This is because the \(H^+\) ions from the acid can react with the anions of the salt, forming more soluble compounds or gases like \(CO_2\) or \(H_2S\). In the context of the exercise provided, compounds like \(\mathrm{MgCO}_3\) and \(\mathrm{FeS}\) become more soluble in acidic solutions because their anions react with \(H^+\) ions, enabling further dissolution.

Salt solubility

Salt solubility refers to how well a salt (ionic compound) dissolves in a solvent, typically water. It is affected by the nature of the salt, temperature, and the presence of other chemicals in the solution.
For each salt, there is a specific solubility product constant (Ksp) that dictates how much can dissolve before the solution becomes saturated. Some salts that contain basic anions, such as carbonate (\(CO_3^{2-}\)) or sulfide (\(S^{2-}\)), increase their solubility in acidic solutions.

• The presence of \(H^+\) ions can react with these anions, producing soluble species and shifting the equilibrium to dissolve more salt.
• For example, \(\mathrm{MgCO}_3\), when exposed to acid, undergoes a reaction that forms magnesium ions and carbon dioxide, increasing the salt's solubility.

Chemical equilibrium

Chemical equilibrium is the state of a system where the concentrations of the reactants and products remain constant over time. It occurs when the forward and reverse reactions balance each other precisely.
In the context of salt solubility, equilibrium is reached when no more salt can dissolve in the solution, and an equilibrium between the solid salt and its ions in solution exists.

• When an acid is added to a solution containing a basic salt like \(\mathrm{MgCO}_3\), this equilibrium is disturbed. The added \(H^+\) ions react with \(CO_3^{2-}\) ions to form \(H_2CO_3\), which decomposes to \(CO_2\) and water.
• This reaction removes \(CO_3^{2-}\) from the equilibrium, allowing more \(\mathrm{MgCO}_3\) to dissolve.

Le Chatelier's Principle

Le Chatelier's Principle is a cornerstone of chemical equilibrium. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.
Applying this principle helps explain why some salts are more soluble in acidic solutions. When an acid is added to a salt solution, it increases the concentration of \(H^+\) ions.

• This shift in concentration forces the equilibrium to adjust, often increasing the solubility of salts containing basic anions.
• For example, when \(H^+\) ions react with \(\mathrm{CO}_3^{2-}\) ions to form \(CO_2\) and water, Le Chatelier's Principle predicts that more \(\mathrm{MgCO}_3\) will dissolve to restore equilibrium.
• This response helps restore balance and increases the overall dissolution of the salt in the acidic environment.