Question

Which of the following will be more soluble if acid is added to the mixture? Will adding base increase the solubility of any of these? Are any of their solubilities independent of the \(\mathrm{pH}\) of the solution? (a) \(\mathrm{ZnS},\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) \(\mathrm{MgCO}_{3}\) (d) \(\mathrm{AgCl}\), (e) \(\mathrm{PbF}_{2}\)

Short answer

Acid increases solubility of ZnS, Ca(OH)2, and MgCO3. Base doesn't increase solubility of any compound listed. Solubilities of AgCl and PbF2 are independent of pH.

Step-by-step solution

- Understanding Acid-Base Effect on Solubility

We must consider the chemical reactions of these compounds with acids and bases. For a salt with a basic anion (that forms a weak base when hydrolysis occurs), the solubility will increase in acid as it reacts with H+ to form water and reduce the concentration of the anion. Conversely, a salt with a cation that forms a weak acid may have increased solubility with the addition of base due to reaction with OH- to form water and reduce the concentration of the cation. Salts derived from strong acids and strong bases will not be affected by pH.

- Analyzing Each Compound with Added Acid

We need to assess each compound and determine if adding acid will affect its solubility. A hint is to look for compounds with anions that could react with added H+. (a) ZnS has an anion that could form H2S, a weak acid, thus acid can increase its solubility. (b) For Ca(OH)2, adding acid neutralizes OH-, increasing solubility. (c) MgCO3 has CO32- which reacts with acid to form H2CO3, a weak acid, increasing solubility. (d) AgCl and (e) PbF2 are derived from strong acids, thus their solubility will not increase by adding acid.

- Analyzing Each Compound with Added Base

Next, we need to assess the impact of adding base on the solubility of each compound. Compounds with cations that are derived from weak acids (e.g., NH4+) may see increased solubility with base. However, none of the provided compounds have cations which will form a weak acid upon hydrolysis, so adding base will not increase their solubility.

- Determining pH Independence

To determine if any compound's solubility is independent of pH, we need to check if both the anion and cation form strong acids and strong bases respectively upon hydrolysis. AgCl and PbF2 don't react with either acid or base significantly, as Cl- and F- come from strong acids (HCl and HF respectively), and Ag+ and Pb2+ do not form weak acids. Therefore, the solubility of AgCl and PbF2 is independent of the pH.

Key concepts

Acid-Base Effect on Solubility

Understanding how the presence of acids or bases in a solution affects the solubility of compounds is vital for students tackling chemistry problems. Consider a substance like ZnS or MgCO3; when an acid is added to the mixture containing such salts, the solubility often increases. This happens because the acid provides H+ ions that react with the basic anions (like S2- in ZnS) to form H2S, which is a weak acid. This reaction reduces the concentration of the anion in the solution, essentially pulling more of the solid into the dissolved state to maintain equilibrium.

Consider Ca(OH)2, a compound that is slightly soluble in water. When you add an acid to a mixture containing Ca(OH)2, you provide a source of H+ ions, which readily react with the OH- ions to form water. This reaction again decreases the concentration of the OH- ions, promoting the dissolution of more Ca(OH)2 to achieve equilibrium. It's a simple cause and effect – the addition of acid increases the solubility of salts that behave as weak bases.

Hydrolysis of Salts

Now, diving into hydrolysis – the reaction between a salt (ionic compound) and water to form an acidic or basic solution. When discussing solubility, it's essential to understand hydronium (H3O+) and hydroxide (OH-) ions. Take for instance AgCl and PbF2, these salts are composed of ions that come from strong acids and do not undergo significant hydrolysis. That means, their solubility remains largely unchanged whether you add acid or base to the solution.

In contrast, salts like ZnS, Ca(OH)2, and MgCO3 undergo hydrolysis, which affects their solubility based on the pH of the solution. For instance, MgCO3 will react with the H+ ions from the acid to form H2CO3, showing that a chemical reaction can influence solubility directly through the process of hydrolysis. Salts derived from weak acids or bases can change the pH of a solution, emphasizing the interplay between solubility, hydrolysis, and the acid-base properties of the constituents.

Chemical Reactions in Solubility

Lastly, it's crucial to link chemical reactions directly to solubility. Whether a compound becomes more or less soluble can be predicted by understanding the possible chemical reactions between the ions in the salt and the H+ or OH- ions present in solution. Taking our previous examples, adding acid to ZnS increases its solubility owing to a chemical reaction forming H2S. Similar reactions can predict the solubility behavior in different pH conditions.

In contrast, for AgCl and PbF2, the lack of reaction with added H+ or OH- ions underscores their pH-independent solubility. Such insights are fundamental in various applications, including environmental chemistry, pharmacology, and industrial processing, where controlling the solubility of substances is crucial. Solubility not only depends on physical parameters but is also a direct consequence of chemical reactivity and equilibrium dynamics.