Question

Explain with equations and calculations, when necessary, whether an aqueous solution of each of these salts is acidic, basic, or neutral: (a) \(\mathrm{KBr} ;\) (b) \(\mathrm{NH}_{4} \mathrm{I} ;\) (c) KCN.

Short answer

(a) Neutral; (b) Acidic; (c) Basic.

Step-by-step solution

- Determine the parent acids and bases of the salts

To understand the nature of the aqueous solution of each salt, identify the parent acid and base that form each salt. Check if these parent compounds are strong or weak acids and bases.

- Analyze \(\text{KBr}\)

The salt \(\text{KBr}\) is formed from \( \text{KOH} \), a strong base, and \( \text{HBr} \), a strong acid. Both the cation (\( \text{K}^+ \)) and the anion (\( \text{Br}^- \)) do not hydrolyze in water. Hence, the solution of \(\text{KBr}\) is neutral.

- Analyze \(\text{NH}_4\text{I}\)

The salt \(\text{NH}_4\text{I}\) is formed from \( \text{NH}_3 \), a weak base, and \( \text{HI} \), a strong acid. Here, the cation (\( \text{NH}_4^+ \)) can hydrolyze to produce \( \text{H}^+ \) ions, making the solution acidic: \[ \text{NH}_4^+ + \text{H}_2\text{O} \rightarrow \text{NH}_3 + \text{H}_3\text{O}^+ \].

- Analyze \(\text{KCN}\)

The salt \(\text{KCN}\) is formed from \( \text{KOH} \), a strong base, and \( \text{HCN} \), a weak acid. Here, the anion (\( \text{CN}^- \)) can hydrolyze to produce \( \text{OH}^- \) ions, making the solution basic: \[ \text{CN}^- + \text{H}_2\text{O} \rightarrow \text{HCN} + \text{OH}^- \].

Key concepts

salt hydrolysis

Salt hydrolysis is when a salt interacts with water and undergoes a chemical reaction. This reaction can make the solution acidic, basic, or neutral, depending on the nature of the salt's ions. To determine this, you can look at the parent acid and base from which the salt is derived. For example, if a salt comes from a strong acid and a strong base, it will not hydrolyze, and the solution will remain neutral. However, if one or both of the parent compounds are weak, the salt may hydrolyze, affecting the pH of the solution.

parent acids and bases

The nature of a salt's aqueous solution depends on the parent acids and bases from which it is formed. A parent acid is the acid that contributed the anion (negative ion) to the salt. A parent base is the base that contributed the cation (positive ion) to the salt.

• If both the parent acid and base are strong, the salt solution will be neutral. For instance, KBr is formed from KOH (a strong base) and HBr (a strong acid), resulting in no hydrolysis and a neutral solution.
• If the parent acid is strong and the base is weak, the solution will be acidic. For example, NH₄I is formed from NH₃ (a weak base) and HI (a strong acid), leading to an acidic solution due to the hydrolysis of NH₄⁺.
• If the parent acid is weak and the base is strong, the solution will be basic. KCN, formed from KOH (a strong base) and HCN (a weak acid), will result in a basic solution due to the hydrolysis of CN^-.

aqueous solution chemistry

Aqueous solution chemistry involves understanding how different substances dissolve and interact in water. Knowing the behavior of salts in water helps predict if a solution becomes acidic, basic, or neutral. Here's why it's important:

• When a salt dissolves in water, it splits into its constituent ions.
• These ions might react (hydrolyze) with water, changing the solution's pH.
• Knowing the interactions lets you predict chemical behaviors crucial for sciences and industries. For instance, in oceanography, chemistry, and environmental science, predicting the pH of solutions helps in understanding natural processes.

strong and weak acids and bases

Understanding the strength of acids and bases is key to predicting the behavior of salt solutions. Here are some basics:

• Strong acids and bases completely dissociate in water. Examples include HCl (strong acid) and NaOH (strong base).
• Weak acids and bases only partially dissociate. Examples are acetic acid (CH₃COOH, a weak acid) and ammonia (NH₃, a weak base).
• The degree of dissociation affects whether the ions formed will react with water. Strong acids and bases will result in neutral solutions because their ions do not hydrolyze.
• In contrast, salts derived from weak acids or weak bases can cause hydrolysis, changing the pH of the solution to either acidic or basic.