Question

The \(\mathrm{NH}_{4}^{+}\) ion forms acidic solutions, and the \(\mathrm{CH}_{3} \mathrm{COO}^{-}\) ion forms basic solutions. However, a solution of ammonium acetate is almost neutral. Do all of the ammonium salts of weak acids form neutral solutions? Explain your answer.

Short answer

No, not all ammonium salts of weak acids form neutral solutions; it depends on the specific strengths of the weak acid and weak base.

Step-by-step solution

Identify the Ions and Solutions

Consider the ions \(\text{NH}_4^+\) and \( \text{CH}_3\text{COO}^- \). \( \text{NH}_4^+ \) ion forms acidic solutions while \( \text{CH}_3\text{COO}^- \) ion forms basic solutions. However, when combined in ammonium acetate \( \text{NH}_4\text{CH}_3\text{COO} \), the solution is almost neutral.

Assessing the Nature of Ammonium Salts

Consider an ammonium salt formed with a weak acid, for example, ammonium acetate. The acidic \( \text{NH}_4^+ \) ion comes from the weak base ammonia \( \text{NH}_3 \), and the basic \( \text{CH}_3\text{COO}^- \) ion comes from the weak acid acetic acid \( \text{CH}_3\text{COOH} \).

Buffer Action in Ammonium Acetate Solution

A solution with both a weak acid (acetic acid) and its conjugate base (acetate ion) can act as a buffer. It resists changes to pH upon the addition of small amounts of acid or base, which helps in maintaining a nearly neutral pH.

Analyze the pH of Ammonium Salts of Weak Acids

Not all ammonium salts of weak acids will form neutral solutions. The neutrality depends on the relative strengths of the weak acid and the weak base. The specific dissociation constants \( K_a \) for the acid and \( K_b \) for the conjugate base determine if the solution will be neutral, acidic or basic.

Conclusion

In conclusion, ammonium salts of weak acids do not necessarily form neutral solutions. The nature of the resulting solution (acidic, neutral, or basic) hinges on the specific weak acid and weak base involved and their corresponding strengths.

Key concepts

Ammonium ion acidity

The ammonium ion (\text{NH}_4^+) is known to form acidic solutions when dissolved in water. This is because \text{NH}_4^+ is the conjugate acid of ammonia (\text{NH}_3), a weak base. When \text{NH}_4^+ ion dissociates in water, it donates a hydrogen ion (\text{H}^+) to the solution, which increases the concentration of hydrogen ions, making the solution acidic. The reaction can be represented as:
\( \text{NH}_4^+ + \text{H}_2\text{O} \rightarrow \text{NH}_3 + \text{H}_3\text{O}^+ \)
This behavior is a key characteristic to remember when dealing with ammonium salts and their effects on pH. Understanding why \text{NH}_4^+ forms acidic solutions helps in predicting the nature of solutions containing ammonium salts.

Acetate ion basicity

The acetate ion (\text{CH}_3\text{COO}^-) forms basic solutions in water. It is the conjugate base of acetic acid (\text{CH}_3\text{COOH}), a weak acid. When acetate ions are present in water, they tend to accept hydrogen ions (\text{H}^+), thus reducing the concentration of hydrogen ions and increasing the hydroxide ion (\text{OH}^-) concentration, making the solution basic. The reaction can be represented as:
\( \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{OH}^- \)
This weak basic behavior is crucial for understanding the actions of acetate ions in buffer solutions and other chemical contexts. Knowing how \text{CH}_3\text{COO}^- affects pH helps in predicting the outcomes when combined with other ions.

Buffer Solution

A buffer solution is one that resists changes to its pH when small amounts of an acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid. In the case of ammonium acetate (\text{NH}_4\text{CH}_3\text{COO}), it contains both \text{NH}_4^+, which is a weak acid, and \text{CH}_3\text{COO}^-, which is a weak base. This combination allows the solution to maintain a relatively stable pH by neutralizing added acids or bases. The reactions in a buffer solution can be shown as:

• For added acid: \( \text{CH}_3\text{COO}^- + \text{H}^+ \rightarrow \text{CH}_3\text{COOH} \)
• For added base: \( \text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O} \)

Understanding how buffer solutions work helps explain why some combinations of ions result in nearly neutral solutions despite their individual acidic or basic properties.

Dissociation constants

Dissociation constants (\text{K}_a for acids and \text{K}_b for bases) provide a measure of the strength of acids and bases in water. For weak acids and bases, these constants are key in determining the extent of dissociation into ions. The relation between \text{K}_a and \text{K}_b is given by the water dissociation constant (\text{K}_w):
\( \text{K}_w = \text{K}_a \times \text{K}_b \)
This relationship is instrumental in predicting the pH of solutions containing weak acids and bases. When considering ammonium salts of weak acids, the specific values of \text{K}_a for the acid and \text{K}_b for the conjugate base determine the resulting pH. If \text{K}_a of the weak acid is greater than \text{K}_b of the weak base, the solution will be acidic, and vice versa. Dissociation constants thus provide valuable insights into the behavior of different ionic species in solution.

Weak acids and bases

Weak acids and bases do not completely dissociate in water. This partial dissociation is what characterizes them as 'weak'. For weak acids, like acetic acid (\text{CH}_3\text{COOH}), a small fraction of molecules release hydrogen ions in solution, represented as:
\( \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \)
Similarly, weak bases like ammonia (\text{NH}_3) only partially accept hydrogen ions:
\( \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \)
The partial dissociation leads to the formation of equilibrium in solutions, indicating that both reactants and products are present at equilibrium. The interplay between weak acids and bases is essential in forming buffer solutions and understanding the pH behavior of different substances. This equilibrium context is key for predicting how ammonium salts of weak acids will behave in solutions.